Semaphore学习一

一、定义
是JUC包下的一个工具类，我们可以通过其限制执行的线程数量，达到限流的效果。
当一个线程执行时先通过其方法进行获取许可操作，获取到许可的线程继续执行业务逻辑，当线程执行完成后进行释放许可操作，未获取达到许可的线程进行等待或者直接结束。
可以把它简单的理解成我们停车场入口立着的那个显示屏，每有一辆车进入停车场显示屏就会显示剩余车位减1，每有一辆车从停车场出去，显示屏上显示的剩余车辆就会加1，当显示屏上的剩余车位为0时，停车场入口的栏杆就不会再打开，车辆就无法进入停车场了，直到有一辆车从停车场出去为止。

二、方法
构造方法

    /**
     * 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);
    }

表示许可线程的数量

    /**
     * Creates a {@code Semaphore} with the given number of
     * permits and the given 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.
     * @param fair {@code true} if this semaphore will guarantee
     *        first-in first-out granting of permits under contention,
     *        else {@code false}
     */
    public Semaphore(int permits, boolean fair) {
        sync = fair ? new FairSync(permits) : new NonfairSync(permits);
    }

fair表示公平性，如果设置为true，表示公平，那么等待最久的线程先执行

    /**
     * Acquires a permit from this semaphore, blocking until one is
     * available, or the thread is {@linkplain Thread#interrupt interrupted}.
     *
     * <p>Acquires a permit, if one is available and returns immediately,
     * reducing the number of available permits by one.
     *
     * <p>If no permit is available then the current thread becomes
     * disabled for thread scheduling purposes and lies dormant until
     * one of two things happens:
     * <ul>
     * <li>Some other thread invokes the {@link #release} method for this
     * semaphore and the current thread is next to be assigned a permit; or
     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
     * the current thread.
     * </ul>
     *
     * <p>If the current thread:
     * <ul>
     * <li>has its interrupted status set on entry to this method; or
     * <li>is {@linkplain Thread#interrupt interrupted} while waiting
     * for a permit,
     * </ul>
     * then {@link InterruptedException} is thrown and the current thread's
     * interrupted status is cleared.
     *
     * @throws InterruptedException if the current thread is interrupted
     */
    public void acquire() throws InterruptedException {
        sync.acquireSharedInterruptibly(1);
    }

表示线程获取1个许可，那么线程许可数量相应减少一个，获取一个令牌，在获取到令牌、或者被其他线程调用中断之前线程一直处于阻塞状态。

    /**
     * Releases a permit, returning it to the semaphore.
     *
     * <p>Releases a permit, increasing the number of available permits by
     * one.  If any threads are trying to acquire a permit, then one is
     * selected and given the permit that was just released.  That thread
     * is (re)enabled for thread scheduling purposes.
     *
     * <p>There is no requirement that a thread that releases a permit must
     * have acquired that permit by calling {@link #acquire}.
     * Correct usage of a semaphore is established by programming convention
     * in the application.
     */
    public void release() {
        sync.releaseShared(1);
    }

表示释放一个许可，那么线程许可数量相应增加，释放一个令牌，唤醒一个获取令牌不成功的阻塞线程。
?

    /**
     * Releases the given number of permits, returning them to the semaphore.
     *
     * <p>Releases the given number of permits, increasing the number of
     * available permits by that amount.
     * If any threads are trying to acquire permits, then one
     * is selected and given the permits that were just released.
     * If the number of available permits satisfies that thread's request
     * then that thread is (re)enabled for thread scheduling purposes;
     * otherwise the thread will wait until sufficient permits are available.
     * If there are still permits available
     * after this thread's request has been satisfied, then those permits
     * are assigned in turn to other threads trying to acquire permits.
     *
     * <p>There is no requirement that a thread that releases a permit must
     * have acquired that permit by calling {@link Semaphore#acquire acquire}.
     * Correct usage of a semaphore is established by programming convention
     * in the application.
     *
     * @param permits the number of permits to release
     * @throws IllegalArgumentException if {@code permits} is negative
     */
    public void release(int permits) {
        if (permits < 0) throw new IllegalArgumentException();
        sync.releaseShared(permits);
    }

表示一个线程释放n个许可，这个数量有参数permits决定，获取一个令牌，在获取到令牌、或者被其他线程调用中断、或超时之前线程一直处于阻塞状态。

/**
     * Acquires the given number of permits from this semaphore,
     * blocking until all are available,
     * or the thread is {@linkplain Thread#interrupt interrupted}.
     *
     * <p>Acquires the given number of permits, if they are available,
     * and returns immediately, reducing the number of available permits
     * by the given amount.
     *
     * <p>If insufficient permits are available then the current thread becomes
     * disabled for thread scheduling purposes and lies dormant until
     * one of two things happens:
     * <ul>
     * <li>Some other thread invokes one of the {@link #release() release}
     * methods for this semaphore, the current thread is next to be assigned
     * permits and the number of available permits satisfies this request; or
     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
     * the current thread.
     * </ul>
     *
     * <p>If the current thread:
     * <ul>
     * <li>has its interrupted status set on entry to this method; or
     * <li>is {@linkplain Thread#interrupt interrupted} while waiting
     * for a permit,
     * </ul>
     * then {@link InterruptedException} is thrown and the current thread's
     * interrupted status is cleared.
     * Any permits that were to be assigned to this thread are instead
     * assigned to other threads trying to acquire permits, as if
     * permits had been made available by a call to {@link #release()}.
     *
     * @param permits the number of permits to acquire
     * @throws InterruptedException if the current thread is interrupted
     * @throws IllegalArgumentException if {@code permits} is negative
     */
    public void acquire(int permits) throws InterruptedException {
        if (permits < 0) throw new IllegalArgumentException();
        sync.acquireSharedInterruptibly(permits);
    }

表示一个线程获取n个许可，这个数量有参数permits决定

/**
     * Returns the current number of permits available in this semaphore.
     *
     * <p>This method is typically used for debugging and testing purposes.
     *
     * @return the number of permits available in this semaphore
     */
    public int availablePermits() {
        return sync.getPermits();
    }

返回当前信号量线程许可数量

 /**
     * Returns an estimate of the number of threads waiting to acquire.
     * The value is only an estimate because the number of threads may
     * change dynamically while this method traverses internal data
     * structures.  This method is designed for use in monitoring of the
     * system state, not for synchronization control.
     *
     * @return the estimated number of threads waiting for this lock
     */
    public final int getQueueLength() {
        return sync.getQueueLength();
    }

返回等待获取许可的线程数的预估值

/**
     * Acquires a permit from this semaphore, blocking until one is
     * available.
     *
     * <p>Acquires a permit, if one is available and returns immediately,
     * reducing the number of available permits by one.
     *
     * <p>If no permit is available then the current thread becomes
     * disabled for thread scheduling purposes and lies dormant until
     * some other thread invokes the {@link #release} method for this
     * semaphore and the current thread is next to be assigned a permit.
     *
     * <p>If the current thread is {@linkplain Thread#interrupt interrupted}
     * while waiting for a permit then it will continue to wait, but the
     * time at which the thread is assigned a permit may change compared to
     * the time it would have received the permit had no interruption
     * occurred.  When the thread does return from this method its interrupt
     * status will be set.
     */
    public void acquireUninterruptibly() {
        sync.acquireShared(1);
    }

获取一个令牌，在获取到令牌之前线程一直处于阻塞状态（忽略中断）

/**
     * Acquires a permit from this semaphore, only if one is available at the
     * time of invocation.
     *
     * <p>Acquires a permit, if one is available and returns immediately,
     * with the value {@code true},
     * reducing the number of available permits by one.
     *
     * <p>If no permit is available then this method will return
     * immediately with the value {@code false}.
     *
     * <p>Even when this semaphore has been set to use a
     * fair ordering policy, a call to {@code tryAcquire()} <em>will</em>
     * immediately acquire a permit if one is available, whether or not
     * other threads are currently waiting.
     * This &quot;barging&quot; behavior can be useful in certain
     * circumstances, even though it breaks fairness. If you want to honor
     * the fairness setting, then use
     * {@link #tryAcquire(long, TimeUnit) tryAcquire(0, TimeUnit.SECONDS) }
     * which is almost equivalent (it also detects interruption).
     *
     * @return {@code true} if a permit was acquired and {@code false}
     *         otherwise
     */
    public boolean tryAcquire() {
        return sync.nonfairTryAcquireShared(1) >= 0;
    }

尝试获得令牌，返回获取令牌成功或失败，不阻塞线程。

    /**
     * Acquires a permit from this semaphore, if one becomes available
     * within the given waiting time and the current thread has not
     * been {@linkplain Thread#interrupt interrupted}.
     *
     * <p>Acquires a permit, if one is available and returns immediately,
     * with the value {@code true},
     * reducing the number of available permits by one.
     *
     * <p>If no permit is available then the current thread becomes
     * disabled for thread scheduling purposes and lies dormant until
     * one of three things happens:
     * <ul>
     * <li>Some other thread invokes the {@link #release} method for this
     * semaphore and the current thread is next to be assigned a permit; or
     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
     * the current thread; or
     * <li>The specified waiting time elapses.
     * </ul>
     *
     * <p>If a permit is acquired then the value {@code true} is returned.
     *
     * <p>If the current thread:
     * <ul>
     * <li>has its interrupted status set on entry to this method; or
     * <li>is {@linkplain Thread#interrupt interrupted} while waiting
     * to acquire a permit,
     * </ul>
     * then {@link InterruptedException} is thrown and the current thread's
     * interrupted status is cleared.
     *
     * <p>If the specified waiting time elapses then the value {@code false}
     * is returned.  If the time is less than or equal to zero, the method
     * will not wait at all.
     *
     * @param timeout the maximum time to wait for a permit
     * @param unit the time unit of the {@code timeout} argument
     * @return {@code true} if a permit was acquired and {@code false}
     *         if the waiting time elapsed before a permit was acquired
     * @throws InterruptedException if the current thread is interrupted
     */
    public boolean tryAcquire(long timeout, TimeUnit unit)
        throws InterruptedException {
        return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
    }

尝试获得令牌，在超时时间内循环尝试获取，直到尝试获取成功或超时返回，不阻塞线程。

    /**
     * Queries whether any threads are waiting to acquire. Note that
     * because cancellations may occur at any time, a {@code true}
     * return does not guarantee that any other thread will ever
     * acquire.  This method is designed primarily for use in
     * monitoring of the system state.
     *
     * @return {@code true} if there may be other threads waiting to
     *         acquire the lock
     */
    public final boolean hasQueuedThreads() {
        return sync.hasQueuedThreads();
    }

等待队列里是否还存在等待线程。

/**
     * Acquires and returns all permits that are immediately available.
     *
     * @return the number of permits acquired
     */
    public int drainPermits() {
        return sync.drainPermits();
    }

清空令牌把可用令牌数置为0，返回清空令牌的数量。

三、使用Semaphore实现停车场指示牌功能
每个停车场入口都有一个提示牌，上面显示着停车场的剩余车位还有多少，当剩余车位为0时，不允许车辆进入停车场，直到停车场里面有车离开停车场，这时提示牌上会显示新的剩余车位数。
业务场景 ：

1、停车场容纳总停车量20。

2、当一辆车进入停车场后，显示牌的剩余车位数响应的减1.

3、每有一辆车驶出停车场后，显示牌的剩余车位数响应的加1。

4、停车场剩余车位不足时，车辆只能在外面等待。

实现代码

package com.util;

import lombok.extern.slf4j.Slf4j;

import java.util.Random;
import java.util.concurrent.Semaphore;
import static java.lang.Thread.sleep;

/**
 * @author : lssffy
 * @Description : 线程的信号量控制
 * @date : 2023/12/17 21:32
 */
@Slf4j
public class SemaphoreTest {

    public static void main(String[] args) {
        //创建Semaphore对象数量20
        Semaphore sp = new Semaphore(20);
        //100个线程同时运行
        for (int i = 0; i < 8; i++) {
           new Thread(new Runnable() {
                @Override
                public void run() {
                    try{
                        System.out.println("===" + Thread.currentThread().getName()+"来到停车场");
                        if(sp.availablePermits() == 0){
                            System.out.println("车位不足，请耐心等待");
                        }
                        sp.acquire();//获取令牌尝试进入停车场
                        System.out.println(Thread.currentThread().getName() + "成功进入停车场");
                        Thread.sleep(new Random().nextInt(10000));//模拟车辆在停车场停留的时间
                        System.out.println(Thread.currentThread().getName() + "驶出停车场");
                        sp.release();//释放令牌，腾出停车场车位
                    }catch (InterruptedException e){
                        e.printStackTrace();
                    }
                }
            },i+"号车").start();
        }
    }
}

执行结果

Semaphore实现原理
Semaphore初始化

Semaphore sp = new Semaphore(20);

1、当调用new Semaphore(20)方式时，默认会创建一个非公平的锁的同步阻塞队列
2、把初始化令牌数量赋值给同步队列的state状态，state的值代表当前所剩余的令牌数量
获取令牌

sp.acquire();

1、当前线程会尝试去同步队列获取一个令牌，获取令牌的过程就是使用原子的操作去修改同步队列的state，获取一个令牌则修改为state=state-1
2、当计算出来的state<0，则代表令牌数量不足，此时会创建一个Node节点加入阻塞队列，挂起当前线程
3、当计算出来的state>=0，则表示获取令牌成功

    /**
     * Acquires in shared mode, aborting if interrupted.  Implemented
     * by first checking interrupt status, then invoking at least once
     * {@link #tryAcquireShared}, returning on success.  Otherwise the
     * thread is queued, possibly repeatedly blocking and unblocking,
     * invoking {@link #tryAcquireShared} until success or the thread
     * is interrupted.
     * @param arg the acquire argument.
     * This value is conveyed to {@link #tryAcquireShared} but is
     * otherwise uninterpreted and can represent anything
     * you like.
     * @throws InterruptedException if the current thread is interrupted
     */
    public final void acquireSharedInterruptibly(int arg)
            throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
            //尝试获取令牌，arg为获取令牌个数，当可用令牌数量减去当前令牌数量结果小于0，则创建一个节点加入阻塞队列，挂起当前线程
        if (tryAcquireShared(arg) < 0)
            doAcquireSharedInterruptibly(arg);
    }

    /**
     * 1、创建节点，加入阻塞队列
     * 2、重双向链表的head、tail节点关系，清空无效节点
     * 3、挂起当前节点线程
     * Acquires in shared interruptible mode.
     * @param arg the acquire argument
     */
    private void doAcquireSharedInterruptibly(int arg)
        throws InterruptedException {
        //创建节点加入阻塞队列
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            for (;;) {
            	//获得当前节点pre节点
                final Node p = node.predecessor();
                if (p == head) {
                	//返回锁的state
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        failed = false;
                        return;
                    }
                }
                //重组双向链表，清空无效节点，挂起当前线程
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

释放令牌

sp.release();

当调用sp.release();方法时
1、线程会尝试释放一个令牌，释放令牌的过程就是把同步队列的state修改为state=state+1的过程
2、释放令牌成功之后，同时唤醒同步队列的一个线程
3、被唤醒的节点会重新尝试去修改state=state-1的操作，如果state>=0则获取令牌成功，否则重新进入阻塞队列，挂起线程

    /**
     * Releases in shared mode.  Implemented by unblocking one or more
     * threads if {@link #tryReleaseShared} returns true.
     * 释放共享锁，同时会唤醒同步队列的一个线程
     * @param arg the release argument.  This value is conveyed to
     *        {@link #tryReleaseShared} but is otherwise uninterpreted
     *        and can represent anything you like.
     * @return the value returned from {@link #tryReleaseShared}
     */
    public final boolean releaseShared(int arg) {
    	//释放共享锁
        if (tryReleaseShared(arg)) {
        	//唤醒所有共享节点线程
            doReleaseShared();
            return true;
        }
        return false;
    }

    /**
     * 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.)
     * 唤醒同步队列中的一个线程
     */
    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;
                //是否需要唤醒后续节点
                if (ws == Node.SIGNAL) {
                	//修改状态为初始0
                    if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
                        continue;            // loop to recheck cases
                    unparkSuccessor(h);//唤醒h.nex节点线程
                }
                else if (ws == 0 &&
                         !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
                    continue;                // loop on failed CAS
            }
            if (h == head)                   // loop if head changed
                break;
        }
    }