摘要
Lock在硬件层面依赖CPU指令，完全由Java代码完成，底层利用LockSupport类和Unsafe类进行操作；
虽然锁有很多实现，但是都依赖AbstractQueuedSynchronizer类，我们用ReentrantLock进行讲解；
ReentrantLock调用过程
ReentrantLock类的API调用都委托给一个内部类 Sync ,而该类继承了 AbstractQueuedSynchronizer类；
public class ReentrantLock implements Lock, java.io.Serializable {
......
abstract static class Sync extends AbstractQueuedSynchronizer {
......
而Sync又分为两个子类：公平锁和非公平锁，默认为非公平锁
/** * Sync object for non-fair locks */ static final class NonfairSync extends Sync {
/** * Sync object for fair locks */ static final class FairSync extends Sync {
Lock的调用过程如下图(其中涉及到 ReentrantLock类、Sync(抽象类)、AbstractQueuedSynchronizer类，NofairSync类，这些类将 Template方法用的淋漓尽致，相当赞)：
先来一张类依赖图：
再来一张lock调用图：
Lock API详解
自底而上来看，由被调用一步步向上分析
nofairTryAcquire
/**
* Performs non-fair tryLock. tryAcquire is implemented in
* subclasses, but both need nonfair try for trylock method.
*/
final boolean nonfairTryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) {
if (compareAndSetState(0, acquires)) {
setExclusiveOwnerThread(current);
return true;
}
}
else if (current == getExclusiveOwnerThread()) {
int nextc = c + acquires;
if (nextc < 0) // overflow
throw new Error("Maximum lock count exceeded");
setState(nextc);
return true;
}
return false;
}
来看这段代码，首先获取当前状态（初始化为0），当它等于0的时候，代表还没有任何线程获得该锁，然后通过CAS（底层是通过CompareAndSwapInt实现）改变state，并且设置当前线程为持有锁的线程；其他线程会直接返回false;当该线程重入的时候，state已经不等于0，这个时候并不需要CAS，因为该线程已经持有锁，然后会重新通过setState设置state的值，这里就实现了一个偏向锁的功能，即锁偏向该线程；
addWaiter
只有当锁被一个线程持有，另外一个线程请求获得该锁的时候才会进入这个方法
/**
* 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;
}
首先持有该锁之外的线程进入到该方法，这里涉及到一个CLH（三个人的名字首字母：Craig, Landin, and Hagersten）队列，其实就是一个链表，
简单说下CLH队列：
CLH队列由node节点组成，mode代表每个Node有两种模式：共享模式和排他模式，并且维护了一个状态：waitStatus，可取值如下：
- CANCELLED = 1 由于超时或者被打断，该线程被取消，将不会被block；
- SIGNAL = -1 当前线程的后继节点线程通过park正处于或即将处于block状态；
- CONDITION = -2 当前线程正处于条件队列，正式因为调用了condition.await造成阻塞；
- PROPAGATE = -3 共享锁应该被传播出去
首先，new一个节点，这个时候模式为：mode为 Node.EXCLUSIVE，默认为null即排它锁；
然后：
如果该队列已经有node即tail!=null，则将新节点的前驱节点置为tail，再通过CAS将tail指向当前节点，前驱节点的后继节点指向当前节点，然后返回当前节点；
如果队列为空或者CAS失败，则通过enq入队：
/**
* 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;
}
}
}
}
进队的时候，要么是第一个入队并且设置head节点并且循环设置tail，要么是add tail，如果CAS不成功，则会无限循环，直到设置成功，即使高并发的场景，也最终能够保证设置成功，然后返回包装好的node节点；
acquireQueued
/**
* 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);
}
}
该方法的主要作用就是将已经进入虚拟队列的节点进行阻塞，我们看到，如果当前节点的前驱节点是head并且尝试获取锁的时候成功了，则直接返回，不需要阻塞；
如果前驱节点不是头节点或者获取锁的时候失败了，则进行判定是否需要阻塞：
/**
* 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;
}
这段代码对该节点的前驱节点的状态进行判断，如果前驱节点已经处于signal状态，则返回true，表明当前节点可以进入阻塞状态；
否则，将前驱节点状态CAS置为signal状态，然后通过上层的for循环进入parkAndCheckInterrupt代码块park：
/**
* Convenience method to park and then check if interrupted
*
* @return {@code true} if interrupted
*/
private final boolean parkAndCheckInterrupt() {
LockSupport.park(this);
return Thread.interrupted();
}
这个时候将该线程交给操作系统内核进行阻塞；
总体来讲，acquireQueued就是依靠前驱节点的状态来决定当前线程是否应该处于阻塞状态，如果前驱节点处于cancel状态，则丢弃这些节点，重新构建队列；
Unlock API详解
流程类似lock api相关类的流程，这里讲主要的代码，unlock相对的比较简单
首先 ReentrantLock 调用 Sync的release接口也就是AbstractQueuedSynchronizer的release接口
/**
* 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;
}
这个时候会先调用Sync的tryRelease，如果返回true，则释放锁成功
protected final boolean tryRelease(int releases) {
int c = getState() - releases;
if (Thread.currentThread() != getExclusiveOwnerThread())
throw new IllegalMonitorStateException();
boolean free = false;
if (c == 0) {
free = true;
setExclusiveOwnerThread(null);
}
setState(c);
return free;
}
这个接口的作用很简单，如果不是获得锁的线程调用直接抛出异常，否则，如果当前state-releases==0也就是lock已经完全释放，返回true，清除资源；
这个返回free之后，release拿到head节点，进入以下代码：
/**
* 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);
}
这个作用即：当头结点的状态小于0，则将头结点的状态CAS为0，然后通过链表获取下一个节点，如果下一个节点为null或者不符合要求的状态，则从队尾遍历整个链表，直到遍历到离head节点最近的一个节点并且
等待状态符合预期，则将头结点的后继节点置为该节点；
对刚刚筛出来的符合要求的节点唤醒，也就是该节点获得 争夺 锁的权利；
这就是非公平锁的特点：在队列一直等待的线程不一定比后来的线程先获得锁，至此，unlock 已经解释完成；