前言

Semaphore是用于限制指定个数线程控制访问资源的同步工具，也是基于AQS实现的共享锁机制。它能控制每个时刻最多有多少个线程能访问共享资源，实现限流的功能。

使用效果

创建一个Semaphore，令牌数是3个，使用5个线程模拟获取令牌

java.util.concurrent.Semaphore semaphore = new java.util.concurrent.Semaphore(3);
        for (int i=0; i< 5; i++) {            int finalI = i;            new Thread(new Runnable() {                @SneakyThrows                @Override                public void run() {                    semaphore.acquire(1);                    System.out.println(Thread.currentThread().getName() + " 在"+ DateFormatUtils.format(new Date(),"yyyy/MM/dd HH:mm:ss") + " 申请到一个令牌");                    Thread.sleep((finalI +1) *3000);                    semaphore.release(1);                }            }).start();        }

控制台输出：

从日志发现，同时最多有3个线程获取到令牌，等释放令牌后，其他线程才能获取到令牌，起到限流的作用。

源码分析

AQS实现

abstract static class Sync extends AbstractQueuedSynchronizer {        private static final long serialVersionUID = 1192457210091910933L;
        Sync(int permits) {            setState(permits);        }
        final int getPermits() {            return getState();        }        //非公平方式获取共享锁，不排队，直接获取锁        final int nonfairTryAcquireShared(int acquires) {            for (;;) {                int available = getState();                int remaining = available - acquires;                if (remaining < 0 ||                    compareAndSetState(available, remaining))                    return remaining;            }        }
        //释放共享锁，归还令牌        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;            }        }        //减少令牌        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;            }        }        //令牌清零        final int drainPermits() {            for (;;) {                int current = getState();                if (current == 0 || compareAndSetState(current, 0))                    return current;            }        }    }        //非公平版本     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);        }    }           /**     * 公平版本     */    static final class FairSync extends Sync {        private static final long serialVersionUID = 2014338818796000944L;
        FairSync(int permits) {            super(permits);        }        //公平获取共享锁，一开始就要先判断队列中是否有线程已经在等待，        //如果有等待的线程，则直接获取失败。        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1个令牌：

public void acquire() throws InterruptedException {        sync.acquireSharedInterruptibly(1);    }

获取多个令牌：

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

释放一个令牌：

public void release() {        sync.releaseShared(1);    }

释放多个令牌：

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

总结：

令牌获取，令牌释放是配对的，体现了令牌资源复用。它既支持公平获取令牌，也支持非公平获取令牌，可以根据实际业务场景选择，在接口限流场景中使用很多。

ITPub博客

Java并发编程之Semaphore知识整理

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