JUC锁——CountDownLatch_countdownlatch属于锁吗-优快云博客

本文围绕CountDownLatch展开，它是一个同步辅助类，本质是“共享锁”。介绍了其与CyclicBarrier的区别，分析了数据结构，通过源码剖析了核心函数，如CountDownLatch(int count)、await()、countDown()，还给出使用示例，展示主线程等待子线程完成工作的实现。

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概要

和ReadWriteLock.ReadLock一样，CountDownLatch的本质也是一个"共享锁"。

CountDownLatch简介

CountDownLatch是一个同步辅助类，在完成一组正在其他线程中执行的操作之前，它允许一个或多个线程一直等待。
CountDownLatch和CyclicBarrier的区别：
①CountDownLatch的作用是允许1或N个线程等待其他线程完成执行；而CyclicBarrier则是允许N个线程相互等待。
② CountDownLatch的计数器无法被重置；CyclicBarrier的计数器可以被重置后使用，因此它被称为是循环的barrier。

CountDownLatch方法列表

CountDownLatch(int count)//构造一个用给定计数初始化的 CountDownLatch
// 使当前线程在锁存器倒计数至零之前一直等待，除非线程被中断
void await()
// 使当前线程在锁存器倒计数至零之前一直等待，除非线程被中断或超出了指定的等待时间
boolean await(long timeout, TimeUnit unit)
// 递减锁存器的计数，如果计数到达零，则释放所有等待的线程
void countDown()
// 返回当前计数
long getCount()
// 返回标识此锁存器及其状态的字符串
String toString()

CountDownLatch数据结构

CountDownLatch的UML类图：

CountDownLatch的数据结构很简单，它是通过"共享锁"实现的。它包含了sync对象，sync是Sync类型。Sync是实例类，它继承于AQS。

CountDownLatch源码分析

public class CountDownLatch {
    /**
     * Synchronization control For CountDownLatch.
     * Uses AQS state to represent count.
     */
    private static final class Sync extends AbstractQueuedSynchronizer {
        private static final long serialVersionUID = 4982264981922014374L;

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

        int getCount() {
            return getState();
        }

        protected int tryAcquireShared(int acquires) {
            return (getState() == 0) ? 1 : -1;
        }

        protected boolean tryReleaseShared(int releases) {
            // Decrement count; signal when transition to zero
            for (;;) {
                int c = getState();
                if (c == 0)
                    return false;
                int nextc = c-1;
                if (compareAndSetState(c, nextc))
                    return nextc == 0;
            }
        }
    }

    private final Sync sync;

    /**
     * Constructs a {@code CountDownLatch} initialized with the given count.
     *
     * @param count the number of times {@link #countDown} must be invoked
     *        before threads can pass through {@link #await}
     * @throws IllegalArgumentException if {@code count} is negative
     */
    public CountDownLatch(int count) {
        if (count < 0) throw new IllegalArgumentException("count < 0");
        this.sync = new Sync(count);
    }

    /**
     * Causes the current thread to wait until the latch has counted down to
     * zero, unless the thread is {@linkplain Thread#interrupt interrupted}.
     *
     * <p>If the current count is zero then this method returns immediately.
     *
     * <p>If the current count is greater than zero then the current
     * thread becomes disabled for thread scheduling purposes and lies
     * dormant until one of two things happen:
     * <ul>
     * <li>The count reaches zero due to invocations of the
     * {@link #countDown} method; 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,
     * </ul>
     * then {@link InterruptedException} is thrown and the current thread's
     * interrupted status is cleared.
     *
     * @throws InterruptedException if the current thread is interrupted
     *         while waiting
     */
    public void await() throws InterruptedException {
        sync.acquireSharedInterruptibly(1);
    }

    /**
     * Causes the current thread to wait until the latch has counted down to
     * zero, unless the thread is {@linkplain Thread#interrupt interrupted},
     * or the specified waiting time elapses.
     *
     * <p>If the current count is zero then this method returns immediately
     * with the value {@code true}.
     *
     * <p>If the current count is greater than zero then the current
     * thread becomes disabled for thread scheduling purposes and lies
     * dormant until one of three things happen:
     * <ul>
     * <li>The count reaches zero due to invocations of the
     * {@link #countDown} method; or
     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
     * the current thread; or
     * <li>The specified waiting time elapses.
     * </ul>
     *
     * <p>If the count reaches zero then the method returns with the
     * value {@code true}.
     *
     * <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,
     * </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
     * @param unit the time unit of the {@code timeout} argument
     * @return {@code true} if the count reached zero and {@code false}
     *         if the waiting time elapsed before the count reached zero
     * @throws InterruptedException if the current thread is interrupted
     *         while waiting
     */
    public boolean await(long timeout, TimeUnit unit)
        throws InterruptedException {
        return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
    }

    /**
     * Decrements the count of the latch, releasing all waiting threads if
     * the count reaches zero.
     *
     * <p>If the current count is greater than zero then it is decremented.
     * If the new count is zero then all waiting threads are re-enabled for
     * thread scheduling purposes.
     *
     * <p>If the current count equals zero then nothing happens.
     */
    public void countDown() {
        sync.releaseShared(1);
    }

    /**
     * Returns the current count.
     *
     * <p>This method is typically used for debugging and testing purposes.
     *
     * @return the current count
     */
    public long getCount() {
        return sync.getCount();
    }

    /**
     * Returns a string identifying this latch, as well as its state.
     * The state, in brackets, includes the String {@code "Count ="}
     * followed by the current count.
     *
     * @return a string identifying this latch, as well as its state
     */
    public String toString() {
        return super.toString() + "[Count = " + sync.getCount() + "]";
    }
}

CountDownLatch是通过“共享锁”实现的。下面，分析一下CountDownLatch中的3个核心函数: CountDownLatch(int count)、await()、countDown()。
1、CountDownLatch(int count)

public CountDownLatch(int count) {
    if (count < 0) throw new IllegalArgumentException("count < 0");
    this.sync = new Sync(count);
}

说明：该函数是创建一个Sync对象，而Sync是继承于AQS类。Sync构造函数如下：

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

setState()在AQS中实现，源码如下：

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

说明：在AQS中，state是一个private volatile long类型的对象。对于CountDownLatch而言，state表示的是”锁计数器“。CountDownLatch中的getCount()最终是调用AQS中的getState()，返回的就是state对象，即”锁计数器“。
2、await()

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

说明：该函数实际上是调用的AQS的acquireSharedInterruptibly(1)，AQS中的acquireSharedInterruptibly()的源码如下：

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

说明：acquireSharedInterruptibly()的作用是获取共享锁。如果当前线程是中断状态，则抛出异常InterruptedException。否则，调用tryAcquireShared(arg)尝试获取共享锁，尝试成功则返回，否则就调用doAcquireSharedInterruptibly()。doAcquireSharedInterruptibly()会使当前线程一直等待，直到当前线程获取到共享锁(或被中断)才返回。tryAcquireShared()在CountDownLatch.java中被重写，它的源码如下：

protected int tryAcquireShared(int acquires) {
    return (getState() == 0) ? 1 : -1;
}

说明：tryAcquireShared()的作用是尝试获取共享锁。如果"锁计数器=0"，即锁是可获取状态，则返回1；否则，锁是不可获取状态，则返回-1。

private void doAcquireSharedInterruptibly(long arg) throws InterruptedException {
    // 创建"当前线程"的Node节点，且Node中记录的锁是"共享锁"类型；并将该节点添加到CLH队列末尾
    final Node node = addWaiter(Node.SHARED);
    boolean failed = true;
    try {
        for (;;) {
            // 获取上一个节点
            // 如果上一节点是CLH队列的表头，则"尝试获取共享锁"
            final Node p = node.predecessor();
            if (p == head) {
                long r = tryAcquireShared(arg);
                if (r >= 0) {
                    setHeadAndPropagate(node, r);
                    p.next = null; // help GC
                    failed = false;
                    return;
                }
            }
            // (上一节点不是CLH队列的表头) 当前线程一直等待，直到获取到共享锁
            // 如果线程在等待过程中被中断过，则再次中断该线程(还原之前的中断状态)
            if (shouldParkAfterFailedAcquire(p, node) &&
                parkAndCheckInterrupt())
                throw new InterruptedException();
        }
    } finally {
        if (failed)
            cancelAcquire(node);
    }
}

说明：
①addWaiter(Node.SHARED)的作用是，创建”当前线程“的Node节点，且Node中记录的锁的类型是”共享锁“(Node.SHARED)；并将该节点添加到CLH队列末尾。
②node.predecessor()的作用是，获取上一个节点。如果上一节点是CLH队列的表头，则”尝试获取共享锁“。
③shouldParkAfterFailedAcquire()的作用和它的名称一样，如果在尝试获取锁失败之后，线程应该等待，则返回true；否则，返回false。
④当shouldParkAfterFailedAcquire()返回ture时，则调用parkAndCheckInterrupt()，当前线程会进入等待状态，直到获取到共享锁才继续运行。
3、countDown()

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

说明：该函数实际上调用releaseShared(1)释放共享锁。releaseShared()在AQS中实现，源码如下：

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

说明：releaseShared()的目的是让当前线程释放它所持有的共享锁。它首先会通过tryReleaseShared()去尝试释放共享锁。尝试成功，则直接返回；尝试失败，则通过doReleaseShared()去释放共享锁。tryReleaseShared()在CountDownLatch.java中被重写，源码如下：

protected boolean tryReleaseShared(int releases) {
    // Decrement count; signal when transition to zero
    for (;;) {
        // 获取“锁计数器”的状态
        int c = getState();
        if (c == 0)
            return false;
        // “锁计数器”-1
        int nextc = c-1;
        // 通过CAS函数进行赋值
        if (compareAndSetState(c, nextc))
            return nextc == 0;
    }
}

说明：tryReleaseShared()的作用是释放共享锁，将“锁计数器”的值-1。
总结：CountDownLatch是通过“共享锁”实现的。在创建CountDownLatch时，会传递一个int类型参数count，该参数是“锁计数器”的初始状态，表示该“共享锁”最多能被count个线程同时获取。当某线程调用该CountDownLatch对象的await()方法时，该线程会等待“共享锁”可用时，才能获取“共享锁”进而继续运行。而“共享锁”可用的条件，就是“锁计数器”的值为0！而“锁计数器”的初始值为count，每当一个线程调用该CountDownLatch对象的countDown()方法时，才将“锁计数器”-1；通过这种方式，必须有count个线程调用countDown()之后，“锁计数器”才为0，而前面提到的等待线程才能继续运行。这就是CountDownLatch的实现原理。

CountDownLatch的使用示例

下面通过CountDownLatch实现："主线程"等待"5个子线程"全部都完成"指定的工作(休眠1000ms)"之后，再继续运行：

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.CyclicBarrier;

public class CountDownLatchTest1 {

    private static int LATCH_SIZE = 5;
    private static CountDownLatch doneSignal;
    public static void main(String[] args) {

        try {
            doneSignal = new CountDownLatch(LATCH_SIZE);

            // 新建5个任务
            for(int i=0; i<LATCH_SIZE; i++)
                new InnerThread().start();

            System.out.println("main await begin.");
            // "主线程"等待线程池中5个任务的完成
            doneSignal.await();

            System.out.println("main await finished.");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    static class InnerThread extends Thread{
        public void run() {
            try {
                Thread.sleep(1000);
                System.out.println(Thread.currentThread().getName() + " sleep 1000ms.");
                // 将CountDownLatch的数值减1
                doneSignal.countDown();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}
//结果
main await begin.
Thread-0 sleep 1000ms.
Thread-2 sleep 1000ms.
Thread-1 sleep 1000ms.
Thread-4 sleep 1000ms.
Thread-3 sleep 1000ms.
main await finished.