JUC代码浅析[4]——基于AQS的信号量Semaphore

       Semaphore是基于AQS共享模式实现的计数信号量，它维护一个资源一个时期内最多访问者个数。超过限制数量的线程被阻塞。使用state表示许可的个数。acquire操作减少计数，release增加计数，许可计数为0时就不允许新的访问进入。

获取许可，

    public void acquire() throws InterruptedException {

    //acquireSharedInterruptibly方法参考AQS的介绍

        sync.acquireSharedInterruptibly(1);

    }

信号量sync也分为公平和非公平的实现，其中非公平sync，剩余许可小于0时线程就进入队列阻塞等待AQS调度

        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;

            }

        }

公平sync，只要当前线程不处于AQS队列的首位就进入队列阻塞等待调度，在首位时跟非公平一样

        protected int tryAcquireShared(int acquires) {

            Thread current = Thread.currentThread();

            for (;;) {

                Thread first = getFirstQueuedThread();

                if (first != null && first != current)

                    return -1;

                int available = getState();

                int remaining = available – acquires;

                if (remaining < 0 ||

                    compareAndSetState(available, remaining))

                    return remaining;

            }

        }

释放许可，比较简单，增加许可

        protected final boolean tryReleaseShared(int releases) {

            for (;;) {

                int p = getState();

                if (compareAndSetState(p, p + releases))

                    return true;

            }

        }