Semaphore源码分析

Semaphore可以控制并发数量，通过分发许可证的方式，acquire就是去获取许可证，如果获取到了的话，就可以执行，如果获取不到的话，就会去同步队列里阻塞.

release会释放许可证并唤醒同步队列的线程.

public Semaphore(int permits) {
        sync = new NonfairSync(permits);
    }

默认是非公平，permits就是AQS中state的值

 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);
    }

protected int tryAcquireShared(int acquires) {
            return nonfairTryAcquireShared(acquires);
        }

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

可以看出就是通过减少state的值，如果剩下的值小于0的话，直接返回remaining，线程就会进入阻塞队列，大于0的话，就通过CAS改变state的值

因为并发可能有多个线程来修改state的值，所以通过cas和自璇来保证线程安全

再来看release方法

public void release() {
        sync.releaseShared(1);
    }

 public final boolean releaseShared(int arg) {
        if (tryReleaseShared(arg)) {
            doReleaseShared();
            return true;
        }
        return false;
    }

 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;
            }
        }

同样是通过cas的方式来归还许可证，并通过cas重新设置state的值

private void doReleaseShared() {
        /*
         * Ensure that a release propagates, even if there are other
         * in-progress acquires/releases.  This proceeds in the usual
         * way of trying to unparkSuccessor of head if it needs
         * signal. But if it does not, status is set to PROPAGATE to
         * ensure that upon release, propagation continues.
         * Additionally, we must loop in case a new node is added
         * while we are doing this. Also, unlike other uses of
         * unparkSuccessor, we need to know if CAS to reset status
         * fails, if so rechecking.
         */
        for (;;) {
            Node h = head;
            if (h != null && h != tail) {
                int ws = h.waitStatus;
                if (ws == Node.SIGNAL) {
                    if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
                        continue;            // loop to recheck cases
                    unparkSuccessor(h);
                }
                else if (ws == 0 &&
                         !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
                    continue;                // loop on failed CAS
            }
            if (h == head)                   // loop if head changed
                break;
        }
    }

唤醒等待队列里的第一个节点