「这是我参与2022首次更文挑战的第17天，活动详情查看：2022首次更文挑战」。
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。
