Semaphore使用及原理解读

import java.util.concurrent.Semaphore;

Semaphore semaphore = new Semaphore(n);

try {
    semaphore.acquire(); // 获取信号量，如果没有可用的许可证，线程将被阻塞
    // 访问共享资源的代码
} catch (InterruptedException e) {
    // 处理中断异常
} finally {
    semaphore.release(); // 释放信号量，增加一个许可证
}

    public static void main(String[] args) {
        // 1. 创建 semaphore 对象
        Semaphore semaphore = new Semaphore(3);
        // 2. 10个线程同时运行
        for (int i = 0; i < 10; i++) {
            new Thread(() -> {
                // 3. 获取许可

                try {
                    semaphore.acquire();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                try {
                    log.debug("running...");
                    sleep(1);
                    log.debug("end...");
                } finally {
                    // 4. 释放许可

                    semaphore.release();
                }
            }).start();
        }

    }

  public Semaphore(int permits) {
        sync = new NonfairSync(permits);
    }

    public Semaphore(int permits, boolean fair) {
        sync = fair ? new FairSync(permits) : new NonfairSync(permits);
    }

    static final class NonfairSync extends Sync {
        private static final long serialVersionUID = -2694183684443567898L;

        NonfairSync(int permits) {
            // permits 即 state

            super(permits);
        }

        // Semaphore 方法, 方便阅读, 放在此处

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

        // AQS 继承过来的方法, 方便阅读, 放在此处

        public final void acquireSharedInterruptibly(int arg)
                throws InterruptedException {
            if (Thread.interrupted())
                throw new InterruptedException();
            if (tryAcquireShared(arg) < 0)
                doAcquireSharedInterruptibly(arg);
        }

        // 尝试获得共享锁

        protected int tryAcquireShared(int acquires) {
            return nonfairTryAcquireShared(acquires);
        }

        // Sync 继承过来的方法, 方便阅读, 放在此处

        final int nonfairTryAcquireShared(int acquires) {
            for (;;) {
                int available = getState();
                int remaining = available - acquires;
                if (
                    // 如果许可已经用完, 返回负数, 表示获取失败, 进入 doAcquireSharedInterruptibly

                        remaining < 0 ||

                                // 如果 cas 重试成功, 返回正数, 表示获取成功

                                compareAndSetState(available, remaining)
                ) {
                    return remaining;
                }
            }
        }

        // AQS 继承过来的方法, 方便阅读, 放在此处

        private void doAcquireSharedInterruptibly(int arg) throws InterruptedException {
            final Node node = addWaiter(Node.SHARED);
            boolean failed = true;
            try {
                for (;;) {
                    final Node p = node.predecessor();
                    if (p == head) {
                        // 再次尝试获取许可

                        int r = tryAcquireShared(arg);
                        if (r >= 0) {
                            // 成功后本线程出队（AQS）, 所在 Node设置为 head

                            // 如果 head.waitStatus == Node.SIGNAL ==> 0 成功, 下一个节点 unpark

                            // 如果 head.waitStatus == 0 ==> Node.PROPAGATE

                            // r 表示可用资源数, 为 0 则不会继续传播

                            setHeadAndPropagate(node, r);
                            p.next = null; // help GC

                            failed = false;
                            return;
                        }
                    }
                    // 不成功, 设置上一个节点 waitStatus = Node.SIGNAL, 下轮进入 park 阻塞

                    if (shouldParkAfterFailedAcquire(p, node) &&

                            parkAndCheckInterrupt())
                        throw new InterruptedException();
                }
            } finally {
                if (failed)
                    cancelAcquire(node);
            }
        }

        // Semaphore 方法, 方便阅读, 放在此处

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

        // AQS 继承过来的方法, 方便阅读, 放在此处

        public final boolean releaseShared(int arg) {
            if (tryReleaseShared(arg)) {
                doReleaseShared();
                return true;
            }
            return false;
        }

        // Sync 继承过来的方法, 方便阅读, 放在此处
        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;
            }
        }
    }