Java—CountDownLatch使用详解

CountDownLatch介绍

CountDownLatch概述

1. CountDownLatch一般用作多线程倒计时计数器，强制它们等待其他一组（CountDownLatch的初始化决定）任务执行完成。
2. 有一点要说明的是CountDownLatch初始化后计数器值递减到0的时候，不能再复原的，这一点区别于Semaphore，Semaphore是可以通过release操作恢复信号量的。

CountDownLatch使用原理

使用原理

1. 创建CountDownLatch并设置计数器值。
2. 启动多线程并且调用CountDownLatch实例的countDown()方法。
3. 主线程调用 await() 方法，这样主线程的操作就会在这个方法上阻塞，直到其他线程完成各自的任务，count值为0，停止阻塞，主线程继续执行。

使用模板

public class CountDownLatchModule {

    //线程数
    private static int N = 10;

    // 单位：min
    private static int countDownLatchTimeout = 5;

    public static void main(String[] args) {
        //创建CountDownLatch并设置计数值，该count值可以根据线程数的需要设置
        CountDownLatch countDownLatch = new CountDownLatch(N);

		//创建线程池
        ExecutorService cachedThreadPool = Executors.newCachedThreadPool();

        for (int i = 0; i < N; i++) {
            cachedThreadPool.execute(() ->{
                try {
                    System.out.println(Thread.currentThread().getName() + " do something!");
                } catch (Exception e) {
                    System.out.println("Exception: do something exception");
                } finally {
                    //该线程执行完毕-1
                    countDownLatch.countDown();
                }
            });
        }

        System.out.println("main thread do something-1");
        try {
            countDownLatch.await(countDownLatchTimeout, TimeUnit.MINUTES);
        } catch (InterruptedException e) {
            System.out.println("Exception: await interrupted exception");
        } finally {
            System.out.println("countDownLatch: " + countDownLatch.toString());
        }
        System.out.println("main thread do something-2");
        //若需要停止线程池可关闭;
//        cachedThreadPool.shutdown();

    }

运行结果：

main thread do something-1
pool-1-thread-1 do something!
pool-1-thread-2 do something!
pool-1-thread-3 do something!
pool-1-thread-5 do something!
pool-1-thread-6 do something!
pool-1-thread-7 do something!
pool-1-thread-8 do something!
pool-1-thread-4 do something!
pool-1-thread-9 do something!
pool-1-thread-10 do something!
countDownLatch: java.util.concurrent.CountDownLatch@76fb509a[Count = 0]
main thread do something-2

CountDownLatch常用方法

• public void await() throws InterruptedException：调用await()方法的线程会被挂起，等待直到count值为0再继续执行。
• public boolean await(long timeout, TimeUnit unit) throws InterruptedException：同await()，若等待timeout时长后，count值还是没有变为0，不再等待，继续执行。时间单位如下常用的毫秒、天、小时、微秒、分钟、纳秒、秒。
  
• public void countDown()： count值递减1.
• public long getCount()：获取当前count值。
• public String toString()：重写了toString()方法，多打印了count值，具体参考源码。

CountDownLatch使用场景

一个程序中有N个任务在执行，我们可以创建值为N的CountDownLatch，当每个任务完成后，调用一下countDown()方法进行递减count值，再在主线程中使用await()方法等待任务执行完成，主线程继续执行。

CountDownLatch源码

构造方法源码

    /**
     * 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);
    }

toString()方法源码

    /**
     * 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示例

作为线程启动信号

代码

public class CountDownLatchTest {

    /**
     * a start signal that prevents any worker from proceeding
     * until the driver is ready for them to proceed;
     * @param args
     * @throws InterruptedException
     */
    public static void main(String[] args) throws InterruptedException {
        CountDownLatch startSignal = new CountDownLatch(1);
        CountDownLatch doneSignal = new CountDownLatch(10);
        for (int i = 0; i < 10; i++) {
            // create and start threads
            new Thread(new Worker(startSignal, doneSignal)).start();
        }
        // don't let run yet
        System.out.println("do something else 1");
        // let all threads proceed
        startSignal.countDown();
        System.out.println("do something else 2");
        // wait for all to finish
        doneSignal.await();
        System.out.println("wait for all to finsh");
    }

    static class Worker implements Runnable{

        private final CountDownLatch startSignal;
        private final CountDownLatch doneSignal;

        Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {
            this.startSignal = startSignal;
            this.doneSignal = doneSignal;
        }

        @Override
        public void run() {
            try {
                startSignal.await();
                doWork();
                doneSignal.countDown();
            } catch (InterruptedException ex) {
                ex.printStackTrace();
            }
        }

        void doWork() {
            System.out.println("do work!");
        }
    }

}

运行结果

do something else 1
do something else 2
do work!
do work!
do work!
do work!
do work!
do work!
do work!
do work!
do work!
do work!
wait for all to finsh

从运行结果可以看出：

1. 主线程先打印do something else 1和do something else 2。因为startSignal.countDown();完后，count才为0，子线程才能打印。
2. 因为startSignal.await();是在子线程内，所有子线程都等待startSignal.countDown()执行后才能打印do work!。
3. doneSignal.await();等待所有子线程执行后，每次都doneSignal.countDown()，最后count为0，主线程才执行打印wait for all to finsh。

作为线程等待完成信号

代码

public class CountDownLatchTest2 {

    /**
     * a completion signal that allows the driver to wait
     * until all workers have completed.
     * @param args
     * @throws InterruptedException
     */
    public static void main(String[] args) throws InterruptedException {
        CountDownLatch doneSignal = new CountDownLatch(5);
        ExecutorService cachedThreadPool = Executors.newCachedThreadPool();
        for (int i = 0; i < 10; i++) {
            // create and start threads
            cachedThreadPool.execute(new Worker(doneSignal, i));
        }
        // don't let run yet
        System.out.println("do something else 1");
        // wait for all to finish
        doneSignal.await();
        System.out.println("===========================count: " + doneSignal.getCount());
        System.out.println("do something else 2");
        cachedThreadPool.shutdown();
    }

    static class Worker implements Runnable{

        private final CountDownLatch doneSignal;
        private final int i;

        Worker(CountDownLatch doneSignal, int i) {
            this.doneSignal = doneSignal;
            this.i = i;
        }

        @Override
        public void run() {
            try {
                doWork();
                doneSignal.countDown();
                System.out.println("i = " + i + ", " + doneSignal.toString());
            } catch (Exception ex) {
                ex.printStackTrace();
            }
        }

        void doWork() {
            System.out.println("do work!");
        }
    }

}

运行结果

do something else 1
do work!
i = 0, java.util.concurrent.CountDownLatch@128abd43[Count = 4]
do work!
i = 1, java.util.concurrent.CountDownLatch@128abd43[Count = 3]
do work!
i = 2, java.util.concurrent.CountDownLatch@128abd43[Count = 2]
do work!
i = 3, java.util.concurrent.CountDownLatch@128abd43[Count = 1]
do work!
i = 4, java.util.concurrent.CountDownLatch@128abd43[Count = 0]
===========================count: 0
do something else 2
do work!
i = 5, java.util.concurrent.CountDownLatch@128abd43[Count = 0]
do work!
i = 6, java.util.concurrent.CountDownLatch@128abd43[Count = 0]
do work!
i = 7, java.util.concurrent.CountDownLatch@128abd43[Count = 0]
do work!
i = 8, java.util.concurrent.CountDownLatch@128abd43[Count = 0]
do work!
i = 9, java.util.concurrent.CountDownLatch@128abd43[Count = 0]

从运行结果可以看出，主线程是等待其他线程运行了5次结束后就打印了do something else 2信息，因为CountDownLatch数值为5。