Semaphore

Semaphore的结构

  构造函数：

	public Semaphore(int permits) {
        sync = new NonfairSync(permits);
    }
    
	public Semaphore(int permits, boolean fair) {
        sync = fair ? new FairSync(permits) : new NonfairSync(permits);
    }

  Semaphore内部有一个Sync类型的锁对象，该类继承于AQS，类似于ReentrantLock有公平锁和非公平锁子类，根据构造函数传入的参数决定公平性策略，默认是非公平锁。构造函数中的permits为初始化的许可数。Semaphore的另一个属性是serialVersionUID，版本号。

  Sync是关键，大多数的操作都转移到Sync的操作上。

Sync源码

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

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

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

		// 非公平策略获取许可
        final int nonfairTryAcquireShared(int acquires) {
            for (;;) {
                int available = getState();
                int remaining = available - acquires;
                if (remaining < 0 ||
                    compareAndSetState(available, remaining))
                    return remaining;
            }
        }

		// 释放许可
        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;
            }
        }

		// 减少指定数量的许可
        final void reducePermits(int reductions) {
            for (;;) {
                int current = getState();
                int next = current - reductions;
                if (next > current) // underflow
                    throw new Error("Permit count underflow");
                if (compareAndSetState(current, next))
                    return;
            }
        }

		// 获取所有的许可
        final int drainPermits() {
            for (;;) {
                int current = getState();
                if (current == 0 || compareAndSetState(current, 0))
                    return current;
            }
        }
    }

Sync的子类NoFairSync和FairSync

static final class NonfairSync extends Sync {
        private static final long serialVersionUID = -2694183684443567898L;

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

        protected int tryAcquireShared(int acquires) {
            return nonfairTryAcquireShared(acquires); // 调用父类的非公平策略获取许可
        }
    }

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()) // AQS队列中还有节点直接返回（公平策略）
                    return -1;
                int available = getState();
                int remaining = available - acquires;
                if (remaining < 0 ||
                    compareAndSetState(available, remaining))
                    return remaining;
            }
        }
    }

  两个子类都比较简单，实现了不同策略的锁。

相关方法

  先看看获取许可的方法：

	public void acquire() throws InterruptedException {
        sync.acquireSharedInterruptibly(1);
    }
    public void acquire(int permits) throws InterruptedException {
        if (permits < 0) throw new IllegalArgumentException();
        sync.acquireSharedInterruptibly(permits);
    }

  带参数和不带参数的方法差不多，不带参数的默认获取一个许可。两个方法都调用了Sync类的acquireSharedInterruptibly(int arg)方法，该方法继承自AQS：

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

  又调用了doAcquireSharedInterruptibly(arg)方法：

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);
        }
    }

  doAcquireSharedInterruptibly(int arg)中的tryAcquireShared(arg)是一个只抛出了异常的方法，Semaphore中的Sync的子类重写了该方法（上面有相关源码），根据锁的类型进行不同的获取许可策略。

  释放许可的方法：

	public void release() {
        sync.releaseShared(1);
    }
    public void release(int permits) {
        if (permits < 0) throw new IllegalArgumentException();
        sync.releaseShared(permits);
    }

  两个方法调用了同一个方法，该方法同样来自AQS

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

  tryReleaseShared(int releases)由Sync进行重写，将许可数加回去。

	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;
            }
        }

  从源码来看许可数的释放没有特别的限定条件，即使没有获取许可，也可以通过release方法增加许可数，所以即使调用release后许可数超出构造方法指定的初始许可数也是可以的。