Semaphore

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1.类声明：

public class Semaphore implements java.io.Serializable {}

2.变量：

//同步器，用来实现Semaphoreprivate final Sync sync;

3.方法：

//自定义同步器 abstract static class Sync extends AbstractQueuedSynchronizer {        private static final long serialVersionUID = 1192457210091910933L;        Sync(int permits) {            setState(permits);        }        final int getPermits() {            return getState();        }        final int nonfairTryAcquireShared(int acquires) {            for (;;) {                int available = getState();                int remaining = available - acquires;                if (remaining < 0 ||                    compareAndSetState(available, remaining))                    return remaining;            }        }        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;            }        }        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;            }        }        final int drainPermits() {            for (;;) {                int current = getState();                if (current == 0 || compareAndSetState(current, 0))                    return current;            }        }    }    //不公平的版本，阻塞之后被唤醒时，不管先来的还是后来的一律等同试待    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);        }    }    /**     * Fair version     */     //公平版本的同步器，阻塞之后，按照先来先访问的原则访问    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())                    return -1;                int available = getState();                int remaining = available - acquires;                if (remaining < 0 ||                    compareAndSetState(available, remaining))                    return remaining;            }        }    }    //获取通行证，若没有则一直阻塞直到获取通行证或者被中断        public void acquire() throws InterruptedException {        sync.acquireSharedInterruptibly(1);    }    //释放通行证      public void release() {        sync.releaseShared(1);    }

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