概要
Java的JUC(java.util.concurrent)包中的锁包括"独占锁"和"共享锁"。在“Java多线程系列--“JUC锁”02之 互斥锁ReentrantLock”中，对Java的独占锁进行了说明。本章对Java的“共享锁”进行介绍，JUC中的共享锁有CountDownLatch, CyclicBarrier, Semaphore, ReentrantReadWriteLock等；本章会以ReentrantReadWriteLock为蓝本对共享锁进行说明。内容包括：
ReadWriteLock 和 ReentrantReadWriteLock介绍
ReadWriteLock 和 ReentrantReadWriteLock函数列表ReentrantReadWriteLock数据结构
参考代码(基于JDK1.7.0_40)
获取共享锁
释放共享锁
公平共享锁和非公平共享锁
ReentrantReadWriteLock示例
转载请注明出处：http://www.cnblogs.com/skywang12345/p/3505809.html
ReadWriteLock 和 ReentrantReadWriteLock介绍
ReadWriteLock，顾名思义，是读写锁。它维护了一对相关的锁 — — “读取锁”和“写入锁”，一个用于读取操作，另一个用于写入操作。
“读取锁”用于只读操作，它是“共享锁”，能同时被多个线程获取。
“写入锁”用于写入操作，它是“独占锁”，写入锁只能被一个线程锁获取。
注意：不能同时存在读取锁和写入锁！
ReadWriteLock是一个接口。ReentrantReadWriteLock是它的实现类，ReentrantReadWriteLock包括子类ReadLock和WriteLock。
ReadWriteLock 和 ReentrantReadWriteLock函数列表
ReadWriteLock函数列表
// 返回用于读取操作的锁。
Lock readLock()
// 返回用于写入操作的锁。
Lock writeLock()
ReentrantReadWriteLock函数列表
// 创建一个新的 ReentrantReadWriteLock，默认是采用“非公平策略”。
ReentrantReadWriteLock()
// 创建一个新的 ReentrantReadWriteLock，fair是“公平策略”。fair为true，意味着公平策略；否则，意味着非公平策略。
ReentrantReadWriteLock(boolean fair) // 返回当前拥有写入锁的线程，如果没有这样的线程，则返回 null。
protected Thread getOwner()
// 返回一个 collection，它包含可能正在等待获取读取锁的线程。
protected Collection<Thread> getQueuedReaderThreads()
// 返回一个 collection，它包含可能正在等待获取读取或写入锁的线程。
protected Collection<Thread> getQueuedThreads()
// 返回一个 collection，它包含可能正在等待获取写入锁的线程。
protected Collection<Thread> getQueuedWriterThreads()
// 返回等待获取读取或写入锁的线程估计数目。
int getQueueLength()
// 查询当前线程在此锁上保持的重入读取锁数量。
int getReadHoldCount()
// 查询为此锁保持的读取锁数量。
int getReadLockCount()
// 返回一个 collection，它包含可能正在等待与写入锁相关的给定条件的那些线程。
protected Collection<Thread> getWaitingThreads(Condition condition)
// 返回正等待与写入锁相关的给定条件的线程估计数目。
int getWaitQueueLength(Condition condition)
// 查询当前线程在此锁上保持的重入写入锁数量。
int getWriteHoldCount()
// 查询是否给定线程正在等待获取读取或写入锁。
boolean hasQueuedThread(Thread thread)
// 查询是否所有的线程正在等待获取读取或写入锁。
boolean hasQueuedThreads()
// 查询是否有些线程正在等待与写入锁有关的给定条件。
boolean hasWaiters(Condition condition)
// 如果此锁将公平性设置为 ture，则返回 true。
boolean isFair()
// 查询是否某个线程保持了写入锁。
boolean isWriteLocked()
// 查询当前线程是否保持了写入锁。
boolean isWriteLockedByCurrentThread()
// 返回用于读取操作的锁。
ReentrantReadWriteLock.ReadLock readLock()
// 返回用于写入操作的锁。
ReentrantReadWriteLock.WriteLock writeLock()
ReentrantReadWriteLock数据结构
ReentrantReadWriteLock的UML类图如下：
从中可以看出：
(01) ReentrantReadWriteLock实现了ReadWriteLock接口。ReadWriteLock是一个读写锁的接口，提供了"获取读锁的readLock()函数" 和 "获取写锁的writeLock()函数"。
(02) ReentrantReadWriteLock中包含：sync对象，读锁readerLock和写锁writerLock。读锁ReadLock和写锁WriteLock都实现了Lock接口。读锁ReadLock和写锁WriteLock中也都分别包含了"Sync对象"，它们的Sync对象和ReentrantReadWriteLock的Sync对象 是一样的，就是通过sync，读锁和写锁实现了对同一个对象的访问。
(03) 和"ReentrantLock"一样，sync是Sync类型；而且，Sync也是一个继承于AQS的抽象类。Sync也包括"公平锁"FairSync和"非公平锁"NonfairSync。sync对象是"FairSync"和"NonfairSync"中的一个，默认是"NonfairSync"。
参考代码(基于JDK1.7.0_40)
ReentrantReadWriteLock的完整源码
/*
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*/ /*
*
*
*
*
*
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/publicdomain/zero/1.0/
*/ package java.util.concurrent.locks;
import java.util.concurrent.*;
import java.util.concurrent.atomic.*;
import java.util.*; /**
* An implementation of {@link ReadWriteLock} supporting similar
* semantics to {@link ReentrantLock}.
* <p>This class has the following properties:
*
* <ul>
* <li><b>Acquisition order</b>
*
* <p> This class does not impose a reader or writer preference
* ordering for lock access. However, it does support an optional
* <em>fairness</em> policy.
*
* <dl>
* <dt><b><i>Non-fair mode (default)</i></b>
* <dd>When constructed as non-fair (the default), the order of entry
* to the read and write lock is unspecified, subject to reentrancy
* constraints. A nonfair lock that is continuously contended may
* indefinitely postpone one or more reader or writer threads, but
* will normally have higher throughput than a fair lock.
* <p>
*
* <dt><b><i>Fair mode</i></b>
* <dd> When constructed as fair, threads contend for entry using an
* approximately arrival-order policy. When the currently held lock
* is released either the longest-waiting single writer thread will
* be assigned the write lock, or if there is a group of reader threads
* waiting longer than all waiting writer threads, that group will be
* assigned the read lock.
*
* <p>A thread that tries to acquire a fair read lock (non-reentrantly)
* will block if either the write lock is held, or there is a waiting
* writer thread. The thread will not acquire the read lock until
* after the oldest currently waiting writer thread has acquired and
* released the write lock. Of course, if a waiting writer abandons
* its wait, leaving one or more reader threads as the longest waiters
* in the queue with the write lock free, then those readers will be
* assigned the read lock.
*
* <p>A thread that tries to acquire a fair write lock (non-reentrantly)
* will block unless both the read lock and write lock are free (which
* implies there are no waiting threads). (Note that the non-blocking
* {@link ReadLock#tryLock()} and {@link WriteLock#tryLock()} methods
* do not honor this fair setting and will acquire the lock if it is
* possible, regardless of waiting threads.)
* <p>
* </dl>
*
* <li><b>Reentrancy</b>
*
* <p>This lock allows both readers and writers to reacquire read or
* write locks in the style of a {@link ReentrantLock}. Non-reentrant
* readers are not allowed until all write locks held by the writing
* thread have been released.
*
* <p>Additionally, a writer can acquire the read lock, but not
* vice-versa. Among other applications, reentrancy can be useful
* when write locks are held during calls or callbacks to methods that
* perform reads under read locks. If a reader tries to acquire the
* write lock it will never succeed.
*
* <li><b>Lock downgrading</b>
* <p>Reentrancy also allows downgrading from the write lock to a read lock,
* by acquiring the write lock, then the read lock and then releasing the
* write lock. However, upgrading from a read lock to the write lock is
* <b>not</b> possible.
*
* <li><b>Interruption of lock acquisition</b>
* <p>The read lock and write lock both support interruption during lock
* acquisition.
*
* <li><b>{@link Condition} support</b>
* <p>The write lock provides a {@link Condition} implementation that
* behaves in the same way, with respect to the write lock, as the
* {@link Condition} implementation provided by
* {@link ReentrantLock#newCondition} does for {@link ReentrantLock}.
* This {@link Condition} can, of course, only be used with the write lock.
*
* <p>The read lock does not support a {@link Condition} and
* {@code readLock().newCondition()} throws
* {@code UnsupportedOperationException}.
*
* <li><b>Instrumentation</b>
* <p>This class supports methods to determine whether locks
* are held or contended. These methods are designed for monitoring
* system state, not for synchronization control.
* </ul>
*
* <p>Serialization of this class behaves in the same way as built-in
* locks: a deserialized lock is in the unlocked state, regardless of
* its state when serialized.
*
* <p><b>Sample usages</b>. Here is a code sketch showing how to perform
* lock downgrading after updating a cache (exception handling is
* particularly tricky when handling multiple locks in a non-nested
* fashion):
*
* <pre> {@code
* class CachedData {
* Object data;
* volatile boolean cacheValid;
* final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();
*
* void processCachedData() {
* rwl.readLock().lock();
* if (!cacheValid) {
* // Must release read lock before acquiring write lock
* rwl.readLock().unlock();
* rwl.writeLock().lock();
* try {
* // Recheck state because another thread might have
* // acquired write lock and changed state before we did.
* if (!cacheValid) {
* data = ...
* cacheValid = true;
* }
* // Downgrade by acquiring read lock before releasing write lock
* rwl.readLock().lock();
* } finally {
* rwl.writeLock().unlock(); // Unlock write, still hold read
* }
* }
*
* try {
* use(data);
* } finally {
* rwl.readLock().unlock();
* }
* }
* }}</pre>
*
* ReentrantReadWriteLocks can be used to improve concurrency in some
* uses of some kinds of Collections. This is typically worthwhile
* only when the collections are expected to be large, accessed by
* more reader threads than writer threads, and entail operations with
* overhead that outweighs synchronization overhead. For example, here
* is a class using a TreeMap that is expected to be large and
* concurrently accessed.
*
* <pre>{@code
* class RWDictionary {
* private final Map<String, Data> m = new TreeMap<String, Data>();
* private final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();
* private final Lock r = rwl.readLock();
* private final Lock w = rwl.writeLock();
*
* public Data get(String key) {
* r.lock();
* try { return m.get(key); }
* finally { r.unlock(); }
* }
* public String[] allKeys() {
* r.lock();
* try { return m.keySet().toArray(); }
* finally { r.unlock(); }
* }
* public Data put(String key, Data value) {
* w.lock();
* try { return m.put(key, value); }
* finally { w.unlock(); }
* }
* public void clear() {
* w.lock();
* try { m.clear(); }
* finally { w.unlock(); }
* }
* }}</pre>
*
* <h3>Implementation Notes</h3>
*
* <p>This lock supports a maximum of 65535 recursive write locks
* and 65535 read locks. Attempts to exceed these limits result in
* {@link Error} throws from locking methods.
*
* @since 1.5
* @author Doug Lea
*
*/
public class ReentrantReadWriteLock
implements ReadWriteLock, java.io.Serializable {
private static final long serialVersionUID = -6992448646407690164L;
/** Inner class providing readlock */
private final ReentrantReadWriteLock.ReadLock readerLock;
/** Inner class providing writelock */
private final ReentrantReadWriteLock.WriteLock writerLock;
/** Performs all synchronization mechanics */
final Sync sync; /**
* Creates a new {@code ReentrantReadWriteLock} with
* default (nonfair) ordering properties.
*/
public ReentrantReadWriteLock() {
this(false);
} /**
* Creates a new {@code ReentrantReadWriteLock} with
* the given fairness policy.
*
* @param fair {@code true} if this lock should use a fair ordering policy
*/
public ReentrantReadWriteLock(boolean fair) {
sync = fair ? new FairSync() : new NonfairSync();
readerLock = new ReadLock(this);
writerLock = new WriteLock(this);
} public ReentrantReadWriteLock.WriteLock writeLock() { return writerLock; }
public ReentrantReadWriteLock.ReadLock readLock() { return readerLock; } /**
* Synchronization implementation for ReentrantReadWriteLock.
* Subclassed into fair and nonfair versions.
*/
abstract static class Sync extends AbstractQueuedSynchronizer {
private static final long serialVersionUID = 6317671515068378041L; /*
* Read vs write count extraction constants and functions.
* Lock state is logically divided into two unsigned shorts:
* The lower one representing the exclusive (writer) lock hold count,
* and the upper the shared (reader) hold count.
*/ static final int SHARED_SHIFT = 16;
static final int SHARED_UNIT = (1 << SHARED_SHIFT);
static final int MAX_COUNT = (1 << SHARED_SHIFT) - 1;
static final int EXCLUSIVE_MASK = (1 << SHARED_SHIFT) - 1; /** Returns the number of shared holds represented in count */
static int sharedCount(int c) { return c >>> SHARED_SHIFT; }
/** Returns the number of exclusive holds represented in count */
static int exclusiveCount(int c) { return c & EXCLUSIVE_MASK; } /**
* A counter for per-thread read hold counts.
* Maintained as a ThreadLocal; cached in cachedHoldCounter
*/
static final class HoldCounter {
int count = 0;
// Use id, not reference, to avoid garbage retention
final long tid = Thread.currentThread().getId();
} /**
* ThreadLocal subclass. Easiest to explicitly define for sake
* of deserialization mechanics.
*/
static final class ThreadLocalHoldCounter
extends ThreadLocal<HoldCounter> {
public HoldCounter initialValue() {
return new HoldCounter();
}
} /**
* The number of reentrant read locks held by current thread.
* Initialized only in constructor and readObject.
* Removed whenever a thread's read hold count drops to 0.
*/
private transient ThreadLocalHoldCounter readHolds; /**
* The hold count of the last thread to successfully acquire
* readLock. This saves ThreadLocal lookup in the common case
* where the next thread to release is the last one to
* acquire. This is non-volatile since it is just used
* as a heuristic, and would be great for threads to cache.
*
* <p>Can outlive the Thread for which it is caching the read
* hold count, but avoids garbage retention by not retaining a
* reference to the Thread.
*
* <p>Accessed via a benign data race; relies on the memory
* model's final field and out-of-thin-air guarantees.
*/
private transient HoldCounter cachedHoldCounter; /**
* firstReader is the first thread to have acquired the read lock.
* firstReaderHoldCount is firstReader's hold count.
*
* <p>More precisely, firstReader is the unique thread that last
* changed the shared count from 0 to 1, and has not released the
* read lock since then; null if there is no such thread.
*
* <p>Cannot cause garbage retention unless the thread terminated
* without relinquishing its read locks, since tryReleaseShared
* sets it to null.
*
* <p>Accessed via a benign data race; relies on the memory
* model's out-of-thin-air guarantees for references.
*
* <p>This allows tracking of read holds for uncontended read
* locks to be very cheap.
*/
private transient Thread firstReader = null;
private transient int firstReaderHoldCount; Sync() {
readHolds = new ThreadLocalHoldCounter();
setState(getState()); // ensures visibility of readHolds
} /*
* Acquires and releases use the same code for fair and
* nonfair locks, but differ in whether/how they allow barging
* when queues are non-empty.
*/ /**
* Returns true if the current thread, when trying to acquire
* the read lock, and otherwise eligible to do so, should block
* because of policy for overtaking other waiting threads.
*/
abstract boolean readerShouldBlock(); /**
* Returns true if the current thread, when trying to acquire
* the write lock, and otherwise eligible to do so, should block
* because of policy for overtaking other waiting threads.
*/
abstract boolean writerShouldBlock(); /*
* Note that tryRelease and tryAcquire can be called by
* Conditions. So it is possible that their arguments contain
* both read and write holds that are all released during a
* condition wait and re-established in tryAcquire.
*/ protected final boolean tryRelease(int releases) {
if (!isHeldExclusively())
throw new IllegalMonitorStateException();
int nextc = getState() - releases;
boolean free = exclusiveCount(nextc) == 0;
if (free)
setExclusiveOwnerThread(null);
setState(nextc);
return free;
} protected final boolean tryAcquire(int acquires) {
/*
* Walkthrough:
* 1. If read count nonzero or write count nonzero
* and owner is a different thread, fail.
* 2. If count would saturate, fail. (This can only
* happen if count is already nonzero.)
* 3. Otherwise, this thread is eligible for lock if
* it is either a reentrant acquire or
* queue policy allows it. If so, update state
* and set owner.
*/
Thread current = Thread.currentThread();
int c = getState();
int w = exclusiveCount(c);
if (c != 0) {
// (Note: if c != 0 and w == 0 then shared count != 0)
if (w == 0 || current != getExclusiveOwnerThread())
return false;
if (w + exclusiveCount(acquires) > MAX_COUNT)
throw new Error("Maximum lock count exceeded");
// Reentrant acquire
setState(c + acquires);
return true;
}
if (writerShouldBlock() ||
!compareAndSetState(c, c + acquires))
return false;
setExclusiveOwnerThread(current);
return true;
} protected final boolean tryReleaseShared(int unused) {
Thread current = Thread.currentThread();
if (firstReader == current) {
// assert firstReaderHoldCount > 0;
if (firstReaderHoldCount == 1)
firstReader = null;
else
firstReaderHoldCount--;
} else {
HoldCounter rh = cachedHoldCounter;
if (rh == null || rh.tid != current.getId())
rh = readHolds.get();
int count = rh.count;
if (count <= 1) {
readHolds.remove();
if (count <= 0)
throw unmatchedUnlockException();
}
--rh.count;
}
for (;;) {
int c = getState();
int nextc = c - SHARED_UNIT;
if (compareAndSetState(c, nextc))
// Releasing the read lock has no effect on readers,
// but it may allow waiting writers to proceed if
// both read and write locks are now free.
return nextc == 0;
}
} private IllegalMonitorStateException unmatchedUnlockException() {
return new IllegalMonitorStateException(
"attempt to unlock read lock, not locked by current thread");
} protected final int tryAcquireShared(int unused) {
/*
* Walkthrough:
* 1. If write lock held by another thread, fail.
* 2. Otherwise, this thread is eligible for
* lock wrt state, so ask if it should block
* because of queue policy. If not, try
* to grant by CASing state and updating count.
* Note that step does not check for reentrant
* acquires, which is postponed to full version
* to avoid having to check hold count in
* the more typical non-reentrant case.
* 3. If step 2 fails either because thread
* apparently not eligible or CAS fails or count
* saturated, chain to version with full retry loop.
*/
Thread current = Thread.currentThread();
int c = getState();
if (exclusiveCount(c) != 0 &&
getExclusiveOwnerThread() != current)
return -1;
int r = sharedCount(c);
if (!readerShouldBlock() &&
r < MAX_COUNT &&
compareAndSetState(c, c + SHARED_UNIT)) {
if (r == 0) {
firstReader = current;
firstReaderHoldCount = 1;
} else if (firstReader == current) {
firstReaderHoldCount++;
} else {
HoldCounter rh = cachedHoldCounter;
if (rh == null || rh.tid != current.getId())
cachedHoldCounter = rh = readHolds.get();
else if (rh.count == 0)
readHolds.set(rh);
rh.count++;
}
return 1;
}
return fullTryAcquireShared(current);
} /**
* Full version of acquire for reads, that handles CAS misses
* and reentrant reads not dealt with in tryAcquireShared.
*/
final int fullTryAcquireShared(Thread current) {
/*
* This code is in part redundant with that in
* tryAcquireShared but is simpler overall by not
* complicating tryAcquireShared with interactions between
* retries and lazily reading hold counts.
*/
HoldCounter rh = null;
for (;;) {
int c = getState();
if (exclusiveCount(c) != 0) {
if (getExclusiveOwnerThread() != current)
return -1;
// else we hold the exclusive lock; blocking here
// would cause deadlock.
} else if (readerShouldBlock()) {
// Make sure we're not acquiring read lock reentrantly
if (firstReader == current) {
// assert firstReaderHoldCount > 0;
} else {
if (rh == null) {
rh = cachedHoldCounter;
if (rh == null || rh.tid != current.getId()) {
rh = readHolds.get();
if (rh.count == 0)
readHolds.remove();
}
}
if (rh.count == 0)
return -1;
}
}
if (sharedCount(c) == MAX_COUNT)
throw new Error("Maximum lock count exceeded");
if (compareAndSetState(c, c + SHARED_UNIT)) {
if (sharedCount(c) == 0) {
firstReader = current;
firstReaderHoldCount = 1;
} else if (firstReader == current) {
firstReaderHoldCount++;
} else {
if (rh == null)
rh = cachedHoldCounter;
if (rh == null || rh.tid != current.getId())
rh = readHolds.get();
else if (rh.count == 0)
readHolds.set(rh);
rh.count++;
cachedHoldCounter = rh; // cache for release
}
return 1;
}
}
} /**
* Performs tryLock for write, enabling barging in both modes.
* This is identical in effect to tryAcquire except for lack
* of calls to writerShouldBlock.
*/
final boolean tryWriteLock() {
Thread current = Thread.currentThread();
int c = getState();
if (c != 0) {
int w = exclusiveCount(c);
if (w == 0 || current != getExclusiveOwnerThread())
return false;
if (w == MAX_COUNT)
throw new Error("Maximum lock count exceeded");
}
if (!compareAndSetState(c, c + 1))
return false;
setExclusiveOwnerThread(current);
return true;
} /**
* Performs tryLock for read, enabling barging in both modes.
* This is identical in effect to tryAcquireShared except for
* lack of calls to readerShouldBlock.
*/
final boolean tryReadLock() {
Thread current = Thread.currentThread();
for (;;) {
int c = getState();
if (exclusiveCount(c) != 0 &&
getExclusiveOwnerThread() != current)
return false;
int r = sharedCount(c);
if (r == MAX_COUNT)
throw new Error("Maximum lock count exceeded");
if (compareAndSetState(c, c + SHARED_UNIT)) {
if (r == 0) {
firstReader = current;
firstReaderHoldCount = 1;
} else if (firstReader == current) {
firstReaderHoldCount++;
} else {
HoldCounter rh = cachedHoldCounter;
if (rh == null || rh.tid != current.getId())
cachedHoldCounter = rh = readHolds.get();
else if (rh.count == 0)
readHolds.set(rh);
rh.count++;
}
return true;
}
}
} protected final boolean isHeldExclusively() {
// While we must in general read state before owner,
// we don't need to do so to check if current thread is owner
return getExclusiveOwnerThread() == Thread.currentThread();
} // Methods relayed to outer class final ConditionObject newCondition() {
return new ConditionObject();
} final Thread getOwner() {
// Must read state before owner to ensure memory consistency
return ((exclusiveCount(getState()) == 0) ?
null :
getExclusiveOwnerThread());
} final int getReadLockCount() {
return sharedCount(getState());
} final boolean isWriteLocked() {
return exclusiveCount(getState()) != 0;
} final int getWriteHoldCount() {
return isHeldExclusively() ? exclusiveCount(getState()) : 0;
} final int getReadHoldCount() {
if (getReadLockCount() == 0)
return 0; Thread current = Thread.currentThread();
if (firstReader == current)
return firstReaderHoldCount; HoldCounter rh = cachedHoldCounter;
if (rh != null && rh.tid == current.getId())
return rh.count; int count = readHolds.get().count;
if (count == 0) readHolds.remove();
return count;
} /**
* Reconstitute this lock instance from a stream
* @param s the stream
*/
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
s.defaultReadObject();
readHolds = new ThreadLocalHoldCounter();
setState(0); // reset to unlocked state
} final int getCount() { return getState(); }
} /**
* Nonfair version of Sync
*/
static final class NonfairSync extends Sync {
private static final long serialVersionUID = -8159625535654395037L;
final boolean writerShouldBlock() {
return false; // writers can always barge
}
final boolean readerShouldBlock() {
/* As a heuristic to avoid indefinite writer starvation,
* block if the thread that momentarily appears to be head
* of queue, if one exists, is a waiting writer. This is
* only a probabilistic effect since a new reader will not
* block if there is a waiting writer behind other enabled
* readers that have not yet drained from the queue.
*/
return apparentlyFirstQueuedIsExclusive();
}
} /**
* Fair version of Sync
*/
static final class FairSync extends Sync {
private static final long serialVersionUID = -2274990926593161451L;
final boolean writerShouldBlock() {
return hasQueuedPredecessors();
}
final boolean readerShouldBlock() {
return hasQueuedPredecessors();
}
} /**
* The lock returned by method {@link ReentrantReadWriteLock#readLock}.
*/
public static class ReadLock implements Lock, java.io.Serializable {
private static final long serialVersionUID = -5992448646407690164L;
private final Sync sync; /**
* Constructor for use by subclasses
*
* @param lock the outer lock object
* @throws NullPointerException if the lock is null
*/
protected ReadLock(ReentrantReadWriteLock lock) {
sync = lock.sync;
} /**
* Acquires the read lock.
*
* <p>Acquires the read lock if the write lock is not held by
* another thread and returns immediately.
*
* <p>If the write lock is held by another thread then
* the current thread becomes disabled for thread scheduling
* purposes and lies dormant until the read lock has been acquired.
*/
public void lock() {
sync.acquireShared(1);
} /**
* Acquires the read lock unless the current thread is
* {@linkplain Thread#interrupt interrupted}.
*
* <p>Acquires the read lock if the write lock is not held
* by another thread and returns immediately.
*
* <p>If the write lock is held by another thread then the
* current thread becomes disabled for thread scheduling
* purposes and lies dormant until one of two things happens:
*
* <ul>
*
* <li>The read lock is acquired by the current thread; or
*
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread.
*
* </ul>
*
* <p>If the current thread:
*
* <ul>
*
* <li>has its interrupted status set on entry to this method; or
*
* <li>is {@linkplain Thread#interrupt interrupted} while
* acquiring the read lock,
*
* </ul>
*
* then {@link InterruptedException} is thrown and the current
* thread's interrupted status is cleared.
*
* <p>In this implementation, as this method is an explicit
* interruption point, preference is given to responding to
* the interrupt over normal or reentrant acquisition of the
* lock.
*
* @throws InterruptedException if the current thread is interrupted
*/
public void lockInterruptibly() throws InterruptedException {
sync.acquireSharedInterruptibly(1);
} /**
* Acquires the read lock only if the write lock is not held by
* another thread at the time of invocation.
*
* <p>Acquires the read lock if the write lock is not held by
* another thread and returns immediately with the value
* {@code true}. Even when this lock has been set to use a
* fair ordering policy, a call to {@code tryLock()}
* <em>will</em> immediately acquire the read lock if it is
* available, whether or not other threads are currently
* waiting for the read lock. This "barging" behavior
* can be useful in certain circumstances, even though it
* breaks fairness. If you want to honor the fairness setting
* for this lock, then use {@link #tryLock(long, TimeUnit)
* tryLock(0, TimeUnit.SECONDS) } which is almost equivalent
* (it also detects interruption).
*
* <p>If the write lock is held by another thread then
* this method will return immediately with the value
* {@code false}.
*
* @return {@code true} if the read lock was acquired
*/
public boolean tryLock() {
return sync.tryReadLock();
} /**
* Acquires the read lock if the write lock is not held by
* another thread within the given waiting time and the
* current thread has not been {@linkplain Thread#interrupt
* interrupted}.
*
* <p>Acquires the read lock if the write lock is not held by
* another thread and returns immediately with the value
* {@code true}. If this lock has been set to use a fair
* ordering policy then an available lock <em>will not</em> be
* acquired if any other threads are waiting for the
* lock. This is in contrast to the {@link #tryLock()}
* method. If you want a timed {@code tryLock} that does
* permit barging on a fair lock then combine the timed and
* un-timed forms together:
*
* <pre>if (lock.tryLock() || lock.tryLock(timeout, unit) ) { ... }
* </pre>
*
* <p>If the write lock is held by another thread then the
* current thread becomes disabled for thread scheduling
* purposes and lies dormant until one of three things happens:
*
* <ul>
*
* <li>The read lock is acquired by the current thread; or
*
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread; or
*
* <li>The specified waiting time elapses.
*
* </ul>
*
* <p>If the read lock is acquired then the value {@code true} is
* returned.
*
* <p>If the current thread:
*
* <ul>
*
* <li>has its interrupted status set on entry to this method; or
*
* <li>is {@linkplain Thread#interrupt interrupted} while
* acquiring the read lock,
*
* </ul> then {@link InterruptedException} is thrown and the
* current thread's interrupted status is cleared.
*
* <p>If the specified waiting time elapses then the value
* {@code false} is returned. If the time is less than or
* equal to zero, the method will not wait at all.
*
* <p>In this implementation, as this method is an explicit
* interruption point, preference is given to responding to
* the interrupt over normal or reentrant acquisition of the
* lock, and over reporting the elapse of the waiting time.
*
* @param timeout the time to wait for the read lock
* @param unit the time unit of the timeout argument
* @return {@code true} if the read lock was acquired
* @throws InterruptedException if the current thread is interrupted
* @throws NullPointerException if the time unit is null
*
*/
public boolean tryLock(long timeout, TimeUnit unit)
throws InterruptedException {
return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
} /**
* Attempts to release this lock.
*
* <p> If the number of readers is now zero then the lock
* is made available for write lock attempts.
*/
public void unlock() {
sync.releaseShared(1);
} /**
* Throws {@code UnsupportedOperationException} because
* {@code ReadLocks} do not support conditions.
*
* @throws UnsupportedOperationException always
*/
public Condition newCondition() {
throw new UnsupportedOperationException();
} /**
* Returns a string identifying this lock, as well as its lock state.
* The state, in brackets, includes the String {@code "Read locks ="}
* followed by the number of held read locks.
*
* @return a string identifying this lock, as well as its lock state
*/
public String toString() {
int r = sync.getReadLockCount();
return super.toString() +
"[Read locks = " + r + "]";
}
} /**
* The lock returned by method {@link ReentrantReadWriteLock#writeLock}.
*/
public static class WriteLock implements Lock, java.io.Serializable {
private static final long serialVersionUID = -4992448646407690164L;
private final Sync sync; /**
* Constructor for use by subclasses
*
* @param lock the outer lock object
* @throws NullPointerException if the lock is null
*/
protected WriteLock(ReentrantReadWriteLock lock) {
sync = lock.sync;
} /**
* Acquires the write lock.
*
* <p>Acquires the write lock if neither the read nor write lock
* are held by another thread
* and returns immediately, setting the write lock hold count to
* one.
*
* <p>If the current thread already holds the write lock then the
* hold count is incremented by one and the method returns
* immediately.
*
* <p>If the lock is held by another thread then the current
* thread becomes disabled for thread scheduling purposes and
* lies dormant until the write lock has been acquired, at which
* time the write lock hold count is set to one.
*/
public void lock() {
sync.acquire(1);
} /**
* Acquires the write lock unless the current thread is
* {@linkplain Thread#interrupt interrupted}.
*
* <p>Acquires the write lock if neither the read nor write lock
* are held by another thread
* and returns immediately, setting the write lock hold count to
* one.
*
* <p>If the current thread already holds this lock then the
* hold count is incremented by one and the method returns
* immediately.
*
* <p>If the lock is held by another thread then the current
* thread becomes disabled for thread scheduling purposes and
* lies dormant until one of two things happens:
*
* <ul>
*
* <li>The write lock is acquired by the current thread; or
*
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread.
*
* </ul>
*
* <p>If the write lock is acquired by the current thread then the
* lock hold count is set to one.
*
* <p>If the current thread:
*
* <ul>
*
* <li>has its interrupted status set on entry to this method;
* or
*
* <li>is {@linkplain Thread#interrupt interrupted} while
* acquiring the write lock,
*
* </ul>
*
* then {@link InterruptedException} is thrown and the current
* thread's interrupted status is cleared.
*
* <p>In this implementation, as this method is an explicit
* interruption point, preference is given to responding to
* the interrupt over normal or reentrant acquisition of the
* lock.
*
* @throws InterruptedException if the current thread is interrupted
*/
public void lockInterruptibly() throws InterruptedException {
sync.acquireInterruptibly(1);
} /**
* Acquires the write lock only if it is not held by another thread
* at the time of invocation.
*
* <p>Acquires the write lock if neither the read nor write lock
* are held by another thread
* and returns immediately with the value {@code true},
* setting the write lock hold count to one. Even when this lock has
* been set to use a fair ordering policy, a call to
* {@code tryLock()} <em>will</em> immediately acquire the
* lock if it is available, whether or not other threads are
* currently waiting for the write lock. This "barging"
* behavior can be useful in certain circumstances, even
* though it breaks fairness. If you want to honor the
* fairness setting for this lock, then use {@link
* #tryLock(long, TimeUnit) tryLock(0, TimeUnit.SECONDS) }
* which is almost equivalent (it also detects interruption).
*
* <p> If the current thread already holds this lock then the
* hold count is incremented by one and the method returns
* {@code true}.
*
* <p>If the lock is held by another thread then this method
* will return immediately with the value {@code false}.
*
* @return {@code true} if the lock was free and was acquired
* by the current thread, or the write lock was already held
* by the current thread; and {@code false} otherwise.
*/
public boolean tryLock( ) {
return sync.tryWriteLock();
} /**
* Acquires the write lock if it is not held by another thread
* within the given waiting time and the current thread has
* not been {@linkplain Thread#interrupt interrupted}.
*
* <p>Acquires the write lock if neither the read nor write lock
* are held by another thread
* and returns immediately with the value {@code true},
* setting the write lock hold count to one. If this lock has been
* set to use a fair ordering policy then an available lock
* <em>will not</em> be acquired if any other threads are
* waiting for the write lock. This is in contrast to the {@link
* #tryLock()} method. If you want a timed {@code tryLock}
* that does permit barging on a fair lock then combine the
* timed and un-timed forms together:
*
* <pre>if (lock.tryLock() || lock.tryLock(timeout, unit) ) { ... }
* </pre>
*
* <p>If the current thread already holds this lock then the
* hold count is incremented by one and the method returns
* {@code true}.
*
* <p>If the lock is held by another thread then the current
* thread becomes disabled for thread scheduling purposes and
* lies dormant until one of three things happens:
*
* <ul>
*
* <li>The write lock is acquired by the current thread; or
*
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread; or
*
* <li>The specified waiting time elapses
*
* </ul>
*
* <p>If the write lock is acquired then the value {@code true} is
* returned and the write lock hold count is set to one.
*
* <p>If the current thread:
*
* <ul>
*
* <li>has its interrupted status set on entry to this method;
* or
*
* <li>is {@linkplain Thread#interrupt interrupted} while
* acquiring the write lock,
*
* </ul>
*
* then {@link InterruptedException} is thrown and the current
* thread's interrupted status is cleared.
*
* <p>If the specified waiting time elapses then the value
* {@code false} is returned. If the time is less than or
* equal to zero, the method will not wait at all.
*
* <p>In this implementation, as this method is an explicit
* interruption point, preference is given to responding to
* the interrupt over normal or reentrant acquisition of the
* lock, and over reporting the elapse of the waiting time.
*
* @param timeout the time to wait for the write lock
* @param unit the time unit of the timeout argument
*
* @return {@code true} if the lock was free and was acquired
* by the current thread, or the write lock was already held by the
* current thread; and {@code false} if the waiting time
* elapsed before the lock could be acquired.
*
* @throws InterruptedException if the current thread is interrupted
* @throws NullPointerException if the time unit is null
*
*/
public boolean tryLock(long timeout, TimeUnit unit)
throws InterruptedException {
return sync.tryAcquireNanos(1, unit.toNanos(timeout));
} /**
* Attempts to release this lock.
*
* <p>If the current thread is the holder of this lock then
* the hold count is decremented. If the hold count is now
* zero then the lock is released. If the current thread is
* not the holder of this lock then {@link
* IllegalMonitorStateException} is thrown.
*
* @throws IllegalMonitorStateException if the current thread does not
* hold this lock.
*/
public void unlock() {
sync.release(1);
} /**
* Returns a {@link Condition} instance for use with this
* {@link Lock} instance.
* <p>The returned {@link Condition} instance supports the same
* usages as do the {@link Object} monitor methods ({@link
* Object#wait() wait}, {@link Object#notify notify}, and {@link
* Object#notifyAll notifyAll}) when used with the built-in
* monitor lock.
*
* <ul>
*
* <li>If this write lock is not held when any {@link
* Condition} method is called then an {@link
* IllegalMonitorStateException} is thrown. (Read locks are
* held independently of write locks, so are not checked or
* affected. However it is essentially always an error to
* invoke a condition waiting method when the current thread
* has also acquired read locks, since other threads that
* could unblock it will not be able to acquire the write
* lock.)
*
* <li>When the condition {@linkplain Condition#await() waiting}
* methods are called the write lock is released and, before
* they return, the write lock is reacquired and the lock hold
* count restored to what it was when the method was called.
*
* <li>If a thread is {@linkplain Thread#interrupt interrupted} while
* waiting then the wait will terminate, an {@link
* InterruptedException} will be thrown, and the thread's
* interrupted status will be cleared.
*
* <li> Waiting threads are signalled in FIFO order.
*
* <li>The ordering of lock reacquisition for threads returning
* from waiting methods is the same as for threads initially
* acquiring the lock, which is in the default case not specified,
* but for <em>fair</em> locks favors those threads that have been
* waiting the longest.
*
* </ul>
*
* @return the Condition object
*/
public Condition newCondition() {
return sync.newCondition();
} /**
* Returns a string identifying this lock, as well as its lock
* state. The state, in brackets includes either the String
* {@code "Unlocked"} or the String {@code "Locked by"}
* followed by the {@linkplain Thread#getName name} of the owning thread.
*
* @return a string identifying this lock, as well as its lock state
*/
public String toString() {
Thread o = sync.getOwner();
return super.toString() + ((o == null) ?
"[Unlocked]" :
"[Locked by thread " + o.getName() + "]");
} /**
* Queries if this write lock is held by the current thread.
* Identical in effect to {@link
* ReentrantReadWriteLock#isWriteLockedByCurrentThread}.
*
* @return {@code true} if the current thread holds this lock and
* {@code false} otherwise
* @since 1.6
*/
public boolean isHeldByCurrentThread() {
return sync.isHeldExclusively();
} /**
* Queries the number of holds on this write lock by the current
* thread. A thread has a hold on a lock for each lock action
* that is not matched by an unlock action. Identical in effect
* to {@link ReentrantReadWriteLock#getWriteHoldCount}.
*
* @return the number of holds on this lock by the current thread,
* or zero if this lock is not held by the current thread
* @since 1.6
*/
public int getHoldCount() {
return sync.getWriteHoldCount();
}
} // Instrumentation and status /**
* Returns {@code true} if this lock has fairness set true.
*
* @return {@code true} if this lock has fairness set true
*/
public final boolean isFair() {
return sync instanceof FairSync;
} /**
* Returns the thread that currently owns the write lock, or
* {@code null} if not owned. When this method is called by a
* thread that is not the owner, the return value reflects a
* best-effort approximation of current lock status. For example,
* the owner may be momentarily {@code null} even if there are
* threads trying to acquire the lock but have not yet done so.
* This method is designed to facilitate construction of
* subclasses that provide more extensive lock monitoring
* facilities.
*
* @return the owner, or {@code null} if not owned
*/
protected Thread getOwner() {
return sync.getOwner();
} /**
* Queries the number of read locks held for this lock. This
* method is designed for use in monitoring system state, not for
* synchronization control.
* @return the number of read locks held.
*/
public int getReadLockCount() {
return sync.getReadLockCount();
} /**
* Queries if the write lock is held by any thread. This method is
* designed for use in monitoring system state, not for
* synchronization control.
*
* @return {@code true} if any thread holds the write lock and
* {@code false} otherwise
*/
public boolean isWriteLocked() {
return sync.isWriteLocked();
} /**
* Queries if the write lock is held by the current thread.
*
* @return {@code true} if the current thread holds the write lock and
* {@code false} otherwise
*/
public boolean isWriteLockedByCurrentThread() {
return sync.isHeldExclusively();
} /**
* Queries the number of reentrant write holds on this lock by the
* current thread. A writer thread has a hold on a lock for
* each lock action that is not matched by an unlock action.
*
* @return the number of holds on the write lock by the current thread,
* or zero if the write lock is not held by the current thread
*/
public int getWriteHoldCount() {
return sync.getWriteHoldCount();
} /**
* Queries the number of reentrant read holds on this lock by the
* current thread. A reader thread has a hold on a lock for
* each lock action that is not matched by an unlock action.
*
* @return the number of holds on the read lock by the current thread,
* or zero if the read lock is not held by the current thread
* @since 1.6
*/
public int getReadHoldCount() {
return sync.getReadHoldCount();
} /**
* Returns a collection containing threads that may be waiting to
* acquire the write lock. Because the actual set of threads may
* change dynamically while constructing this result, the returned
* collection is only a best-effort estimate. The elements of the
* returned collection are in no particular order. This method is
* designed to facilitate construction of subclasses that provide
* more extensive lock monitoring facilities.
*
* @return the collection of threads
*/
protected Collection<Thread> getQueuedWriterThreads() {
return sync.getExclusiveQueuedThreads();
} /**
* Returns a collection containing threads that may be waiting to
* acquire the read lock. Because the actual set of threads may
* change dynamically while constructing this result, the returned
* collection is only a best-effort estimate. The elements of the
* returned collection are in no particular order. This method is
* designed to facilitate construction of subclasses that provide
* more extensive lock monitoring facilities.
*
* @return the collection of threads
*/
protected Collection<Thread> getQueuedReaderThreads() {
return sync.getSharedQueuedThreads();
} /**
* Queries whether any threads are waiting to acquire the read or
* write lock. Note that because cancellations may occur at any
* time, a {@code true} return does not guarantee that any other
* thread will ever acquire a lock. This method is designed
* primarily for use in monitoring of the system state.
*
* @return {@code true} if there may be other threads waiting to
* acquire the lock
*/
public final boolean hasQueuedThreads() {
return sync.hasQueuedThreads();
} /**
* Queries whether the given thread is waiting to acquire either
* the read or write lock. Note that because cancellations may
* occur at any time, a {@code true} return does not guarantee
* that this thread will ever acquire a lock. This method is
* designed primarily for use in monitoring of the system state.
*
* @param thread the thread
* @return {@code true} if the given thread is queued waiting for this lock
* @throws NullPointerException if the thread is null
*/
public final boolean hasQueuedThread(Thread thread) {
return sync.isQueued(thread);
} /**
* Returns an estimate of the number of threads waiting to acquire
* either the read or write lock. The value is only an estimate
* because the number of threads may change dynamically while this
* method traverses internal data structures. This method is
* designed for use in monitoring of the system state, not for
* synchronization control.
*
* @return the estimated number of threads waiting for this lock
*/
public final int getQueueLength() {
return sync.getQueueLength();
} /**
* Returns a collection containing threads that may be waiting to
* acquire either the read or write lock. Because the actual set
* of threads may change dynamically while constructing this
* result, the returned collection is only a best-effort estimate.
* The elements of the returned collection are in no particular
* order. This method is designed to facilitate construction of
* subclasses that provide more extensive monitoring facilities.
*
* @return the collection of threads
*/
protected Collection<Thread> getQueuedThreads() {
return sync.getQueuedThreads();
} /**
* Queries whether any threads are waiting on the given condition
* associated with the write lock. Note that because timeouts and
* interrupts may occur at any time, a {@code true} return does
* not guarantee that a future {@code signal} will awaken any
* threads. This method is designed primarily for use in
* monitoring of the system state.
*
* @param condition the condition
* @return {@code true} if there are any waiting threads
* @throws IllegalMonitorStateException if this lock is not held
* @throws IllegalArgumentException if the given condition is
* not associated with this lock
* @throws NullPointerException if the condition is null
*/
public boolean hasWaiters(Condition condition) {
if (condition == null)
throw new NullPointerException();
if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
throw new IllegalArgumentException("not owner");
return sync.hasWaiters((AbstractQueuedSynchronizer.ConditionObject)condition);
} /**
* Returns an estimate of the number of threads waiting on the
* given condition associated with the write lock. Note that because
* timeouts and interrupts may occur at any time, the estimate
* serves only as an upper bound on the actual number of waiters.
* This method is designed for use in monitoring of the system
* state, not for synchronization control.
*
* @param condition the condition
* @return the estimated number of waiting threads
* @throws IllegalMonitorStateException if this lock is not held
* @throws IllegalArgumentException if the given condition is
* not associated with this lock
* @throws NullPointerException if the condition is null
*/
public int getWaitQueueLength(Condition condition) {
if (condition == null)
throw new NullPointerException();
if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
throw new IllegalArgumentException("not owner");
return sync.getWaitQueueLength((AbstractQueuedSynchronizer.ConditionObject)condition);
} /**
* Returns a collection containing those threads that may be
* waiting on the given condition associated with the write lock.
* Because the actual set of threads may change dynamically while
* constructing this result, the returned collection is only a
* best-effort estimate. The elements of the returned collection
* are in no particular order. This method is designed to
* facilitate construction of subclasses that provide more
* extensive condition monitoring facilities.
*
* @param condition the condition
* @return the collection of threads
* @throws IllegalMonitorStateException if this lock is not held
* @throws IllegalArgumentException if the given condition is
* not associated with this lock
* @throws NullPointerException if the condition is null
*/
protected Collection<Thread> getWaitingThreads(Condition condition) {
if (condition == null)
throw new NullPointerException();
if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
throw new IllegalArgumentException("not owner");
return sync.getWaitingThreads((AbstractQueuedSynchronizer.ConditionObject)condition);
} /**
* Returns a string identifying this lock, as well as its lock state.
* The state, in brackets, includes the String {@code "Write locks ="}
* followed by the number of reentrantly held write locks, and the
* String {@code "Read locks ="} followed by the number of held
* read locks.
*
* @return a string identifying this lock, as well as its lock state
*/
public String toString() {
int c = sync.getCount();
int w = Sync.exclusiveCount(c);
int r = Sync.sharedCount(c); return super.toString() +
"[Write locks = " + w + ", Read locks = " + r + "]";
} }
AQS的完整源码
/*
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*/ /*
*
*
*
*
*
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/publicdomain/zero/1.0/
*/ package java.util.concurrent.locks;
import java.util.*;
import java.util.concurrent.*;
import java.util.concurrent.atomic.*;
import sun.misc.Unsafe; /**
* Provides a framework for implementing blocking locks and related
* synchronizers (semaphores, events, etc) that rely on
* first-in-first-out (FIFO) wait queues. This class is designed to
* be a useful basis for most kinds of synchronizers that rely on a
* single atomic <tt>int</tt> value to represent state. Subclasses
* must define the protected methods that change this state, and which
* define what that state means in terms of this object being acquired
* or released. Given these, the other methods in this class carry
* out all queuing and blocking mechanics. Subclasses can maintain
* other state fields, but only the atomically updated <tt>int</tt>
* value manipulated using methods {@link #getState}, {@link
* #setState} and {@link #compareAndSetState} is tracked with respect
* to synchronization.
*
* <p>Subclasses should be defined as non-public internal helper
* classes that are used to implement the synchronization properties
* of their enclosing class. Class
* <tt>AbstractQueuedSynchronizer</tt> does not implement any
* synchronization interface. Instead it defines methods such as
* {@link #acquireInterruptibly} that can be invoked as
* appropriate by concrete locks and related synchronizers to
* implement their public methods.
*
* <p>This class supports either or both a default <em>exclusive</em>
* mode and a <em>shared</em> mode. When acquired in exclusive mode,
* attempted acquires by other threads cannot succeed. Shared mode
* acquires by multiple threads may (but need not) succeed. This class
* does not "understand" these differences except in the
* mechanical sense that when a shared mode acquire succeeds, the next
* waiting thread (if one exists) must also determine whether it can
* acquire as well. Threads waiting in the different modes share the
* same FIFO queue. Usually, implementation subclasses support only
* one of these modes, but both can come into play for example in a
* {@link ReadWriteLock}. Subclasses that support only exclusive or
* only shared modes need not define the methods supporting the unused mode.
*
* <p>This class defines a nested {@link ConditionObject} class that
* can be used as a {@link Condition} implementation by subclasses
* supporting exclusive mode for which method {@link
* #isHeldExclusively} reports whether synchronization is exclusively
* held with respect to the current thread, method {@link #release}
* invoked with the current {@link #getState} value fully releases
* this object, and {@link #acquire}, given this saved state value,
* eventually restores this object to its previous acquired state. No
* <tt>AbstractQueuedSynchronizer</tt> method otherwise creates such a
* condition, so if this constraint cannot be met, do not use it. The
* behavior of {@link ConditionObject} depends of course on the
* semantics of its synchronizer implementation.
*
* <p>This class provides inspection, instrumentation, and monitoring
* methods for the internal queue, as well as similar methods for
* condition objects. These can be exported as desired into classes
* using an <tt>AbstractQueuedSynchronizer</tt> for their
* synchronization mechanics.
*
* <p>Serialization of this class stores only the underlying atomic
* integer maintaining state, so deserialized objects have empty
* thread queues. Typical subclasses requiring serializability will
* define a <tt>readObject</tt> method that restores this to a known
* initial state upon deserialization.
*
* <h3>Usage</h3>
*
* <p>To use this class as the basis of a synchronizer, redefine the
* following methods, as applicable, by inspecting and/or modifying
* the synchronization state using {@link #getState}, {@link
* #setState} and/or {@link #compareAndSetState}:
*
* <ul>
* <li> {@link #tryAcquire}
* <li> {@link #tryRelease}
* <li> {@link #tryAcquireShared}
* <li> {@link #tryReleaseShared}
* <li> {@link #isHeldExclusively}
*</ul>
*
* Each of these methods by default throws {@link
* UnsupportedOperationException}. Implementations of these methods
* must be internally thread-safe, and should in general be short and
* not block. Defining these methods is the <em>only</em> supported
* means of using this class. All other methods are declared
* <tt>final</tt> because they cannot be independently varied.
*
* <p>You may also find the inherited methods from {@link
* AbstractOwnableSynchronizer} useful to keep track of the thread
* owning an exclusive synchronizer. You are encouraged to use them
* -- this enables monitoring and diagnostic tools to assist users in
* determining which threads hold locks.
*
* <p>Even though this class is based on an internal FIFO queue, it
* does not automatically enforce FIFO acquisition policies. The core
* of exclusive synchronization takes the form:
*
* <pre>
* Acquire:
* while (!tryAcquire(arg)) {
* <em>enqueue thread if it is not already queued</em>;
* <em>possibly block current thread</em>;
* }
*
* Release:
* if (tryRelease(arg))
* <em>unblock the first queued thread</em>;
* </pre>
*
* (Shared mode is similar but may involve cascading signals.)
*
* <p><a name="barging">Because checks in acquire are invoked before
* enqueuing, a newly acquiring thread may <em>barge</em> ahead of
* others that are blocked and queued. However, you can, if desired,
* define <tt>tryAcquire</tt> and/or <tt>tryAcquireShared</tt> to
* disable barging by internally invoking one or more of the inspection
* methods, thereby providing a <em>fair</em> FIFO acquisition order.
* In particular, most fair synchronizers can define <tt>tryAcquire</tt>
* to return <tt>false</tt> if {@link #hasQueuedPredecessors} (a method
* specifically designed to be used by fair synchronizers) returns
* <tt>true</tt>. Other variations are possible.
*
* <p>Throughput and scalability are generally highest for the
* default barging (also known as <em>greedy</em>,
* <em>renouncement</em>, and <em>convoy-avoidance</em>) strategy.
* While this is not guaranteed to be fair or starvation-free, earlier
* queued threads are allowed to recontend before later queued
* threads, and each recontention has an unbiased chance to succeed
* against incoming threads. Also, while acquires do not
* "spin" in the usual sense, they may perform multiple
* invocations of <tt>tryAcquire</tt> interspersed with other
* computations before blocking. This gives most of the benefits of
* spins when exclusive synchronization is only briefly held, without
* most of the liabilities when it isn't. If so desired, you can
* augment this by preceding calls to acquire methods with
* "fast-path" checks, possibly prechecking {@link #hasContended}
* and/or {@link #hasQueuedThreads} to only do so if the synchronizer
* is likely not to be contended.
*
* <p>This class provides an efficient and scalable basis for
* synchronization in part by specializing its range of use to
* synchronizers that can rely on <tt>int</tt> state, acquire, and
* release parameters, and an internal FIFO wait queue. When this does
* not suffice, you can build synchronizers from a lower level using
* {@link java.util.concurrent.atomic atomic} classes, your own custom
* {@link java.util.Queue} classes, and {@link LockSupport} blocking
* support.
*
* <h3>Usage Examples</h3>
*
* <p>Here is a non-reentrant mutual exclusion lock class that uses
* the value zero to represent the unlocked state, and one to
* represent the locked state. While a non-reentrant lock
* does not strictly require recording of the current owner
* thread, this class does so anyway to make usage easier to monitor.
* It also supports conditions and exposes
* one of the instrumentation methods:
*
* <pre>
* class Mutex implements Lock, java.io.Serializable {
*
* // Our internal helper class
* private static class Sync extends AbstractQueuedSynchronizer {
* // Report whether in locked state
* protected boolean isHeldExclusively() {
* return getState() == 1;
* }
*
* // Acquire the lock if state is zero
* public boolean tryAcquire(int acquires) {
* assert acquires == 1; // Otherwise unused
* if (compareAndSetState(0, 1)) {
* setExclusiveOwnerThread(Thread.currentThread());
* return true;
* }
* return false;
* }
*
* // Release the lock by setting state to zero
* protected boolean tryRelease(int releases) {
* assert releases == 1; // Otherwise unused
* if (getState() == 0) throw new IllegalMonitorStateException();
* setExclusiveOwnerThread(null);
* setState(0);
* return true;
* }
*
* // Provide a Condition
* Condition newCondition() { return new ConditionObject(); }
*
* // Deserialize properly
* private void readObject(ObjectInputStream s)
* throws IOException, ClassNotFoundException {
* s.defaultReadObject();
* setState(0); // reset to unlocked state
* }
* }
*
* // The sync object does all the hard work. We just forward to it.
* private final Sync sync = new Sync();
*
* public void lock() { sync.acquire(1); }
* public boolean tryLock() { return sync.tryAcquire(1); }
* public void unlock() { sync.release(1); }
* public Condition newCondition() { return sync.newCondition(); }
* public boolean isLocked() { return sync.isHeldExclusively(); }
* public boolean hasQueuedThreads() { return sync.hasQueuedThreads(); }
* public void lockInterruptibly() throws InterruptedException {
* sync.acquireInterruptibly(1);
* }
* public boolean tryLock(long timeout, TimeUnit unit)
* throws InterruptedException {
* return sync.tryAcquireNanos(1, unit.toNanos(timeout));
* }
* }
* </pre>
*
* <p>Here is a latch class that is like a {@link CountDownLatch}
* except that it only requires a single <tt>signal</tt> to
* fire. Because a latch is non-exclusive, it uses the <tt>shared</tt>
* acquire and release methods.
*
* <pre>
* class BooleanLatch {
*
* private static class Sync extends AbstractQueuedSynchronizer {
* boolean isSignalled() { return getState() != 0; }
*
* protected int tryAcquireShared(int ignore) {
* return isSignalled() ? 1 : -1;
* }
*
* protected boolean tryReleaseShared(int ignore) {
* setState(1);
* return true;
* }
* }
*
* private final Sync sync = new Sync();
* public boolean isSignalled() { return sync.isSignalled(); }
* public void signal() { sync.releaseShared(1); }
* public void await() throws InterruptedException {
* sync.acquireSharedInterruptibly(1);
* }
* }
* </pre>
*
* @since 1.5
* @author Doug Lea
*/
public abstract class AbstractQueuedSynchronizer
extends AbstractOwnableSynchronizer
implements java.io.Serializable { private static final long serialVersionUID = 7373984972572414691L; /**
* Creates a new <tt>AbstractQueuedSynchronizer</tt> instance
* with initial synchronization state of zero.
*/
protected AbstractQueuedSynchronizer() { } /**
* Wait queue node class.
*
* <p>The wait queue is a variant of a "CLH" (Craig, Landin, and
* Hagersten) lock queue. CLH locks are normally used for
* spinlocks. We instead use them for blocking synchronizers, but
* use the same basic tactic of holding some of the control
* information about a thread in the predecessor of its node. A
* "status" field in each node keeps track of whether a thread
* should block. A node is signalled when its predecessor
* releases. Each node of the queue otherwise serves as a
* specific-notification-style monitor holding a single waiting
* thread. The status field does NOT control whether threads are
* granted locks etc though. A thread may try to acquire if it is
* first in the queue. But being first does not guarantee success;
* it only gives the right to contend. So the currently released
* contender thread may need to rewait.
*
* <p>To enqueue into a CLH lock, you atomically splice it in as new
* tail. To dequeue, you just set the head field.
* <pre>
* +------+ prev +-----+ +-----+
* head | | <---- | | <---- | | tail
* +------+ +-----+ +-----+
* </pre>
*
* <p>Insertion into a CLH queue requires only a single atomic
* operation on "tail", so there is a simple atomic point of
* demarcation from unqueued to queued. Similarly, dequeing
* involves only updating the "head". However, it takes a bit
* more work for nodes to determine who their successors are,
* in part to deal with possible cancellation due to timeouts
* and interrupts.
*
* <p>The "prev" links (not used in original CLH locks), are mainly
* needed to handle cancellation. If a node is cancelled, its
* successor is (normally) relinked to a non-cancelled
* predecessor. For explanation of similar mechanics in the case
* of spin locks, see the papers by Scott and Scherer at
* http://www.cs.rochester.edu/u/scott/synchronization/
*
* <p>We also use "next" links to implement blocking mechanics.
* The thread id for each node is kept in its own node, so a
* predecessor signals the next node to wake up by traversing
* next link to determine which thread it is. Determination of
* successor must avoid races with newly queued nodes to set
* the "next" fields of their predecessors. This is solved
* when necessary by checking backwards from the atomically
* updated "tail" when a node's successor appears to be null.
* (Or, said differently, the next-links are an optimization
* so that we don't usually need a backward scan.)
*
* <p>Cancellation introduces some conservatism to the basic
* algorithms. Since we must poll for cancellation of other
* nodes, we can miss noticing whether a cancelled node is
* ahead or behind us. This is dealt with by always unparking
* successors upon cancellation, allowing them to stabilize on
* a new predecessor, unless we can identify an uncancelled
* predecessor who will carry this responsibility.
*
* <p>CLH queues need a dummy header node to get started. But
* we don't create them on construction, because it would be wasted
* effort if there is never contention. Instead, the node
* is constructed and head and tail pointers are set upon first
* contention.
*
* <p>Threads waiting on Conditions use the same nodes, but
* use an additional link. Conditions only need to link nodes
* in simple (non-concurrent) linked queues because they are
* only accessed when exclusively held. Upon await, a node is
* inserted into a condition queue. Upon signal, the node is
* transferred to the main queue. A special value of status
* field is used to mark which queue a node is on.
*
* <p>Thanks go to Dave Dice, Mark Moir, Victor Luchangco, Bill
* Scherer and Michael Scott, along with members of JSR-166
* expert group, for helpful ideas, discussions, and critiques
* on the design of this class.
*/
static final class Node {
/** Marker to indicate a node is waiting in shared mode */
static final Node SHARED = new Node();
/** Marker to indicate a node is waiting in exclusive mode */
static final Node EXCLUSIVE = null; /** waitStatus value to indicate thread has cancelled */
static final int CANCELLED = 1;
/** waitStatus value to indicate successor's thread needs unparking */
static final int SIGNAL = -1;
/** waitStatus value to indicate thread is waiting on condition */
static final int CONDITION = -2;
/**
* waitStatus value to indicate the next acquireShared should
* unconditionally propagate
*/
static final int PROPAGATE = -3; /**
* Status field, taking on only the values:
* SIGNAL: The successor of this node is (or will soon be)
* blocked (via park), so the current node must
* unpark its successor when it releases or
* cancels. To avoid races, acquire methods must
* first indicate they need a signal,
* then retry the atomic acquire, and then,
* on failure, block.
* CANCELLED: This node is cancelled due to timeout or interrupt.
* Nodes never leave this state. In particular,
* a thread with cancelled node never again blocks.
* CONDITION: This node is currently on a condition queue.
* It will not be used as a sync queue node
* until transferred, at which time the status
* will be set to 0. (Use of this value here has
* nothing to do with the other uses of the
* field, but simplifies mechanics.)
* PROPAGATE: A releaseShared should be propagated to other
* nodes. This is set (for head node only) in
* doReleaseShared to ensure propagation
* continues, even if other operations have
* since intervened.
* 0: None of the above
*
* The values are arranged numerically to simplify use.
* Non-negative values mean that a node doesn't need to
* signal. So, most code doesn't need to check for particular
* values, just for sign.
*
* The field is initialized to 0 for normal sync nodes, and
* CONDITION for condition nodes. It is modified using CAS
* (or when possible, unconditional volatile writes).
*/
volatile int waitStatus; /**
* Link to predecessor node that current node/thread relies on
* for checking waitStatus. Assigned during enqueing, and nulled
* out (for sake of GC) only upon dequeuing. Also, upon
* cancellation of a predecessor, we short-circuit while
* finding a non-cancelled one, which will always exist
* because the head node is never cancelled: A node becomes
* head only as a result of successful acquire. A
* cancelled thread never succeeds in acquiring, and a thread only
* cancels itself, not any other node.
*/
volatile Node prev; /**
* Link to the successor node that the current node/thread
* unparks upon release. Assigned during enqueuing, adjusted
* when bypassing cancelled predecessors, and nulled out (for
* sake of GC) when dequeued. The enq operation does not
* assign next field of a predecessor until after attachment,
* so seeing a null next field does not necessarily mean that
* node is at end of queue. However, if a next field appears
* to be null, we can scan prev's from the tail to
* double-check. The next field of cancelled nodes is set to
* point to the node itself instead of null, to make life
* easier for isOnSyncQueue.
*/
volatile Node next; /**
* The thread that enqueued this node. Initialized on
* construction and nulled out after use.
*/
volatile Thread thread; /**
* Link to next node waiting on condition, or the special
* value SHARED. Because condition queues are accessed only
* when holding in exclusive mode, we just need a simple
* linked queue to hold nodes while they are waiting on
* conditions. They are then transferred to the queue to
* re-acquire. And because conditions can only be exclusive,
* we save a field by using special value to indicate shared
* mode.
*/
Node nextWaiter; /**
* Returns true if node is waiting in shared mode
*/
final boolean isShared() {
return nextWaiter == SHARED;
} /**
* Returns previous node, or throws NullPointerException if null.
* Use when predecessor cannot be null. The null check could
* be elided, but is present to help the VM.
*
* @return the predecessor of this node
*/
final Node predecessor() throws NullPointerException {
Node p = prev;
if (p == null)
throw new NullPointerException();
else
return p;
} Node() { // Used to establish initial head or SHARED marker
} Node(Thread thread, Node mode) { // Used by addWaiter
this.nextWaiter = mode;
this.thread = thread;
} Node(Thread thread, int waitStatus) { // Used by Condition
this.waitStatus = waitStatus;
this.thread = thread;
}
} /**
* Head of the wait queue, lazily initialized. Except for
* initialization, it is modified only via method setHead. Note:
* If head exists, its waitStatus is guaranteed not to be
* CANCELLED.
*/
private transient volatile Node head; /**
* Tail of the wait queue, lazily initialized. Modified only via
* method enq to add new wait node.
*/
private transient volatile Node tail; /**
* The synchronization state.
*/
private volatile int state; /**
* Returns the current value of synchronization state.
* This operation has memory semantics of a <tt>volatile</tt> read.
* @return current state value
*/
protected final int getState() {
return state;
} /**
* Sets the value of synchronization state.
* This operation has memory semantics of a <tt>volatile</tt> write.
* @param newState the new state value
*/
protected final void setState(int newState) {
state = newState;
} /**
* Atomically sets synchronization state to the given updated
* value if the current state value equals the expected value.
* This operation has memory semantics of a <tt>volatile</tt> read
* and write.
*
* @param expect the expected value
* @param update the new value
* @return true if successful. False return indicates that the actual
* value was not equal to the expected value.
*/
protected final boolean compareAndSetState(int expect, int update) {
// See below for intrinsics setup to support this
return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
} // Queuing utilities /**
* The number of nanoseconds for which it is faster to spin
* rather than to use timed park. A rough estimate suffices
* to improve responsiveness with very short timeouts.
*/
static final long spinForTimeoutThreshold = 1000L; /**
* Inserts node into queue, initializing if necessary. See picture above.
* @param node the node to insert
* @return node's predecessor
*/
private Node enq(final Node node) {
for (;;) {
Node t = tail;
if (t == null) { // Must initialize
if (compareAndSetHead(new Node()))
tail = head;
} else {
node.prev = t;
if (compareAndSetTail(t, node)) {
t.next = node;
return t;
}
}
}
} /**
* Creates and enqueues node for current thread and given mode.
*
* @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
* @return the new node
*/
private Node addWaiter(Node mode) {
Node node = new Node(Thread.currentThread(), mode);
// Try the fast path of enq; backup to full enq on failure
Node pred = tail;
if (pred != null) {
node.prev = pred;
if (compareAndSetTail(pred, node)) {
pred.next = node;
return node;
}
}
enq(node);
return node;
} /**
* Sets head of queue to be node, thus dequeuing. Called only by
* acquire methods. Also nulls out unused fields for sake of GC
* and to suppress unnecessary signals and traversals.
*
* @param node the node
*/
private void setHead(Node node) {
head = node;
node.thread = null;
node.prev = null;
} /**
* Wakes up node's successor, if one exists.
*
* @param node the node
*/
private void unparkSuccessor(Node node) {
/*
* If status is negative (i.e., possibly needing signal) try
* to clear in anticipation of signalling. It is OK if this
* fails or if status is changed by waiting thread.
*/
int ws = node.waitStatus;
if (ws < 0)
compareAndSetWaitStatus(node, ws, 0); /*
* Thread to unpark is held in successor, which is normally
* just the next node. But if cancelled or apparently null,
* traverse backwards from tail to find the actual
* non-cancelled successor.
*/
Node s = node.next;
if (s == null || s.waitStatus > 0) {
s = null;
for (Node t = tail; t != null && t != node; t = t.prev)
if (t.waitStatus <= 0)
s = t;
}
if (s != null)
LockSupport.unpark(s.thread);
} /**
* Release action for shared mode -- signal successor and ensure
* propagation. (Note: For exclusive mode, release just amounts
* to calling unparkSuccessor of head if it needs signal.)
*/
private void doReleaseShared() {
/*
* Ensure that a release propagates, even if there are other
* in-progress acquires/releases. This proceeds in the usual
* way of trying to unparkSuccessor of head if it needs
* signal. But if it does not, status is set to PROPAGATE to
* ensure that upon release, propagation continues.
* Additionally, we must loop in case a new node is added
* while we are doing this. Also, unlike other uses of
* unparkSuccessor, we need to know if CAS to reset status
* fails, if so rechecking.
*/
for (;;) {
Node h = head;
if (h != null && h != tail) {
int ws = h.waitStatus;
if (ws == Node.SIGNAL) {
if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
continue; // loop to recheck cases
unparkSuccessor(h);
}
else if (ws == 0 &&
!compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
continue; // loop on failed CAS
}
if (h == head) // loop if head changed
break;
}
} /**
* Sets head of queue, and checks if successor may be waiting
* in shared mode, if so propagating if either propagate > 0 or
* PROPAGATE status was set.
*
* @param node the node
* @param propagate the return value from a tryAcquireShared
*/
private void setHeadAndPropagate(Node node, int propagate) {
Node h = head; // Record old head for check below
setHead(node);
/*
* Try to signal next queued node if:
* Propagation was indicated by caller,
* or was recorded (as h.waitStatus) by a previous operation
* (note: this uses sign-check of waitStatus because
* PROPAGATE status may transition to SIGNAL.)
* and
* The next node is waiting in shared mode,
* or we don't know, because it appears null
*
* The conservatism in both of these checks may cause
* unnecessary wake-ups, but only when there are multiple
* racing acquires/releases, so most need signals now or soon
* anyway.
*/
if (propagate > 0 || h == null || h.waitStatus < 0) {
Node s = node.next;
if (s == null || s.isShared())
doReleaseShared();
}
} // Utilities for various versions of acquire /**
* Cancels an ongoing attempt to acquire.
*
* @param node the node
*/
private void cancelAcquire(Node node) {
// Ignore if node doesn't exist
if (node == null)
return; node.thread = null; // Skip cancelled predecessors
Node pred = node.prev;
while (pred.waitStatus > 0)
node.prev = pred = pred.prev; // predNext is the apparent node to unsplice. CASes below will
// fail if not, in which case, we lost race vs another cancel
// or signal, so no further action is necessary.
Node predNext = pred.next; // Can use unconditional write instead of CAS here.
// After this atomic step, other Nodes can skip past us.
// Before, we are free of interference from other threads.
node.waitStatus = Node.CANCELLED; // If we are the tail, remove ourselves.
if (node == tail && compareAndSetTail(node, pred)) {
compareAndSetNext(pred, predNext, null);
} else {
// If successor needs signal, try to set pred's next-link
// so it will get one. Otherwise wake it up to propagate.
int ws;
if (pred != head &&
((ws = pred.waitStatus) == Node.SIGNAL ||
(ws <= 0 && compareAndSetWaitStatus(pred, ws, Node.SIGNAL))) &&
pred.thread != null) {
Node next = node.next;
if (next != null && next.waitStatus <= 0)
compareAndSetNext(pred, predNext, next);
} else {
unparkSuccessor(node);
} node.next = node; // help GC
}
} /**
* Checks and updates status for a node that failed to acquire.
* Returns true if thread should block. This is the main signal
* control in all acquire loops. Requires that pred == node.prev
*
* @param pred node's predecessor holding status
* @param node the node
* @return {@code true} if thread should block
*/
private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
int ws = pred.waitStatus;
if (ws == Node.SIGNAL)
/*
* This node has already set status asking a release
* to signal it, so it can safely park.
*/
return true;
if (ws > 0) {
/*
* Predecessor was cancelled. Skip over predecessors and
* indicate retry.
*/
do {
node.prev = pred = pred.prev;
} while (pred.waitStatus > 0);
pred.next = node;
} else {
/*
* waitStatus must be 0 or PROPAGATE. Indicate that we
* need a signal, but don't park yet. Caller will need to
* retry to make sure it cannot acquire before parking.
*/
compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
}
return false;
} /**
* Convenience method to interrupt current thread.
*/
private static void selfInterrupt() {
Thread.currentThread().interrupt();
} /**
* Convenience method to park and then check if interrupted
*
* @return {@code true} if interrupted
*/
private final boolean parkAndCheckInterrupt() {
LockSupport.park(this);
return Thread.interrupted();
} /*
* Various flavors of acquire, varying in exclusive/shared and
* control modes. Each is mostly the same, but annoyingly
* different. Only a little bit of factoring is possible due to
* interactions of exception mechanics (including ensuring that we
* cancel if tryAcquire throws exception) and other control, at
* least not without hurting performance too much.
*/ /**
* Acquires in exclusive uninterruptible mode for thread already in
* queue. Used by condition wait methods as well as acquire.
*
* @param node the node
* @param arg the acquire argument
* @return {@code true} if interrupted while waiting
*/
final boolean acquireQueued(final Node node, int arg) {
boolean failed = true;
try {
boolean interrupted = false;
for (;;) {
final Node p = node.predecessor();
if (p == head && tryAcquire(arg)) {
setHead(node);
p.next = null; // help GC
failed = false;
return interrupted;
}
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
interrupted = true;
}
} finally {
if (failed)
cancelAcquire(node);
}
} /**
* Acquires in exclusive interruptible mode.
* @param arg the acquire argument
*/
private void doAcquireInterruptibly(int arg)
throws InterruptedException {
final Node node = addWaiter(Node.EXCLUSIVE);
boolean failed = true;
try {
for (;;) {
final Node p = node.predecessor();
if (p == head && tryAcquire(arg)) {
setHead(node);
p.next = null; // help GC
failed = false;
return;
}
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
throw new InterruptedException();
}
} finally {
if (failed)
cancelAcquire(node);
}
} /**
* Acquires in exclusive timed mode.
*
* @param arg the acquire argument
* @param nanosTimeout max wait time
* @return {@code true} if acquired
*/
private boolean doAcquireNanos(int arg, long nanosTimeout)
throws InterruptedException {
long lastTime = System.nanoTime();
final Node node = addWaiter(Node.EXCLUSIVE);
boolean failed = true;
try {
for (;;) {
final Node p = node.predecessor();
if (p == head && tryAcquire(arg)) {
setHead(node);
p.next = null; // help GC
failed = false;
return true;
}
if (nanosTimeout <= 0)
return false;
if (shouldParkAfterFailedAcquire(p, node) &&
nanosTimeout > spinForTimeoutThreshold)
LockSupport.parkNanos(this, nanosTimeout);
long now = System.nanoTime();
nanosTimeout -= now - lastTime;
lastTime = now;
if (Thread.interrupted())
throw new InterruptedException();
}
} finally {
if (failed)
cancelAcquire(node);
}
} /**
* Acquires in shared uninterruptible mode.
* @param arg the acquire argument
*/
private void doAcquireShared(int arg) {
final Node node = addWaiter(Node.SHARED);
boolean failed = true;
try {
boolean interrupted = false;
for (;;) {
final Node p = node.predecessor();
if (p == head) {
int r = tryAcquireShared(arg);
if (r >= 0) {
setHeadAndPropagate(node, r);
p.next = null; // help GC
if (interrupted)
selfInterrupt();
failed = false;
return;
}
}
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
interrupted = true;
}
} finally {
if (failed)
cancelAcquire(node);
}
} /**
* Acquires in shared interruptible mode.
* @param arg the acquire argument
*/
private void doAcquireSharedInterruptibly(int arg)
throws InterruptedException {
final Node node = addWaiter(Node.SHARED);
boolean failed = true;
try {
for (;;) {
final Node p = node.predecessor();
if (p == head) {
int r = tryAcquireShared(arg);
if (r >= 0) {
setHeadAndPropagate(node, r);
p.next = null; // help GC
failed = false;
return;
}
}
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
throw new InterruptedException();
}
} finally {
if (failed)
cancelAcquire(node);
}
} /**
* Acquires in shared timed mode.
*
* @param arg the acquire argument
* @param nanosTimeout max wait time
* @return {@code true} if acquired
*/
private boolean doAcquireSharedNanos(int arg, long nanosTimeout)
throws InterruptedException { long lastTime = System.nanoTime();
final Node node = addWaiter(Node.SHARED);
boolean failed = true;
try {
for (;;) {
final Node p = node.predecessor();
if (p == head) {
int r = tryAcquireShared(arg);
if (r >= 0) {
setHeadAndPropagate(node, r);
p.next = null; // help GC
failed = false;
return true;
}
}
if (nanosTimeout <= 0)
return false;
if (shouldParkAfterFailedAcquire(p, node) &&
nanosTimeout > spinForTimeoutThreshold)
LockSupport.parkNanos(this, nanosTimeout);
long now = System.nanoTime();
nanosTimeout -= now - lastTime;
lastTime = now;
if (Thread.interrupted())
throw new InterruptedException();
}
} finally {
if (failed)
cancelAcquire(node);
}
} // Main exported methods /**
* Attempts to acquire in exclusive mode. This method should query
* if the state of the object permits it to be acquired in the
* exclusive mode, and if so to acquire it.
*
* <p>This method is always invoked by the thread performing
* acquire. If this method reports failure, the acquire method
* may queue the thread, if it is not already queued, until it is
* signalled by a release from some other thread. This can be used
* to implement method {@link Lock#tryLock()}.
*
* <p>The default
* implementation throws {@link UnsupportedOperationException}.
*
* @param arg the acquire argument. This value is always the one
* passed to an acquire method, or is the value saved on entry
* to a condition wait. The value is otherwise uninterpreted
* and can represent anything you like.
* @return {@code true} if successful. Upon success, this object has
* been acquired.
* @throws IllegalMonitorStateException if acquiring would place this
* synchronizer in an illegal state. This exception must be
* thrown in a consistent fashion for synchronization to work
* correctly.
* @throws UnsupportedOperationException if exclusive mode is not supported
*/
protected boolean tryAcquire(int arg) {
throw new UnsupportedOperationException();
} /**
* Attempts to set the state to reflect a release in exclusive
* mode.
*
* <p>This method is always invoked by the thread performing release.
*
* <p>The default implementation throws
* {@link UnsupportedOperationException}.
*
* @param arg the release argument. This value is always the one
* passed to a release method, or the current state value upon
* entry to a condition wait. The value is otherwise
* uninterpreted and can represent anything you like.
* @return {@code true} if this object is now in a fully released
* state, so that any waiting threads may attempt to acquire;
* and {@code false} otherwise.
* @throws IllegalMonitorStateException if releasing would place this
* synchronizer in an illegal state. This exception must be
* thrown in a consistent fashion for synchronization to work
* correctly.
* @throws UnsupportedOperationException if exclusive mode is not supported
*/
protected boolean tryRelease(int arg) {
throw new UnsupportedOperationException();
} /**
* Attempts to acquire in shared mode. This method should query if
* the state of the object permits it to be acquired in the shared
* mode, and if so to acquire it.
*
* <p>This method is always invoked by the thread performing
* acquire. If this method reports failure, the acquire method
* may queue the thread, if it is not already queued, until it is
* signalled by a release from some other thread.
*
* <p>The default implementation throws {@link
* UnsupportedOperationException}.
*
* @param arg the acquire argument. This value is always the one
* passed to an acquire method, or is the value saved on entry
* to a condition wait. The value is otherwise uninterpreted
* and can represent anything you like.
* @return a negative value on failure; zero if acquisition in shared
* mode succeeded but no subsequent shared-mode acquire can
* succeed; and a positive value if acquisition in shared
* mode succeeded and subsequent shared-mode acquires might
* also succeed, in which case a subsequent waiting thread
* must check availability. (Support for three different
* return values enables this method to be used in contexts
* where acquires only sometimes act exclusively.) Upon
* success, this object has been acquired.
* @throws IllegalMonitorStateException if acquiring would place this
* synchronizer in an illegal state. This exception must be
* thrown in a consistent fashion for synchronization to work
* correctly.
* @throws UnsupportedOperationException if shared mode is not supported
*/
protected int tryAcquireShared(int arg) {
throw new UnsupportedOperationException();
} /**
* Attempts to set the state to reflect a release in shared mode.
*
* <p>This method is always invoked by the thread performing release.
*
* <p>The default implementation throws
* {@link UnsupportedOperationException}.
*
* @param arg the release argument. This value is always the one
* passed to a release method, or the current state value upon
* entry to a condition wait. The value is otherwise
* uninterpreted and can represent anything you like.
* @return {@code true} if this release of shared mode may permit a
* waiting acquire (shared or exclusive) to succeed; and
* {@code false} otherwise
* @throws IllegalMonitorStateException if releasing would place this
* synchronizer in an illegal state. This exception must be
* thrown in a consistent fashion for synchronization to work
* correctly.
* @throws UnsupportedOperationException if shared mode is not supported
*/
protected boolean tryReleaseShared(int arg) {
throw new UnsupportedOperationException();
} /**
* Returns {@code true} if synchronization is held exclusively with
* respect to the current (calling) thread. This method is invoked
* upon each call to a non-waiting {@link ConditionObject} method.
* (Waiting methods instead invoke {@link #release}.)
*
* <p>The default implementation throws {@link
* UnsupportedOperationException}. This method is invoked
* internally only within {@link ConditionObject} methods, so need
* not be defined if conditions are not used.
*
* @return {@code true} if synchronization is held exclusively;
* {@code false} otherwise
* @throws UnsupportedOperationException if conditions are not supported
*/
protected boolean isHeldExclusively() {
throw new UnsupportedOperationException();
} /**
* Acquires in exclusive mode, ignoring interrupts. Implemented
* by invoking at least once {@link #tryAcquire},
* returning on success. Otherwise the thread is queued, possibly
* repeatedly blocking and unblocking, invoking {@link
* #tryAcquire} until success. This method can be used
* to implement method {@link Lock#lock}.
*
* @param arg the acquire argument. This value is conveyed to
* {@link #tryAcquire} but is otherwise uninterpreted and
* can represent anything you like.
*/
public final void acquire(int arg) {
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
} /**
* Acquires in exclusive mode, aborting if interrupted.
* Implemented by first checking interrupt status, then invoking
* at least once {@link #tryAcquire}, returning on
* success. Otherwise the thread is queued, possibly repeatedly
* blocking and unblocking, invoking {@link #tryAcquire}
* until success or the thread is interrupted. This method can be
* used to implement method {@link Lock#lockInterruptibly}.
*
* @param arg the acquire argument. This value is conveyed to
* {@link #tryAcquire} but is otherwise uninterpreted and
* can represent anything you like.
* @throws InterruptedException if the current thread is interrupted
*/
public final void acquireInterruptibly(int arg)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
if (!tryAcquire(arg))
doAcquireInterruptibly(arg);
} /**
* Attempts to acquire in exclusive mode, aborting if interrupted,
* and failing if the given timeout elapses. Implemented by first
* checking interrupt status, then invoking at least once {@link
* #tryAcquire}, returning on success. Otherwise, the thread is
* queued, possibly repeatedly blocking and unblocking, invoking
* {@link #tryAcquire} until success or the thread is interrupted
* or the timeout elapses. This method can be used to implement
* method {@link Lock#tryLock(long, TimeUnit)}.
*
* @param arg the acquire argument. This value is conveyed to
* {@link #tryAcquire} but is otherwise uninterpreted and
* can represent anything you like.
* @param nanosTimeout the maximum number of nanoseconds to wait
* @return {@code true} if acquired; {@code false} if timed out
* @throws InterruptedException if the current thread is interrupted
*/
public final boolean tryAcquireNanos(int arg, long nanosTimeout)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
return tryAcquire(arg) ||
doAcquireNanos(arg, nanosTimeout);
} /**
* Releases in exclusive mode. Implemented by unblocking one or
* more threads if {@link #tryRelease} returns true.
* This method can be used to implement method {@link Lock#unlock}.
*
* @param arg the release argument. This value is conveyed to
* {@link #tryRelease} but is otherwise uninterpreted and
* can represent anything you like.
* @return the value returned from {@link #tryRelease}
*/
public final boolean release(int arg) {
if (tryRelease(arg)) {
Node h = head;
if (h != null && h.waitStatus != 0)
unparkSuccessor(h);
return true;
}
return false;
} /**
* Acquires in shared mode, ignoring interrupts. Implemented by
* first invoking at least once {@link #tryAcquireShared},
* returning on success. Otherwise the thread is queued, possibly
* repeatedly blocking and unblocking, invoking {@link
* #tryAcquireShared} until success.
*
* @param arg the acquire argument. This value is conveyed to
* {@link #tryAcquireShared} but is otherwise uninterpreted
* and can represent anything you like.
*/
public final void acquireShared(int arg) {
if (tryAcquireShared(arg) < 0)
doAcquireShared(arg);
} /**
* Acquires in shared mode, aborting if interrupted. Implemented
* by first checking interrupt status, then invoking at least once
* {@link #tryAcquireShared}, returning on success. Otherwise the
* thread is queued, possibly repeatedly blocking and unblocking,
* invoking {@link #tryAcquireShared} until success or the thread
* is interrupted.
* @param arg the acquire argument
* This value is conveyed to {@link #tryAcquireShared} but is
* otherwise uninterpreted and can represent anything
* you like.
* @throws InterruptedException if the current thread is interrupted
*/
public final void acquireSharedInterruptibly(int arg)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
if (tryAcquireShared(arg) < 0)
doAcquireSharedInterruptibly(arg);
} /**
* Attempts to acquire in shared mode, aborting if interrupted, and
* failing if the given timeout elapses. Implemented by first
* checking interrupt status, then invoking at least once {@link
* #tryAcquireShared}, returning on success. Otherwise, the
* thread is queued, possibly repeatedly blocking and unblocking,
* invoking {@link #tryAcquireShared} until success or the thread
* is interrupted or the timeout elapses.
*
* @param arg the acquire argument. This value is conveyed to
* {@link #tryAcquireShared} but is otherwise uninterpreted
* and can represent anything you like.
* @param nanosTimeout the maximum number of nanoseconds to wait
* @return {@code true} if acquired; {@code false} if timed out
* @throws InterruptedException if the current thread is interrupted
*/
public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
return tryAcquireShared(arg) >= 0 ||
doAcquireSharedNanos(arg, nanosTimeout);
} /**
* Releases in shared mode. Implemented by unblocking one or more
* threads if {@link #tryReleaseShared} returns true.
*
* @param arg the release argument. This value is conveyed to
* {@link #tryReleaseShared} but is otherwise uninterpreted
* and can represent anything you like.
* @return the value returned from {@link #tryReleaseShared}
*/
public final boolean releaseShared(int arg) {
if (tryReleaseShared(arg)) {
doReleaseShared();
return true;
}
return false;
} // Queue inspection methods /**
* Queries whether any threads are waiting to acquire. Note that
* because cancellations due to interrupts and timeouts may occur
* at any time, a {@code true} return does not guarantee that any
* other thread will ever acquire.
*
* <p>In this implementation, this operation returns in
* constant time.
*
* @return {@code true} if there may be other threads waiting to acquire
*/
public final boolean hasQueuedThreads() {
return head != tail;
} /**
* Queries whether any threads have ever contended to acquire this
* synchronizer; that is if an acquire method has ever blocked.
*
* <p>In this implementation, this operation returns in
* constant time.
*
* @return {@code true} if there has ever been contention
*/
public final boolean hasContended() {
return head != null;
} /**
* Returns the first (longest-waiting) thread in the queue, or
* {@code null} if no threads are currently queued.
*
* <p>In this implementation, this operation normally returns in
* constant time, but may iterate upon contention if other threads are
* concurrently modifying the queue.
*
* @return the first (longest-waiting) thread in the queue, or
* {@code null} if no threads are currently queued
*/
public final Thread getFirstQueuedThread() {
// handle only fast path, else relay
return (head == tail) ? null : fullGetFirstQueuedThread();
} /**
* Version of getFirstQueuedThread called when fastpath fails
*/
private Thread fullGetFirstQueuedThread() {
/*
* The first node is normally head.next. Try to get its
* thread field, ensuring consistent reads: If thread
* field is nulled out or s.prev is no longer head, then
* some other thread(s) concurrently performed setHead in
* between some of our reads. We try this twice before
* resorting to traversal.
*/
Node h, s;
Thread st;
if (((h = head) != null && (s = h.next) != null &&
s.prev == head && (st = s.thread) != null) ||
((h = head) != null && (s = h.next) != null &&
s.prev == head && (st = s.thread) != null))
return st; /*
* Head's next field might not have been set yet, or may have
* been unset after setHead. So we must check to see if tail
* is actually first node. If not, we continue on, safely
* traversing from tail back to head to find first,
* guaranteeing termination.
*/ Node t = tail;
Thread firstThread = null;
while (t != null && t != head) {
Thread tt = t.thread;
if (tt != null)
firstThread = tt;
t = t.prev;
}
return firstThread;
} /**
* Returns true if the given thread is currently queued.
*
* <p>This implementation traverses the queue to determine
* presence of the given thread.
*
* @param thread the thread
* @return {@code true} if the given thread is on the queue
* @throws NullPointerException if the thread is null
*/
public final boolean isQueued(Thread thread) {
if (thread == null)
throw new NullPointerException();
for (Node p = tail; p != null; p = p.prev)
if (p.thread == thread)
return true;
return false;
} /**
* Returns {@code true} if the apparent first queued thread, if one
* exists, is waiting in exclusive mode. If this method returns
* {@code true}, and the current thread is attempting to acquire in
* shared mode (that is, this method is invoked from {@link
* #tryAcquireShared}) then it is guaranteed that the current thread
* is not the first queued thread. Used only as a heuristic in
* ReentrantReadWriteLock.
*/
final boolean apparentlyFirstQueuedIsExclusive() {
Node h, s;
return (h = head) != null &&
(s = h.next) != null &&
!s.isShared() &&
s.thread != null;
} /**
* Queries whether any threads have been waiting to acquire longer
* than the current thread.
*
* <p>An invocation of this method is equivalent to (but may be
* more efficient than):
* <pre> {@code
* getFirstQueuedThread() != Thread.currentThread() &&
* hasQueuedThreads()}</pre>
*
* <p>Note that because cancellations due to interrupts and
* timeouts may occur at any time, a {@code true} return does not
* guarantee that some other thread will acquire before the current
* thread. Likewise, it is possible for another thread to win a
* race to enqueue after this method has returned {@code false},
* due to the queue being empty.
*
* <p>This method is designed to be used by a fair synchronizer to
* avoid <a href="AbstractQueuedSynchronizer#barging">barging</a>.
* Such a synchronizer's {@link #tryAcquire} method should return
* {@code false}, and its {@link #tryAcquireShared} method should
* return a negative value, if this method returns {@code true}
* (unless this is a reentrant acquire). For example, the {@code
* tryAcquire} method for a fair, reentrant, exclusive mode
* synchronizer might look like this:
*
* <pre> {@code
* protected boolean tryAcquire(int arg) {
* if (isHeldExclusively()) {
* // A reentrant acquire; increment hold count
* return true;
* } else if (hasQueuedPredecessors()) {
* return false;
* } else {
* // try to acquire normally
* }
* }}</pre>
*
* @return {@code true} if there is a queued thread preceding the
* current thread, and {@code false} if the current thread
* is at the head of the queue or the queue is empty
* @since 1.7
*/
public final boolean hasQueuedPredecessors() {
// The correctness of this depends on head being initialized
// before tail and on head.next being accurate if the current
// thread is first in queue.
Node t = tail; // Read fields in reverse initialization order
Node h = head;
Node s;
return h != t &&
((s = h.next) == null || s.thread != Thread.currentThread());
} // Instrumentation and monitoring methods /**
* Returns an estimate of the number of threads waiting to
* acquire. The value is only an estimate because the number of
* threads may change dynamically while this method traverses
* internal data structures. This method is designed for use in
* monitoring system state, not for synchronization
* control.
*
* @return the estimated number of threads waiting to acquire
*/
public final int getQueueLength() {
int n = 0;
for (Node p = tail; p != null; p = p.prev) {
if (p.thread != null)
++n;
}
return n;
} /**
* Returns a collection containing threads that may be waiting to
* acquire. Because the actual set of threads may change
* dynamically while constructing this result, the returned
* collection is only a best-effort estimate. The elements of the
* returned collection are in no particular order. This method is
* designed to facilitate construction of subclasses that provide
* more extensive monitoring facilities.
*
* @return the collection of threads
*/
public final Collection<Thread> getQueuedThreads() {
ArrayList<Thread> list = new ArrayList<Thread>();
for (Node p = tail; p != null; p = p.prev) {
Thread t = p.thread;
if (t != null)
list.add(t);
}
return list;
} /**
* Returns a collection containing threads that may be waiting to
* acquire in exclusive mode. This has the same properties
* as {@link #getQueuedThreads} except that it only returns
* those threads waiting due to an exclusive acquire.
*
* @return the collection of threads
*/
public final Collection<Thread> getExclusiveQueuedThreads() {
ArrayList<Thread> list = new ArrayList<Thread>();
for (Node p = tail; p != null; p = p.prev) {
if (!p.isShared()) {
Thread t = p.thread;
if (t != null)
list.add(t);
}
}
return list;
} /**
* Returns a collection containing threads that may be waiting to
* acquire in shared mode. This has the same properties
* as {@link #getQueuedThreads} except that it only returns
* those threads waiting due to a shared acquire.
*
* @return the collection of threads
*/
public final Collection<Thread> getSharedQueuedThreads() {
ArrayList<Thread> list = new ArrayList<Thread>();
for (Node p = tail; p != null; p = p.prev) {
if (p.isShared()) {
Thread t = p.thread;
if (t != null)
list.add(t);
}
}
return list;
} /**
* Returns a string identifying this synchronizer, as well as its state.
* The state, in brackets, includes the String {@code "State ="}
* followed by the current value of {@link #getState}, and either
* {@code "nonempty"} or {@code "empty"} depending on whether the
* queue is empty.
*
* @return a string identifying this synchronizer, as well as its state
*/
public String toString() {
int s = getState();
String q = hasQueuedThreads() ? "non" : "";
return super.toString() +
"[State = " + s + ", " + q + "empty queue]";
} // Internal support methods for Conditions /**
* Returns true if a node, always one that was initially placed on
* a condition queue, is now waiting to reacquire on sync queue.
* @param node the node
* @return true if is reacquiring
*/
final boolean isOnSyncQueue(Node node) {
if (node.waitStatus == Node.CONDITION || node.prev == null)
return false;
if (node.next != null) // If has successor, it must be on queue
return true;
/*
* node.prev can be non-null, but not yet on queue because
* the CAS to place it on queue can fail. So we have to
* traverse from tail to make sure it actually made it. It
* will always be near the tail in calls to this method, and
* unless the CAS failed (which is unlikely), it will be
* there, so we hardly ever traverse much.
*/
return findNodeFromTail(node);
} /**
* Returns true if node is on sync queue by searching backwards from tail.
* Called only when needed by isOnSyncQueue.
* @return true if present
*/
private boolean findNodeFromTail(Node node) {
Node t = tail;
for (;;) {
if (t == node)
return true;
if (t == null)
return false;
t = t.prev;
}
} /**
* Transfers a node from a condition queue onto sync queue.
* Returns true if successful.
* @param node the node
* @return true if successfully transferred (else the node was
* cancelled before signal).
*/
final boolean transferForSignal(Node node) {
/*
* If cannot change waitStatus, the node has been cancelled.
*/
if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))
return false; /*
* Splice onto queue and try to set waitStatus of predecessor to
* indicate that thread is (probably) waiting. If cancelled or
* attempt to set waitStatus fails, wake up to resync (in which
* case the waitStatus can be transiently and harmlessly wrong).
*/
Node p = enq(node);
int ws = p.waitStatus;
if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
LockSupport.unpark(node.thread);
return true;
} /**
* Transfers node, if necessary, to sync queue after a cancelled
* wait. Returns true if thread was cancelled before being
* signalled.
* @param current the waiting thread
* @param node its node
* @return true if cancelled before the node was signalled
*/
final boolean transferAfterCancelledWait(Node node) {
if (compareAndSetWaitStatus(node, Node.CONDITION, 0)) {
enq(node);
return true;
}
/*
* If we lost out to a signal(), then we can't proceed
* until it finishes its enq(). Cancelling during an
* incomplete transfer is both rare and transient, so just
* spin.
*/
while (!isOnSyncQueue(node))
Thread.yield();
return false;
} /**
* Invokes release with current state value; returns saved state.
* Cancels node and throws exception on failure.
* @param node the condition node for this wait
* @return previous sync state
*/
final int fullyRelease(Node node) {
boolean failed = true;
try {
int savedState = getState();
if (release(savedState)) {
failed = false;
return savedState;
} else {
throw new IllegalMonitorStateException();
}
} finally {
if (failed)
node.waitStatus = Node.CANCELLED;
}
} // Instrumentation methods for conditions /**
* Queries whether the given ConditionObject
* uses this synchronizer as its lock.
*
* @param condition the condition
* @return <tt>true</tt> if owned
* @throws NullPointerException if the condition is null
*/
public final boolean owns(ConditionObject condition) {
if (condition == null)
throw new NullPointerException();
return condition.isOwnedBy(this);
} /**
* Queries whether any threads are waiting on the given condition
* associated with this synchronizer. Note that because timeouts
* and interrupts may occur at any time, a <tt>true</tt> return
* does not guarantee that a future <tt>signal</tt> will awaken
* any threads. This method is designed primarily for use in
* monitoring of the system state.
*
* @param condition the condition
* @return <tt>true</tt> if there are any waiting threads
* @throws IllegalMonitorStateException if exclusive synchronization
* is not held
* @throws IllegalArgumentException if the given condition is
* not associated with this synchronizer
* @throws NullPointerException if the condition is null
*/
public final boolean hasWaiters(ConditionObject condition) {
if (!owns(condition))
throw new IllegalArgumentException("Not owner");
return condition.hasWaiters();
} /**
* Returns an estimate of the number of threads waiting on the
* given condition associated with this synchronizer. Note that
* because timeouts and interrupts may occur at any time, the
* estimate serves only as an upper bound on the actual number of
* waiters. This method is designed for use in monitoring of the
* system state, not for synchronization control.
*
* @param condition the condition
* @return the estimated number of waiting threads
* @throws IllegalMonitorStateException if exclusive synchronization
* is not held
* @throws IllegalArgumentException if the given condition is
* not associated with this synchronizer
* @throws NullPointerException if the condition is null
*/
public final int getWaitQueueLength(ConditionObject condition) {
if (!owns(condition))
throw new IllegalArgumentException("Not owner");
return condition.getWaitQueueLength();
} /**
* Returns a collection containing those threads that may be
* waiting on the given condition associated with this
* synchronizer. Because the actual set of threads may change
* dynamically while constructing this result, the returned
* collection is only a best-effort estimate. The elements of the
* returned collection are in no particular order.
*
* @param condition the condition
* @return the collection of threads
* @throws IllegalMonitorStateException if exclusive synchronization
* is not held
* @throws IllegalArgumentException if the given condition is
* not associated with this synchronizer
* @throws NullPointerException if the condition is null
*/
public final Collection<Thread> getWaitingThreads(ConditionObject condition) {
if (!owns(condition))
throw new IllegalArgumentException("Not owner");
return condition.getWaitingThreads();
} /**
* Condition implementation for a {@link
* AbstractQueuedSynchronizer} serving as the basis of a {@link
* Lock} implementation.
*
* <p>Method documentation for this class describes mechanics,
* not behavioral specifications from the point of view of Lock
* and Condition users. Exported versions of this class will in
* general need to be accompanied by documentation describing
* condition semantics that rely on those of the associated
* <tt>AbstractQueuedSynchronizer</tt>.
*
* <p>This class is Serializable, but all fields are transient,
* so deserialized conditions have no waiters.
*/
public class ConditionObject implements Condition, java.io.Serializable {
private static final long serialVersionUID = 1173984872572414699L;
/** First node of condition queue. */
private transient Node firstWaiter;
/** Last node of condition queue. */
private transient Node lastWaiter; /**
* Creates a new <tt>ConditionObject</tt> instance.
*/
public ConditionObject() { } // Internal methods /**
* Adds a new waiter to wait queue.
* @return its new wait node
*/
private Node addConditionWaiter() {
Node t = lastWaiter;
// If lastWaiter is cancelled, clean out.
if (t != null && t.waitStatus != Node.CONDITION) {
unlinkCancelledWaiters();
t = lastWaiter;
}
Node node = new Node(Thread.currentThread(), Node.CONDITION);
if (t == null)
firstWaiter = node;
else
t.nextWaiter = node;
lastWaiter = node;
return node;
} /**
* Removes and transfers nodes until hit non-cancelled one or
* null. Split out from signal in part to encourage compilers
* to inline the case of no waiters.
* @param first (non-null) the first node on condition queue
*/
private void doSignal(Node first) {
do {
if ( (firstWaiter = first.nextWaiter) == null)
lastWaiter = null;
first.nextWaiter = null;
} while (!transferForSignal(first) &&
(first = firstWaiter) != null);
} /**
* Removes and transfers all nodes.
* @param first (non-null) the first node on condition queue
*/
private void doSignalAll(Node first) {
lastWaiter = firstWaiter = null;
do {
Node next = first.nextWaiter;
first.nextWaiter = null;
transferForSignal(first);
first = next;
} while (first != null);
} /**
* Unlinks cancelled waiter nodes from condition queue.
* Called only while holding lock. This is called when
* cancellation occurred during condition wait, and upon
* insertion of a new waiter when lastWaiter is seen to have
* been cancelled. This method is needed to avoid garbage
* retention in the absence of signals. So even though it may
* require a full traversal, it comes into play only when
* timeouts or cancellations occur in the absence of
* signals. It traverses all nodes rather than stopping at a
* particular target to unlink all pointers to garbage nodes
* without requiring many re-traversals during cancellation
* storms.
*/
private void unlinkCancelledWaiters() {
Node t = firstWaiter;
Node trail = null;
while (t != null) {
Node next = t.nextWaiter;
if (t.waitStatus != Node.CONDITION) {
t.nextWaiter = null;
if (trail == null)
firstWaiter = next;
else
trail.nextWaiter = next;
if (next == null)
lastWaiter = trail;
}
else
trail = t;
t = next;
}
} // public methods /**
* Moves the longest-waiting thread, if one exists, from the
* wait queue for this condition to the wait queue for the
* owning lock.
*
* @throws IllegalMonitorStateException if {@link #isHeldExclusively}
* returns {@code false}
*/
public final void signal() {
if (!isHeldExclusively())
throw new IllegalMonitorStateException();
Node first = firstWaiter;
if (first != null)
doSignal(first);
} /**
* Moves all threads from the wait queue for this condition to
* the wait queue for the owning lock.
*
* @throws IllegalMonitorStateException if {@link #isHeldExclusively}
* returns {@code false}
*/
public final void signalAll() {
if (!isHeldExclusively())
throw new IllegalMonitorStateException();
Node first = firstWaiter;
if (first != null)
doSignalAll(first);
} /**
* Implements uninterruptible condition wait.
* <ol>
* <li> Save lock state returned by {@link #getState}.
* <li> Invoke {@link #release} with
* saved state as argument, throwing
* IllegalMonitorStateException if it fails.
* <li> Block until signalled.
* <li> Reacquire by invoking specialized version of
* {@link #acquire} with saved state as argument.
* </ol>
*/
public final void awaitUninterruptibly() {
Node node = addConditionWaiter();
int savedState = fullyRelease(node);
boolean interrupted = false;
while (!isOnSyncQueue(node)) {
LockSupport.park(this);
if (Thread.interrupted())
interrupted = true;
}
if (acquireQueued(node, savedState) || interrupted)
selfInterrupt();
} /*
* For interruptible waits, we need to track whether to throw
* InterruptedException, if interrupted while blocked on
* condition, versus reinterrupt current thread, if
* interrupted while blocked waiting to re-acquire.
*/ /** Mode meaning to reinterrupt on exit from wait */
private static final int REINTERRUPT = 1;
/** Mode meaning to throw InterruptedException on exit from wait */
private static final int THROW_IE = -1; /**
* Checks for interrupt, returning THROW_IE if interrupted
* before signalled, REINTERRUPT if after signalled, or
* 0 if not interrupted.
*/
private int checkInterruptWhileWaiting(Node node) {
return Thread.interrupted() ?
(transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) :
0;
} /**
* Throws InterruptedException, reinterrupts current thread, or
* does nothing, depending on mode.
*/
private void reportInterruptAfterWait(int interruptMode)
throws InterruptedException {
if (interruptMode == THROW_IE)
throw new InterruptedException();
else if (interruptMode == REINTERRUPT)
selfInterrupt();
} /**
* Implements interruptible condition wait.
* <ol>
* <li> If current thread is interrupted, throw InterruptedException.
* <li> Save lock state returned by {@link #getState}.
* <li> Invoke {@link #release} with
* saved state as argument, throwing
* IllegalMonitorStateException if it fails.
* <li> Block until signalled or interrupted.
* <li> Reacquire by invoking specialized version of
* {@link #acquire} with saved state as argument.
* <li> If interrupted while blocked in step 4, throw InterruptedException.
* </ol>
*/
public final void await() throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
Node node = addConditionWaiter();
int savedState = fullyRelease(node);
int interruptMode = 0;
while (!isOnSyncQueue(node)) {
LockSupport.park(this);
if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
break;
}
if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
interruptMode = REINTERRUPT;
if (node.nextWaiter != null) // clean up if cancelled
unlinkCancelledWaiters();
if (interruptMode != 0)
reportInterruptAfterWait(interruptMode);
} /**
* Implements timed condition wait.
* <ol>
* <li> If current thread is interrupted, throw InterruptedException.
* <li> Save lock state returned by {@link #getState}.
* <li> Invoke {@link #release} with
* saved state as argument, throwing
* IllegalMonitorStateException if it fails.
* <li> Block until signalled, interrupted, or timed out.
* <li> Reacquire by invoking specialized version of
* {@link #acquire} with saved state as argument.
* <li> If interrupted while blocked in step 4, throw InterruptedException.
* </ol>
*/
public final long awaitNanos(long nanosTimeout)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
Node node = addConditionWaiter();
int savedState = fullyRelease(node);
long lastTime = System.nanoTime();
int interruptMode = 0;
while (!isOnSyncQueue(node)) {
if (nanosTimeout <= 0L) {
transferAfterCancelledWait(node);
break;
}
LockSupport.parkNanos(this, nanosTimeout);
if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
break; long now = System.nanoTime();
nanosTimeout -= now - lastTime;
lastTime = now;
}
if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
interruptMode = REINTERRUPT;
if (node.nextWaiter != null)
unlinkCancelledWaiters();
if (interruptMode != 0)
reportInterruptAfterWait(interruptMode);
return nanosTimeout - (System.nanoTime() - lastTime);
} /**
* Implements absolute timed condition wait.
* <ol>
* <li> If current thread is interrupted, throw InterruptedException.
* <li> Save lock state returned by {@link #getState}.
* <li> Invoke {@link #release} with
* saved state as argument, throwing
* IllegalMonitorStateException if it fails.
* <li> Block until signalled, interrupted, or timed out.
* <li> Reacquire by invoking specialized version of
* {@link #acquire} with saved state as argument.
* <li> If interrupted while blocked in step 4, throw InterruptedException.
* <li> If timed out while blocked in step 4, return false, else true.
* </ol>
*/
public final boolean awaitUntil(Date deadline)
throws InterruptedException {
if (deadline == null)
throw new NullPointerException();
long abstime = deadline.getTime();
if (Thread.interrupted())
throw new InterruptedException();
Node node = addConditionWaiter();
int savedState = fullyRelease(node);
boolean timedout = false;
int interruptMode = 0;
while (!isOnSyncQueue(node)) {
if (System.currentTimeMillis() > abstime) {
timedout = transferAfterCancelledWait(node);
break;
}
LockSupport.parkUntil(this, abstime);
if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
break;
}
if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
interruptMode = REINTERRUPT;
if (node.nextWaiter != null)
unlinkCancelledWaiters();
if (interruptMode != 0)
reportInterruptAfterWait(interruptMode);
return !timedout;
} /**
* Implements timed condition wait.
* <ol>
* <li> If current thread is interrupted, throw InterruptedException.
* <li> Save lock state returned by {@link #getState}.
* <li> Invoke {@link #release} with
* saved state as argument, throwing
* IllegalMonitorStateException if it fails.
* <li> Block until signalled, interrupted, or timed out.
* <li> Reacquire by invoking specialized version of
* {@link #acquire} with saved state as argument.
* <li> If interrupted while blocked in step 4, throw InterruptedException.
* <li> If timed out while blocked in step 4, return false, else true.
* </ol>
*/
public final boolean await(long time, TimeUnit unit)
throws InterruptedException {
if (unit == null)
throw new NullPointerException();
long nanosTimeout = unit.toNanos(time);
if (Thread.interrupted())
throw new InterruptedException();
Node node = addConditionWaiter();
int savedState = fullyRelease(node);
long lastTime = System.nanoTime();
boolean timedout = false;
int interruptMode = 0;
while (!isOnSyncQueue(node)) {
if (nanosTimeout <= 0L) {
timedout = transferAfterCancelledWait(node);
break;
}
if (nanosTimeout >= spinForTimeoutThreshold)
LockSupport.parkNanos(this, nanosTimeout);
if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
break;
long now = System.nanoTime();
nanosTimeout -= now - lastTime;
lastTime = now;
}
if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
interruptMode = REINTERRUPT;
if (node.nextWaiter != null)
unlinkCancelledWaiters();
if (interruptMode != 0)
reportInterruptAfterWait(interruptMode);
return !timedout;
} // support for instrumentation /**
* Returns true if this condition was created by the given
* synchronization object.
*
* @return {@code true} if owned
*/
final boolean isOwnedBy(AbstractQueuedSynchronizer sync) {
return sync == AbstractQueuedSynchronizer.this;
} /**
* Queries whether any threads are waiting on this condition.
* Implements {@link AbstractQueuedSynchronizer#hasWaiters}.
*
* @return {@code true} if there are any waiting threads
* @throws IllegalMonitorStateException if {@link #isHeldExclusively}
* returns {@code false}
*/
protected final boolean hasWaiters() {
if (!isHeldExclusively())
throw new IllegalMonitorStateException();
for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
if (w.waitStatus == Node.CONDITION)
return true;
}
return false;
} /**
* Returns an estimate of the number of threads waiting on
* this condition.
* Implements {@link AbstractQueuedSynchronizer#getWaitQueueLength}.
*
* @return the estimated number of waiting threads
* @throws IllegalMonitorStateException if {@link #isHeldExclusively}
* returns {@code false}
*/
protected final int getWaitQueueLength() {
if (!isHeldExclusively())
throw new IllegalMonitorStateException();
int n = 0;
for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
if (w.waitStatus == Node.CONDITION)
++n;
}
return n;
} /**
* Returns a collection containing those threads that may be
* waiting on this Condition.
* Implements {@link AbstractQueuedSynchronizer#getWaitingThreads}.
*
* @return the collection of threads
* @throws IllegalMonitorStateException if {@link #isHeldExclusively}
* returns {@code false}
*/
protected final Collection<Thread> getWaitingThreads() {
if (!isHeldExclusively())
throw new IllegalMonitorStateException();
ArrayList<Thread> list = new ArrayList<Thread>();
for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
if (w.waitStatus == Node.CONDITION) {
Thread t = w.thread;
if (t != null)
list.add(t);
}
}
return list;
}
} /**
* Setup to support compareAndSet. We need to natively implement
* this here: For the sake of permitting future enhancements, we
* cannot explicitly subclass AtomicInteger, which would be
* efficient and useful otherwise. So, as the lesser of evils, we
* natively implement using hotspot intrinsics API. And while we
* are at it, we do the same for other CASable fields (which could
* otherwise be done with atomic field updaters).
*/
private static final Unsafe unsafe = Unsafe.getUnsafe();
private static final long stateOffset;
private static final long headOffset;
private static final long tailOffset;
private static final long waitStatusOffset;
private static final long nextOffset; static {
try {
stateOffset = unsafe.objectFieldOffset
(AbstractQueuedSynchronizer.class.getDeclaredField("state"));
headOffset = unsafe.objectFieldOffset
(AbstractQueuedSynchronizer.class.getDeclaredField("head"));
tailOffset = unsafe.objectFieldOffset
(AbstractQueuedSynchronizer.class.getDeclaredField("tail"));
waitStatusOffset = unsafe.objectFieldOffset
(Node.class.getDeclaredField("waitStatus"));
nextOffset = unsafe.objectFieldOffset
(Node.class.getDeclaredField("next")); } catch (Exception ex) { throw new Error(ex); }
} /**
* CAS head field. Used only by enq.
*/
private final boolean compareAndSetHead(Node update) {
return unsafe.compareAndSwapObject(this, headOffset, null, update);
} /**
* CAS tail field. Used only by enq.
*/
private final boolean compareAndSetTail(Node expect, Node update) {
return unsafe.compareAndSwapObject(this, tailOffset, expect, update);
} /**
* CAS waitStatus field of a node.
*/
private static final boolean compareAndSetWaitStatus(Node node,
int expect,
int update) {
return unsafe.compareAndSwapInt(node, waitStatusOffset,
expect, update);
} /**
* CAS next field of a node.
*/
private static final boolean compareAndSetNext(Node node,
Node expect,
Node update) {
return unsafe.compareAndSwapObject(node, nextOffset, expect, update);
}
}
其中，共享锁源码相关的代码如下：
public static class ReadLock implements Lock, java.io.Serializable {
private static final long serialVersionUID = -5992448646407690164L;
// ReentrantReadWriteLock的AQS对象
private final Sync sync;
protected ReadLock(ReentrantReadWriteLock lock) {
sync = lock.sync;
}
// 获取“共享锁”
public void lock() {
sync.acquireShared(1);
}
// 如果线程是中断状态，则抛出一场，否则尝试获取共享锁。
public void lockInterruptibly() throws InterruptedException {
sync.acquireSharedInterruptibly(1);
}
// 尝试获取“共享锁”
public boolean tryLock() {
return sync.tryReadLock();
}
// 在指定时间内，尝试获取“共享锁”
public boolean tryLock(long timeout, TimeUnit unit)
throws InterruptedException {
return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
}
// 释放“共享锁”
public void unlock() {
sync.releaseShared(1);
}
// 新建条件
public Condition newCondition() {
throw new UnsupportedOperationException();
}
public String toString() {
int r = sync.getReadLockCount();
return super.toString() +
"[Read locks = " + r + "]";
}
}
说明：
ReadLock中的sync是一个Sync对象，Sync继承于AQS类，即Sync就是一个锁。ReentrantReadWriteLock中也有一个Sync对象，而且ReadLock中的sync和ReentrantReadWriteLock中的sync是对应关系。即ReentrantReadWriteLock和ReadLock共享同一个AQS对象，共享同一把锁。
ReentrantReadWriteLock中Sync的定义如下：
final Sync sync;
下面，分别从“获取共享锁”和“释放共享锁”两个方面对共享锁进行说明。
获取共享锁
获取共享锁的思想(即lock函数的步骤)，是先通过tryAcquireShared()尝试获取共享锁。尝试成功的话，则直接返回；尝试失败的话，则通过doAcquireShared()不断的循环并尝试获取锁，若有需要，则阻塞等待。doAcquireShared()在循环中每次尝试获取锁时，都是通过tryAcquireShared()来进行尝试的。下面看看“获取共享锁”的详细流程。
1. lock()
lock()在ReadLock中，源码如下：
public void lock() {
sync.acquireShared(1);
}
2. acquireShared()
Sync继承于AQS，acquireShared()定义在AQS中。源码如下：
public final void acquireShared(int arg) {
if (tryAcquireShared(arg) < 0)
doAcquireShared(arg);
}
说明：acquireShared()首先会通过tryAcquireShared()来尝试获取锁。
尝试成功的话，则不再做任何动作(因为已经成功获取到锁了)。
尝试失败的话，则通过doAcquireShared()来获取锁。doAcquireShared()会获取到锁了才返回。
3. tryAcquireShared()
tryAcquireShared()定义在ReentrantReadWriteLock.java的Sync中，源码如下：
protected final int tryAcquireShared(int unused) {
Thread current = Thread.currentThread();
// 获取“锁”的状态
int c = getState();
// 如果“锁”是“互斥锁”，并且获取锁的线程不是current线程；则返回-1。
if (exclusiveCount(c) != 0 &&
getExclusiveOwnerThread() != current)
return -1;
// 获取“读取锁”的共享计数
int r = sharedCount(c);
// 如果“不需要阻塞等待”，并且“读取锁”的共享计数小于MAX_COUNT；
// 则通过CAS函数更新“锁的状态”，将“读取锁”的共享计数+1。
if (!readerShouldBlock() &&
r < MAX_COUNT &&
compareAndSetState(c, c + SHARED_UNIT)) {
// 第1次获取“读取锁”。
if (r == 0) {
firstReader = current;
firstReaderHoldCount = 1;
// 如果想要获取锁的线程(current)是第1个获取锁(firstReader)的线程
} else if (firstReader == current) {
firstReaderHoldCount++;
} else {
// HoldCounter是用来统计该线程获取“读取锁”的次数。
HoldCounter rh = cachedHoldCounter;
if (rh == null || rh.tid != current.getId())
cachedHoldCounter = rh = readHolds.get();
else if (rh.count == 0)
readHolds.set(rh);
// 将该线程获取“读取锁”的次数+1。
rh.count++;
}
return 1;
}
return fullTryAcquireShared(current);
}
说明：tryAcquireShared()的作用是尝试获取“共享锁”。
如果在尝试获取锁时，“不需要阻塞等待”并且“读取锁的共享计数小于MAX_COUNT”，则直接通过CAS函数更新“读取锁的共享计数”，以及将“当前线程获取读取锁的次数+1”。
否则，通过fullTryAcquireShared()获取读取锁。
4. fullTryAcquireShared()
fullTryAcquireShared()在ReentrantReadWriteLock中定义，源码如下：
final int fullTryAcquireShared(Thread current) {
HoldCounter rh = null;
for (;;) {
// 获取“锁”的状态
int c = getState();
// 如果“锁”是“互斥锁”，并且获取锁的线程不是current线程；则返回-1。
if (exclusiveCount(c) != 0) {
if (getExclusiveOwnerThread() != current)
return -1;
// 如果“需要阻塞等待”。
// (01) 当“需要阻塞等待”的线程是第1个获取锁的线程的话，则继续往下执行。
// (02) 当“需要阻塞等待”的线程获取锁的次数=0时，则返回-1。
} else if (readerShouldBlock()) {
// 如果想要获取锁的线程(current)是第1个获取锁(firstReader)的线程
if (firstReader == current) {
} else {
if (rh == null) {
rh = cachedHoldCounter;
if (rh == null || rh.tid != current.getId()) {
rh = readHolds.get();
if (rh.count == 0)
readHolds.remove();
}
}
// 如果当前线程获取锁的计数=0,则返回-1。
if (rh.count == 0)
return -1;
}
}
// 如果“不需要阻塞等待”，则获取“读取锁”的共享统计数；
// 如果共享统计数超过MAX_COUNT，则抛出异常。
if (sharedCount(c) == MAX_COUNT)
throw new Error("Maximum lock count exceeded");
// 将线程获取“读取锁”的次数+1。
if (compareAndSetState(c, c + SHARED_UNIT)) {
// 如果是第1次获取“读取锁”，则更新firstReader和firstReaderHoldCount。
if (sharedCount(c) == 0) {
firstReader = current;
firstReaderHoldCount = 1;
// 如果想要获取锁的线程(current)是第1个获取锁(firstReader)的线程，
// 则将firstReaderHoldCount+1。
} else if (firstReader == current) {
firstReaderHoldCount++;
} else {
if (rh == null)
rh = cachedHoldCounter;
if (rh == null || rh.tid != current.getId())
rh = readHolds.get();
else if (rh.count == 0)
readHolds.set(rh);
// 更新线程的获取“读取锁”的共享计数
rh.count++;
cachedHoldCounter = rh; // cache for release
}
return 1;
}
}
}
说明：fullTryAcquireShared()会根据“是否需要阻塞等待”，“读取锁的共享计数是否超过限制”等等进行处理。如果不需要阻塞等待，并且锁的共享计数没有超过限制，则通过CAS尝试获取锁，并返回1。
5. doAcquireShared()
doAcquireShared()定义在AQS函数中，源码如下：
private void doAcquireShared(int arg) {
// addWaiter(Node.SHARED)的作用是，创建“当前线程”对应的节点，并将该线程添加到CLH队列中。
final Node node = addWaiter(Node.SHARED);
boolean failed = true;
try {
boolean interrupted = false;
for (;;) {
// 获取“node”的前一节点
final Node p = node.predecessor();
// 如果“当前线程”是CLH队列的表头，则尝试获取共享锁。
if (p == head) {
int r = tryAcquireShared(arg);
if (r >= 0) {
setHeadAndPropagate(node, r);
p.next = null; // help GC
if (interrupted)
selfInterrupt();
failed = false;
return;
}
}
// 如果“当前线程”不是CLH队列的表头，则通过shouldParkAfterFailedAcquire()判断是否需要等待，
// 需要的话，则通过parkAndCheckInterrupt()进行阻塞等待。若阻塞等待过程中，线程被中断过，则设置interrupted为true。
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
interrupted = true;
}
} finally {
if (failed)
cancelAcquire(node);
}
}
说明：doAcquireShared()的作用是获取共享锁。
它会首先创建线程对应的CLH队列的节点，然后将该节点添加到CLH队列中。CLH队列是管理获取锁的等待线程的队列。
如果“当前线程”是CLH队列的表头，则尝试获取共享锁；否则，则需要通过shouldParkAfterFailedAcquire()判断是否阻塞等待，需要的话，则通过parkAndCheckInterrupt()进行阻塞等待。
doAcquireShared()会通过for循环，不断的进行上面的操作；目的就是获取共享锁。需要注意的是：doAcquireShared()在每一次尝试获取锁时，是通过tryAcquireShared()来执行的！
shouldParkAfterFailedAcquire(), parkAndCheckInterrupt()等函数已经在“Java多线程系列--“JUC锁”03之 公平锁(一) ”中详细介绍过，这里就不再重复说明了。
释放共享锁
释放共享锁的思想，是先通过tryReleaseShared()尝试释放共享锁。尝试成功的话，则通过doReleaseShared()唤醒“其他等待获取共享锁的线程”，并返回true；否则的话，返回flase。
1. unlock()
public void unlock() {
sync.releaseShared(1);
}
说明：该函数实际上调用releaseShared(1)释放共享锁。
2. releaseShared()
releaseShared()在AQS中实现，源码如下：
public final boolean releaseShared(int arg) {
if (tryReleaseShared(arg)) {
doReleaseShared();
return true;
}
return false;
}
说明：releaseShared()的目的是让当前线程释放它所持有的共享锁。
它首先会通过tryReleaseShared()去尝试释放共享锁。尝试成功，则直接返回；尝试失败，则通过doReleaseShared()去释放共享锁。
3. tryReleaseShared()
tryReleaseShared()定义在ReentrantReadWriteLock中，源码如下：
protected final boolean tryReleaseShared(int unused) {
// 获取当前线程，即释放共享锁的线程。
Thread current = Thread.currentThread();
// 如果想要释放锁的线程(current)是第1个获取锁(firstReader)的线程，
// 并且“第1个获取锁的线程获取锁的次数”=1，则设置firstReader为null；
// 否则，将“第1个获取锁的线程的获取次数”-1。
if (firstReader == current) {
// assert firstReaderHoldCount > 0;
if (firstReaderHoldCount == 1)
firstReader = null;
else
firstReaderHoldCount--;
// 获取rh对象，并更新“当前线程获取锁的信息”。
} else {
HoldCounter rh = cachedHoldCounter;
if (rh == null || rh.tid != current.getId())
rh = readHolds.get();
int count = rh.count;
if (count <= 1) {
readHolds.remove();
if (count <= 0)
throw unmatchedUnlockException();
}
--rh.count;
}
for (;;) {
// 获取锁的状态
int c = getState();
// 将锁的获取次数-1。
int nextc = c - SHARED_UNIT;
// 通过CAS更新锁的状态。
if (compareAndSetState(c, nextc))
return nextc == 0;
}
}
说明：tryReleaseShared()的作用是尝试释放共享锁。
4. doReleaseShared()
doReleaseShared()定义在AQS中，源码如下：
private void doReleaseShared() {
for (;;) {
// 获取CLH队列的头节点
Node h = head;
// 如果头节点不为null，并且头节点不等于tail节点。
if (h != null && h != tail) {
// 获取头节点对应的线程的状态
int ws = h.waitStatus;
// 如果头节点对应的线程是SIGNAL状态，则意味着“头节点的下一个节点所对应的线程”需要被unpark唤醒。
if (ws == Node.SIGNAL) {
// 设置“头节点对应的线程状态”为空状态。失败的话，则继续循环。
if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
continue;
// 唤醒“头节点的下一个节点所对应的线程”。
unparkSuccessor(h);
}
// 如果头节点对应的线程是空状态，则设置“文件点对应的线程所拥有的共享锁”为其它线程获取锁的空状态。
else if (ws == 0 &&
!compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
continue; // loop on failed CAS
}
// 如果头节点发生变化，则继续循环。否则，退出循环。
if (h == head) // loop if head changed
break;
}
}
说明：doReleaseShared()会释放“共享锁”。它会从前往后的遍历CLH队列，依次“唤醒”然后“执行”队列中每个节点对应的线程；最终的目的是让这些线程释放它们所持有的锁。
公平共享锁和非公平共享锁
和互斥锁ReentrantLock一样，ReadLock也分为公平锁和非公平锁。
公平锁和非公平锁的区别，体现在判断是否需要阻塞的函数readerShouldBlock()是不同的。
公平锁的readerShouldBlock()的源码如下：
final boolean readerShouldBlock() {
return hasQueuedPredecessors();
}
在公平共享锁中，如果在当前线程的前面有其他线程在等待获取共享锁，则返回true；否则，返回false。
非公平锁的readerShouldBlock()的源码如下：
final boolean readerShouldBlock() {
return apparentlyFirstQueuedIsExclusive();
}
在非公平共享锁中，它会无视当前线程的前面是否有其他线程在等待获取共享锁。只要该非公平共享锁对应的线程不为null，则返回true。
ReentrantReadWriteLock示例
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock; public class ReadWriteLockTest1 { public static void main(String[] args) {
// 创建账户
MyCount myCount = new MyCount("4238920615242830", 10000);
// 创建用户，并指定账户
User user = new User("Tommy", myCount); // 分别启动3个“读取账户金钱”的线程 和 3个“设置账户金钱”的线程
for (int i=0; i<3; i++) {
user.getCash();
user.setCash((i+1)*1000);
}
}
} class User {
private String name; //用户名
private MyCount myCount; //所要操作的账户
private ReadWriteLock myLock; //执行操作所需的锁对象 User(String name, MyCount myCount) {
this.name = name;
this.myCount = myCount;
this.myLock = new ReentrantReadWriteLock();
} public void getCash() {
new Thread() {
public void run() {
myLock.readLock().lock();
try {
System.out.println(Thread.currentThread().getName() +" getCash start");
myCount.getCash();
Thread.sleep(1);
System.out.println(Thread.currentThread().getName() +" getCash end");
} catch (InterruptedException e) {
} finally {
myLock.readLock().unlock();
}
}
}.start();
} public void setCash(final int cash) {
new Thread() {
public void run() {
myLock.writeLock().lock();
try {
System.out.println(Thread.currentThread().getName() +" setCash start");
myCount.setCash(cash);
Thread.sleep(1);
System.out.println(Thread.currentThread().getName() +" setCash end");
} catch (InterruptedException e) {
} finally {
myLock.writeLock().unlock();
}
}
}.start();
}
} class MyCount {
private String id; //账号
private int cash; //账户余额 MyCount(String id, int cash) {
this.id = id;
this.cash = cash;
} public String getId() {
return id;
} public void setId(String id) {
this.id = id;
} public int getCash() {
System.out.println(Thread.currentThread().getName() +" getCash cash="+ cash);
return cash;
} public void setCash(int cash) {
System.out.println(Thread.currentThread().getName() +" setCash cash="+ cash);
this.cash = cash;
}
}
运行结果：
Thread-0 getCash start
Thread-2 getCash start
Thread-0 getCash cash=10000
Thread-2 getCash cash=10000
Thread-0 getCash end
Thread-2 getCash end
Thread-1 setCash start
Thread-1 setCash cash=1000
Thread-1 setCash end
Thread-3 setCash start
Thread-3 setCash cash=2000
Thread-3 setCash end
Thread-4 getCash start
Thread-4 getCash cash=2000
Thread-4 getCash end
Thread-5 setCash start
Thread-5 setCash cash=3000
Thread-5 setCash end
结果说明：
(01) 观察Thread0和Thread-2的运行结果，我们发现，Thread-0启动并获取到“读取锁”，在它还没运行完毕的时候，Thread-2也启动了并且也成功获取到“读取锁”。
因此，“读取锁”支持被多个线程同时获取。
(02) 观察Thread-1,Thread-3,Thread-5这三个“写入锁”的线程。只要“写入锁”被某线程获取，则该线程运行完毕了，才释放该锁。
因此，“写入锁”不支持被多个线程同时获取。
更多内容
2. Java多线程系列--“JUC锁”02之 互斥锁ReentrantLock
3. Java多线程系列--“JUC锁”03之 公平锁(一)
4. Java多线程系列--“JUC锁”04之 公平锁(二)
6. Java多线程系列--“JUC锁”06之 Condition条件
7. Java多线程系列--“JUC锁”07之 LockSupport
Java多线程系列--“JUC锁”08之 共享锁和ReentrantReadWriteLock的更多相关文章
-
Java多线程系列 JUC锁08 LockSupport
转载 http://www.cnblogs.com/skywang12345/p/3505784.html https://www.cnblogs.com/leesf456/p/5347293.htm ...
-
Java多线程系列--“JUC锁”10之 CyclicBarrier原理和示例
概要 本章介绍JUC包中的CyclicBarrier锁.内容包括:CyclicBarrier简介CyclicBarrier数据结构CyclicBarrier源码分析(基于JDK1.7.0_40)Cyc ...
-
Java多线程系列--“JUC锁”01之 框架
本章,我们介绍锁的架构:后面的章节将会对它们逐个进行分析介绍.目录如下:01. Java多线程系列--“JUC锁”01之 框架02. Java多线程系列--“JUC锁”02之 互斥锁Reentrant ...
-
Java多线程系列--“JUC锁”09之 CountDownLatch原理和示例
概要 前面对"独占锁"和"共享锁"有了个大致的了解:本章,我们对CountDownLatch进行学习.和ReadWriteLock.ReadLock一样,Cou ...
-
Java多线程系列--“JUC锁”11之 Semaphore信号量的原理和示例
概要 本章,我们对JUC包中的信号量Semaphore进行学习.内容包括:Semaphore简介Semaphore数据结构Semaphore源码分析(基于JDK1.7.0_40)Semaphore示例 ...
-
Java多线程系列--“JUC锁”03之 公平锁(一)
概要 本章对“公平锁”的获取锁机制进行介绍(本文的公平锁指的是互斥锁的公平锁),内容包括:基本概念ReentrantLock数据结构参考代码获取公平锁(基于JDK1.7.0_40)一. tryAcqu ...
-
Java多线程系列--“JUC锁”04之 公平锁(二)
概要 前面一章,我们学习了“公平锁”获取锁的详细流程:这里,我们再来看看“公平锁”释放锁的过程.内容包括:参考代码释放公平锁(基于JDK1.7.0_40) “公平锁”的获取过程请参考“Java多线程系 ...
-
Java多线程系列--“JUC锁”06之 Condition条件
概要 前面对JUC包中的锁的原理进行了介绍,本章会JUC中对与锁经常配合使用的Condition进行介绍,内容包括:Condition介绍Condition函数列表Condition示例转载请注明出处 ...
-
Java多线程系列--“JUC锁”02之 互斥锁ReentrantLock
本章对ReentrantLock包进行基本介绍,这一章主要对ReentrantLock进行概括性的介绍,内容包括:ReentrantLock介绍ReentrantLock函数列表ReentrantLo ...
随机推荐
-
Spring JavaMail发送邮件
JavaMail的介绍 JavaMail,顾名思义,提供给开发者处理电子邮件相关的编程接口.它是Sun发布的用来处理email的API.它可以方便地执行一些常用的邮件传输. 虽然JavaMail是 ...
-
计算节点宕机了怎么办？- 每天5分钟玩转 OpenStack（43）
Rebuild 可以恢复损坏的 instance. 那如果是宿主机坏了怎么办呢? 比如硬件故障或者断电造成整台计算节点无法工作,该节点上运行的 instance 如何恢复呢? 用 Shelve 或者 ...
-
添加和删除hadoop集群中的节点
参见 http://www.cnblogs.com/tommyli/p/3418273.html
-
Magento后台订单显示产品图片的修改方法
Magento后台订单原来是没有显示产品图片,客服业务还得到网站前台查找这个产品的图片提供给发货部,这样是很不方便的.为提高工作效率,应客服业务要求,现对Magento后台订单进行修改,使订单页面就显 ...
-
delphi tidhttp 超时的解决方案
现在delphi都发布到xe10.1了,tidhttp还有缺陷,那就是超时设置在没有网络或者连不上服务器的时候是无效的,不管你设置为多少都要10-20秒.connectTimeout和readTime ...
-
【thinkphp 5 在nginx 环境下路由无法生效（404 500错误 ）的解决方法】
非常惭愧的说,由于之前一直使用的是windowservice,安装apache来进行服务器布置的,这种方式也是最简单最直接的方式, 但是由于php的服务大多都是linux栈的,咱们也不能落后呀,在 ...
-
zabbix监控mysql性能
使用zabbix监控mysql的三种方式 1.只是安装agent 2.启用模板监控 3.启用自定义脚本的模板监控 zabbix中默认有mysql的监控模板.默认已经在zabbix2.2及以上的版本中. ...
-
Tomcat的常用内置对象
Tomcat的常用内置对象 1.request内置对象 所谓内置对象就是容器已经创建好了的对象,如果收到一个用户的请求就会自动创建一个对象来处理客户端发送的一些信息,这个内置对象就是request.类 ...
-
kettle性能优化
普通开发电脑,如果没有网络查询步骤,kettle正常的速度应该在3000~20000条/秒.如果速度在2000条/秒一下,就可能需要调优. 性能优化的方式包括如下几种: 1.通过改变开始复制的数量(针 ...
-
BZOJ2160: 拉拉队排练
Description 艾利斯顿商学院篮球队要参加一年一度的市篮球比赛了.拉拉队是篮球比赛的一个看点,好的拉拉队往往能帮助球队增加士气,赢得最终的比赛.所以作为拉拉队队长的楚雨荨同学知道,帮助篮球队训 ...