JUC锁——CountDownLatch

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 zerofor (;;) {
    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 GCfailed = 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 zerofor (;;) {
    // 获取“锁计数器”的状态int c = getState();if (c == 0)return false;// “锁计数器”-1int 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1doneSignal.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.

结果说明：主线程通过doneSignal.await()等待其它线程将doneSignal递减至0。其它的5个InnerThread线程，每一个都通过doneSignal.countDown()将doneSignal的值减1；当doneSignal为0时，main被唤醒后继续执行。