AQS之Semaphore源码解析_aqs semaphore 源码-优快云博客

本文链接：https://blog.youkuaiyun.com/zhangzhen02/article/details/106557920

本文深入探讨了Semaphore和CountDownLatch两种同步工具类的工作原理。Semaphore通过acquire和release方法控制并发线程的数量，CountDownLatch则用于等待一组操作完成。文章详细分析了它们的内部实现，包括如何使用AbstractQueuedSynchronizer进行线程排队和唤醒。

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看几个主要方法

abstract static class Sync extends AbstractQueuedSynchronizer {
        private static final long serialVersionUID = 1192457210091910933L;

        Sync(int permits) {
            setState(permits);
        }

        final int getPermits() {
            return getState();
        }

        final int nonfairTryAcquireShared(int acquires) {
            for (;;) {
                int available = getState();
                int remaining = available - acquires;
                if (remaining < 0 ||
                    compareAndSetState(available, remaining))
                    return remaining;
            }
        }

        protected final boolean tryReleaseShared(int releases) {
            for (;;) {
                int current = getState();
                int next = current + releases;
                if (next < current) // overflow
                    throw new Error("Maximum permit count exceeded");
                if (compareAndSetState(current, next))
                    return true;
            }
        }
   }

AbstractQueuedSychronizer.class

public final void acquireSharedInterruptibly(int arg)
            throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        if (tryAcquireShared(arg) < 0)
            doAcquireSharedInterruptibly(arg);
    }
public final boolean releaseShared(int arg) {
        if (tryReleaseShared(arg)) {
            doReleaseShared();
            return true;
        }
        return false;
    }

可以看到，当tryAcquireShare小于0时，将线程入列。
当tryReleaseShared为true 时，通知后置节点停止挂起。
AQS里的相关代码，详见之前的AQS文章。

通过上面的源代码可以开出，当state被减到小于0时，后续线程入队挂起。
当release后state 加1，并通知后续节点，停止挂起，进行获取锁操作。

跟前文相比，我们看到CountDown是设置一个state,通过await方法获取锁，当state数量不等于0时，将线程入队并挂起线程。然后通过countDown方法，release锁，将state减一，直到state为0时，依次唤醒后续队列中的线程。
而Semaphore是通过acquire方法，将state减一，当state小于0时，将线程入队挂起。
通过release方法，将state加一，并唤醒后继节点，进而保持同时有state个数的线程在执行。

使用示例

public class SemaphoreTest {
    static class MyThread extends Thread{
        Semaphore semaphore;
        public MyThread(Semaphore semaphore) {
            this.semaphore=semaphore;
        }

        @Override
        public void run() {
            try {
                semaphore.acquire();
                Thread.sleep(2000);
                System.out.println("thread "+Thread.currentThread().getName()+" sleep done");
            }catch (InterruptedException e){
                e.printStackTrace();
            }finally {
                semaphore.release();
            }

        }
    }
    public static void main(String[] args) {
        int count=50;
        Semaphore semaphore=new Semaphore(5);
        for (int i=0;i<count;i++){
            new MyThread(semaphore).start();
        }
        System.out.println("main thread run");

    }
}

运行结果

main thread run
thread Thread-0 sleep done
thread Thread-3 sleep done
thread Thread-1 sleep done
thread Thread-4 sleep done
thread Thread-2 sleep done
thread Thread-5 sleep done
thread Thread-7 sleep done
thread Thread-9 sleep done
thread Thread-8 sleep done

每次运行五个线程。