aqs小记

AbstractQueuedSynchronizer是Java中许多同步类的底层实现原理。ReentranLocl、CyclicBarrier、Semaphore、CountdownLatch等都是基于aqs实现的。这个是一种模板模式的实现。使用的时候一般是先声明一个内部类继承aqs，然后实现tryRelease()和tryRequire()方法（独占锁方式），或者tryReleaShared()和tryAcquireShared()（共享方式）内部通过getState和setState对aqs的state进行操作，实现同步控制。

一个例子：

/**
 * 独占式非共享锁
 */
public class Mutex implements Lock{

	@Override
	public void lock() {
		sync.acquire(1);
	}

	@Override
	public void lockInterruptibly() throws InterruptedException {
		sync.acquireInterruptibly(1);
	}

	@Override
	public boolean tryLock() {
		return false;
	}

	@Override
	public boolean tryLock(long time, TimeUnit unit) throws InterruptedException {
		return false;
	}

	@Override
	public void unlock() {
		sync.release(1);
	}

	@Override
	public Condition newCondition() {
		return null;
	}


	private final Sync sync = new Sync();

	private static class Sync extends AbstractQueuedSynchronizer {
		@Override
		protected boolean tryAcquire(int arg) {
			if (compareAndSetState(0, 1)) {
				setExclusiveOwnerThread(Thread.currentThread());
				return true;
			}
			return false;
		}


		@Override
		protected boolean tryRelease(int arg) {
			if (getState() == 0 || Thread.currentThread() != getExclusiveOwnerThread()) {
				throw new IllegalMonitorStateException();
			}
			setExclusiveOwnerThread(null);
			setState(0);
			return true;
		}
	}
}

Written on October 22, 2018