Semaphore

public class Semaphore implements java.io.Serializable {
    private static final long serialVersionUID = -3222578661600680210L;
    /** All mechanics via AbstractQueuedSynchronizer subclass */
    private final Sync sync;

    /**
     * 继承AQS以实现信号量机制，使用AQS的state属性代表允许使用的信号量的数量，该Sync类有两个
     * 派生类，其中一个是公平模式的，另一个是非公平模式的
     */
    abstract static class Sync extends AbstractQueuedSynchronizer {
        private static final long serialVersionUID = 1192457210091910933L;

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

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


        //非公平模式的获取信号量
        final int nonfairTryAcquireShared(int acquires) {
            for (;;) {
                int available = getState();
                int remaining = available - acquires;
                if (remaining < 0 ||
                    compareAndSetState(available, remaining))
                    return remaining;
            }
        }

        //释放信号量
        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;
            }
        }

        //减少信号量
        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;
            }
        }

        清空所有信号量
        final int drainPermits() {
            for (;;) {
                int current = getState();
                if (current == 0 || compareAndSetState(current, 0))
                    return current;
            }
        }
    }
}

public class Semaphore implements java.io.Serializable {

/**
     * 非公平模式
     */
    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);
        }
    }

    /**
     * 公平模式
     */
    static final class FairSync extends Sync {
        private static final long serialVersionUID = 2014338818796000944L;

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

        protected int tryAcquireShared(int acquires) {
            for (;;) {
                //公平模式下，先判断是否有前驱结点
                if (hasQueuedPredecessors())
                    return -1;
                int available = getState();
                int remaining = available - acquires;
                if (remaining < 0 ||
                    compareAndSetState(available, remaining))
                    return remaining;
            }
        }
    }

}

public class Semaphore implements java.io.Serializable {

    //未指定模式，默认生产非公平模式的信号量工具
    public Semaphore(int permits) {
        sync = new NonfairSync(permits);
    }

    //指定模式，生产公平或非公平模式的信号量工具
    public Semaphore(int permits, boolean fair) {
        sync = fair ? new FairSync(permits) : new NonfairSync(permits);
    }

    //使用可中断的方式获取信号量，tryAcquireShared->doAcquireSharedInterruptibly
    public void acquire() throws InterruptedException {
        sync.acquireSharedInterruptibly(1);
    }

    //发起可中断的获取信号量，指定获取信号量的数量
    public void acquire(int permits) throws InterruptedException {
        if (permits < 0) throw new IllegalArgumentException();
        sync.acquireSharedInterruptibly(permits);
    }


    //使用不可中断的方式获取信号量，tryAcquireShared->doAcquireShared
    public void acquireUninterruptibly() {
        sync.acquireShared(1);
    }

    //发起不可中的的获取信号量，指定获取信号量的数量
    public void acquireUninterruptibly(int permits) {
        if (permits < 0) throw new IllegalArgumentException();
        sync.acquireShared(permits);
    }

    //使用非公平模式获取信号量，默认获取一个信号量
    public boolean tryAcquire() {
        return sync.nonfairTryAcquireShared(1) >= 0;
    }

    //使用非公平模式获取信号量，指定获取信号量的数量
    public boolean tryAcquire(int permits) {
        if (permits < 0) throw new IllegalArgumentException();
        return sync.nonfairTryAcquireShared(permits) >= 0;
    }

    //使用超时的模式获取信号量,默认获取一个信号量
    public boolean tryAcquire(long timeout, TimeUnit unit)
        throws InterruptedException {
        return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
    }

    //使用超时的模式获取信号量，指定获取信号量的数量
    public boolean tryAcquire(int permits, long timeout, TimeUnit unit)
        throws InterruptedException {
        if (permits < 0) throw new IllegalArgumentException();
        return sync.tryAcquireSharedNanos(permits, unit.toNanos(timeout));
    }

    //释放信号量
    public void release() {
        sync.releaseShared(1);
    }
}