Semaphore信号量探秘

import java.util.Random;
import java.util.concurrent.Semaphore;

/**
 * 如厕管理系统
 */
class Ruce implements Runnable {
    private String userName;
    private Semaphore pots;

    public Ruce(String userName, Semaphore pots) {
        this.userName = userName;
        this.pots = pots;
    }

    @Override
    public void run() {
        try {
            // 查询剩下的资源(剩下的茅坑)
            int availablePermits = pots.availablePermits();
            if (availablePermits > 0) {
                System.out.println(userName + "有茅坑...");
            } else {
                System.out.println(userName + "没有茅坑了...");
            }
            //申请茅坑 如果资源达到规定的次数，就等待
            pots.acquire();
            System.out.println(userName + "我开始上厕所了..");
            Thread.sleep(new Random().nextInt(3000)); // 模拟上厕所时间。
            System.out.println(userName + "厕所上完了...爽啊！");
            pots.release();

        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

import java.util.concurrent.Semaphore;

public class TestSemaphore {
    public static void main(String[] args) {
        // 一个厕所只有3个坑位，但是有10个人来上厕所，那怎么办？假设10的人的编号分别为1-10，并且1号先到厕所，10号最后到厕所。那么1-3号来的时候必然有可用坑位，顺利如厕，4号来的时候需要看看前面3人是否有人出来了，如果有人出来，进去，否则等待。同样的道理，4-10号也需要等待正在上厕所的人出来后才能进去，并且谁先进去这得看等待的人是否有素质，是否能遵守先来先上的规则。
        Semaphore semaphore = new Semaphore(3);
        for (int i = 1; i <= 10; i++) {
            Ruce parent = new Ruce("第" + i + "个人,", semaphore);
            new Thread(parent).start();
        }
    }
}

/**
 * Creates a {@code Semaphore} with the given number of
 * permits and nonfair fairness setting.
 *
 * @param permits the initial number of permits available.
 *        This value may be negative, in which case releases
 *        must occur before any acquires will be granted.
 */
public Semaphore(int permits) {
    sync = new NonfairSync(permits);
}

NonfairSync(int permits) {
    super(permits);
}

Sync(int permits) {
    setState(permits);
}

protected final void setState(int newState) {
    state = newState;
}

/**
 * The synchronization state.
 */
private volatile int state;

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

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

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

final int nonfairTryAcquireShared(int acquires) {
    for (;;) {
//获取同步状态值
        int available = getState();
//获取指定数量的许可证
        int remaining = available - acquires;
//剩余许可证小于0或者CAS成功
        if (remaining < 0 ||
            compareAndSetState(available, remaining))
//剩余的许可证数量
            return remaining;
    }
}

/**
 * Acquires in shared interruptible mode.
 * @param arg the acquire argument
 */
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) {
                int r = tryAcquireShared(arg);
                if (r >= 0) {
                    setHeadAndPropagate(node, r);
                    p.next = null; // help GC
                    failed = false;
                    return;
                }
            }
            if (shouldParkAfterFailedAcquire(p, node) &&
                parkAndCheckInterrupt())
                throw new InterruptedException();
        }
    } finally {
        if (failed)
            cancelAcquire(node);
    }
}

LockSupport.park(this);

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

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

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;
    }
}

/**
 * Release action for shared mode -- signals successor and ensures
 * propagation. (Note: For exclusive mode, release just amounts
 * to calling unparkSuccessor of head if it needs signal.)
 */
private void doReleaseShared() {
    /*
     * Ensure that a release propagates, even if there are other
     * in-progress acquires/releases.  This proceeds in the usual
     * way of trying to unparkSuccessor of head if it needs
     * signal. But if it does not, status is set to PROPAGATE to
     * ensure that upon release, propagation continues.
     * Additionally, we must loop in case a new node is added
     * while we are doing this. Also, unlike other uses of
     * unparkSuccessor, we need to know if CAS to reset status
     * fails, if so rechecking.
     */
    for (;;) {
        Node h = head;
        if (h != null && h != tail) {
            int ws = h.waitStatus;
            if (ws == Node.SIGNAL) {
                if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
                    continue;            // loop to recheck cases
                unparkSuccessor(h);
            }
            else if (ws == 0 &&
                     !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
                continue;                // loop on failed CAS
        }
        if (h == head)                   // loop if head changed
            break;
    }
}