前两章介绍了锁,那么现在我们介绍新的一个类,锁
一,简介
Lock是一个接口,实现它的类有读锁,写锁,和ReentrantLock,我们可以在类上点击ctrl+t来看看有哪些类实现了这个接口
使用方法
private Lock l = new ReentrantLock();
try{
l.lock();
if(ticketCount > 0){
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName()+ "卖出了第"+(ticketCount--)+"张票");
}
}
finally{
l.unlock();
}
特别注意,要在最后unlock锁,否则这个锁会一直锁着,导致别的无法使用
二,读锁和写锁
这里有一个例子,读写锁的缓存应用,如下所示
package cn.unis; import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock; public class CacheDemo {
public Map<String, Object> cached = new HashMap<String, Object>(); private ReadWriteLock rwl = new ReentrantReadWriteLock(); public Object getobject(String keyname) {
rwl.readLock().lock(); // 读数据前先加锁
Object value = null;
try {
value = cached.get(keyname);
if (value == null) { //发现数据是空的,则写数据,先开启写锁
rwl.readLock().unlock();
rwl.writeLock().lock();
try {
if (value == "" || value == null) {
value = "test";
}
} finally {
rwl.writeLock().lock();
rwl.readLock().lock();
}
}
} finally {
rwl.readLock().unlock();
} return value;
}
}
缓存
三,Condition实现线程同步通信
类似于传统线程技术中的object.wait()和object.notifyall()方法
可以这样说,他替代了wait方法,用的是如下的代码,注意,Condition是基于Lock的
Lock lock = new ReentrantLock();
Condition condition = lock.newCondition();
使用方法是:
condition.await();
condition.signal();
在SDK里面有一个例子,缓冲区的例子,可以参考一下
class BoundedBuffer {
final Lock lock = new ReentrantLock();
final Condition notFull = lock.newCondition();
final Condition notEmpty = lock.newCondition(); final Object[] items = new Object[100];
int putptr, takeptr, count; public void put(Object x) throws InterruptedException {
lock.lock();
try {
while (count == items.length)
notFull.await();
items[putptr] = x;
if (++putptr == items.length) putptr = 0;
++count;
notEmpty.signal();
} finally {
lock.unlock();
}
} public Object take() throws InterruptedException {
lock.lock();
try {
while (count == 0)
notEmpty.await();
Object x = items[takeptr];
if (++takeptr == items.length) takeptr = 0;
--count;
notFull.signal();
return x;
} finally {
lock.unlock();
}
}
}