Semaphore源码分析

ps:源码雷同CountDownLatch，细节不讲解，请查看上一篇CountDownLatch源码分析。
semaphore 也就是我们常说的信号灯，semaphore 可以控制同时访问的线程个数，通过 acquire 获取一个许可，如果没有就等待，通过 release 释放一个许可。有点类似限流的作用。叫信号灯的原因也和他的用处有关，比如某商场就 5 个停车位，每个停车位只能停一辆车，如果这个时候来了 10 辆车，必须要等前面有空的车位才能进入。
Semaphore 比较常见的就是用来做限流操作了。

案例

public class SemaphoreDemo {

    public static void main(String[] args) {
        Semaphore semaphore=new Semaphore(5); //令牌数 state=5
        for(int i=0;i<10;i++){
            new Car(semaphore,i).start();
        }
    }
    static class Car extends Thread{
        Semaphore semaphore;
        int num;

        public Car(Semaphore semaphore, int num) {
            this.semaphore = semaphore;
            this.num = num;
        }
        @Override
        public void run() {
            try {
                semaphore.acquire(); //5-1 获得令牌.(没拿到令牌，会阻塞，拿到了就可以往下执行）
                System.out.println("第"+num+"线程占用一个令牌");
                Thread.sleep(3000);
                System.out.println("第"+num+"线程释放一个令牌");
                semaphore.release(); //释放令牌
            } catch (InterruptedException e) {
                e.printStackTrace();
            }

        }
    }
}

源码分析

从 Semaphore 的功能来看，我们基本能猜测到它的底层实现一定是基于 AQS 的共享锁，因为需要实现多个线程共享一个令牌池
创建 Semaphore 实例的时候，需要一个参数 permits，这个基本上可以确定是设置给 AQS 的 state 的，然后每个线程调用 acquire 的时候，执行 state = state - 1，
走喽");
release 的时候执行 state = state + 1，当然，acquire 的时候，如果 state=0，说明没有资源了，需要等待其他线程 release。

public void acquire() throws InterruptedException {
        sync.acquireSharedInterruptibly(1);
    }
public final void acquireSharedInterruptibly(int arg)
            throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        if (tryAcquireShared(arg) < 0)
            doAcquireSharedInterruptibly(arg);
    }

tryAcquireShared分为公平锁FairSync和非公平锁NonfairSync，区别在于公平锁先判断队列中是否有等待的线程

static final class FairSync extends Sync {
        private static final long serialVersionUID = 2014338818796000944L;

        FairSync(int permits) {
            super(permits);
        }

        protected int tryAcquireShared(int acquires) {
            for (;;) {
                if (hasQueuedPredecessors())
                    return -1;
                int available = getState();
                int remaining = available - acquires;
                if (remaining < 0 ||
                    compareAndSetState(available, remaining))
                    return remaining;
            }
        }
    }
public final boolean hasQueuedPredecessors() {
        // The correctness of this depends on head being initialized
        // before tail and on head.next being accurate if the current
        // thread is first in queue.
        Node t = tail; // Read fields in reverse initialization order
        Node h = head;
        Node s;
        return h != t &&
            ((s = h.next) == null || s.thread != Thread.currentThread());
    }

有的话就将当前线程放入AQS队列。
如果state-1后判断小于0，那么就执行doAcquireSharedInterruptibly阻塞。

private void doAcquireSharedInterruptibly(int arg)
        throws InterruptedException {
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            for (;;) {
                final Node p = node.predecessor();
                if (p == head) {
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        failed = false;
                        return;
                    }
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

release

tryReleaseShared采用cas操作将state+1，然后去释放共享锁doReleaseShared

public void release() {
        sync.releaseShared(1);
    }
public final boolean releaseShared(int arg) {
        if (tryReleaseShared(arg)) {
            doReleaseShared();
            return true;
        }
        return false;
    }

代码与CountDownLatch相同，到此阅读完毕。

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