Java 并发编程之 AQS Semaphore 源码解析

init

// java.util.concurrent.Semaphore.Sync#Sync
abstract static class Sync extends AbstractQueuedSynchronizer {
    private static final long serialVersionUID = 1192457210091910933L;

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

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

}

Semaphore初始化时将AQS的state设置为构造方法中的值。

acquire

// java.util.concurrent.Semaphore#acquire(int)
public void acquire(int permits) throws InterruptedException {
    if (permits < 0) throw new IllegalArgumentException();
    sync.acquireSharedInterruptibly(permits);
}



// java.util.concurrent.locks.AbstractQueuedSynchronizer#acquireSharedInterruptibly
public final void acquireSharedInterruptibly(int arg)
        throws InterruptedException {
    if (Thread.interrupted())
        throw new InterruptedException();
    if (tryAcquireShared(arg) < 0)
        doAcquireSharedInterruptibly(arg);
}

// java.util.concurrent.Semaphore.Sync#nonfairTryAcquireShared
final int nonfairTryAcquireShared(int acquires) {
    for (;;) {
        int available = getState();
        int remaining = available - acquires;
        if (remaining < 0 ||
            compareAndSetState(available, remaining))
            return remaining;
    }
}

// java.util.concurrent.locks.AbstractQueuedSynchronizer#doAcquireSharedInterruptibly
private void doAcquireSharedInterruptibly(int arg)
    throws InterruptedException {
    final Node node = addWaiter(Node.SHARED);
    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
                    return;
                }
            }
            if (shouldParkAfterFailedAcquire(p, node) &&
                parkAndCheckInterrupt())
                throw new InterruptedException();
        }
    } catch (Throwable t) {
        cancelAcquire(node);
        throw t;
    }
}

Semaphore判断是否需要进入同步队列的条件比较宽松，如果剩余许可小于0了，不进行CAS操作，直接进行入队操作，如果还是没有获取到就进行阻塞。

此处很多逻辑与ReentrantLock、ReentrantReadWriteLock中的读锁相似不再赘述。

release

// java.util.concurrent.Semaphore#release(int)
public void release(int permits) {
    if (permits < 0) throw new IllegalArgumentException();
    sync.releaseShared(permits);
}

// java.util.concurrent.locks.AbstractQueuedSynchronizer#releaseShared
public final boolean releaseShared(int arg) {
    if (tryReleaseShared(arg)) {
        doReleaseShared();
        return true;
    }
    return false;
}

// java.util.concurrent.Semaphore.Sync#tryReleaseShared
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;
    }
}

// java.util.concurrent.locks.AbstractQueuedSynchronizer#doReleaseShared
private void doReleaseShared() {
    for (;;) {
        Node h = head;
        if (h != null && h != tail) {
            int ws = h.waitStatus;
            if (ws == Node.SIGNAL) {
                if (!h.compareAndSetWaitStatus(Node.SIGNAL, 0))
                    continue;            // loop to recheck cases
                unparkSuccessor(h);
            }
            else if (ws == 0 &&
                     !h.compareAndSetWaitStatus(0, Node.PROPAGATE))
                continue;                // loop on failed CAS
        }
        if (h == head)                   // loop if head changed
            break;
    }
}

此处的doReleaseShared释放锁流程与ReentrantReadWriteLock中的读锁释放一样。恢复了一个节点运行后，在setHeadAndPropagate中，会继续调用doReleaseShared，释放连续的共享锁。