一、AbstractQueuedSynchronizer就不多说了,Lock就是基于它来实现锁的功能的,而AbstractQueuedSynchronizer底层则是通过cas操作来实现。
通过AbstractQueuedSynchronizer来实现共享锁其实很简单,只需要对AbstractQueuedSynchronizer的状态进行控制就行了,其他的AbstractQueuedSynchronizer都做好了封装,举个例子吧。
package com.pinnet.test; import java.util.concurrent.TimeUnit; import java.util.concurrent.locks.AbstractQueuedSynchronizer; import java.util.concurrent.locks.Condition; import java.util.concurrent.locks.Lock; public class ShareLock implements Lock { private Syn sync = new Syn(5); /** * 在同步装置内部实现一个AbstractQueuedSynchronizer的子类 * */ private static final class Syn extends AbstractQueuedSynchronizer { private static final long serialVersionUID = 5854536238831876527L; //构造方法,同时进入的线程数量必须为整数 Syn(int count) { if (count <= 0) { throw new IllegalArgumentException("count must large than zero."); } //设置AQS的状态为count setState(count); } /** * 加锁的时候会调用此方法,redececount参数为改变的状态数量,一般传1即可 */ public int tryAcquireShared(int reduceCount) { //加锁和释放的方法都必须要放在循环中,避免失败的操作 for (;;) { //获取当前的状态 int current = getState(); //计算过后的状态 int newCount = current - reduceCount; //如果新的状态小于0 则返回值,在AQS的doAcquireShared方法中,如果返回的值小于0,则会将该线程封装为node放入队列 //如果大于0 表示该线程能执行则线程会继续执行不受影响 if (newCount < 0 || compareAndSetState(current, newCount)) { return newCount; } } } public boolean tryReleaseShared(int retrunCount) { for (;;) { int current = getState(); int newCount = current + retrunCount; if (compareAndSetState(current, newCount)) { return true; } } } } public void lock(){ sync.acquireShared(1); } public void lockInterruptibly() throws InterruptedException{ sync.acquireSharedInterruptibly(1); } public boolean tryLock(){ return sync.tryAcquireShared(1)>=0; } public boolean tryLock(long time, TimeUnit unit) throws InterruptedException { return sync.tryAcquireSharedNanos(1, unit.toNanos(time)); } public void unlock(){ sync.releaseShared(1); } public Condition newCondition(){ return null; } }
注释很清楚,具体的可以看AbstractQueuedSynchronizer实现,其实都是很简单地