CountDownLatch是JDK 5+里面闭锁的一个实现,允许一个或者多个线程等待某个事件的发生。今天我们通过一些实例来学习一下它的用法。
CountDownLatch的简单使用
CountDownLatch的简单介绍:
CountDownLatch类是一个同步计数器,构造时传入int参数,该参数就是计数器的初始值,每调用一次countDown()方法,计数器减1,计数器大于0 时,await()方法会阻塞程序继续执行 CountDownLatch是一个倒计数的锁存器,当计数减至0时触发特定的事件。
CountDownLatch被用来同步一个或多个任务,强制它们等待由其他任务执行的一组操作完成。你可以向CountDownLatch对象设置一个初始计数值,任何在这个对象上调用wait()方法都将阻塞,直至这个计数值达到0。其他任务在结束工作时,可以在该对象上调用凑你countDown()来减小这个计数值。CountDownLatch被设计为只触发一次,计数值不能被重置。如果你需要能够重置计数值的版本,则可以使用CyclicBarrier。
以下是jdk文档上的两个例子,我们可以学习一下。
一、开始信号,阻止任何工人进行,直到司机准备好他们继续进行
package com.linux.huhx.concurreny; import java.util.concurrent.CountDownLatch; import java.util.concurrent.TimeUnit; public class Driver1 { public static void main(String[] args) throws Exception { CountDownLatch startSignal = new CountDownLatch(1); CountDownLatch doneSignal = new CountDownLatch(5); for (int i = 0; i < 5; ++i) // create and start threads new Thread(new TaskWorker(startSignal, doneSignal)).start(); System.out.println("countDown before."); // don't let run yet startSignal.countDown(); // let all threads proceed TimeUnit.SECONDS.sleep(1); System.out.println("await before"); /** * 1、由于doneSignal里面的初始count=5,所以主线程在此会阻塞。 * 2、当所有的子线程执行完(doneSignal.countDown())之后,此时的count=0。所以后面代码可以继续执行。 */ doneSignal.await(); System.out.println("await after"); } private static class TaskWorker implements Runnable { private final CountDownLatch startSignal; private final CountDownLatch doneSignal; public TaskWorker(CountDownLatch startSignal, CountDownLatch doneSignal) { this.startSignal = startSignal; this.doneSignal = doneSignal; } @Override public void run() { try { /** * 1、刚进来的时候,由于startSignal的count=1。所以await()方法阻塞于此。 * 2、当主线程执行了startSignal.countDown();此时的startSignal的count=0。所以子线程可以正常运行。 */ startSignal.await(); doSomeWork(); doneSignal.countDown(); } catch (InterruptedException e) { e.printStackTrace(); } } private void doSomeWork() { System.out.println("in run method: " + Thread.currentThread().getName()); } } }
执行的结果如下:子线程的执行顺序不固定
countDown before. in run method: Thread-0 in run method: Thread-1 in run method: Thread-2 in run method: Thread-4 in run method: Thread-3 await before await after
调用countDown()的任务在产生这个调用时并没有阻塞,只有对await()的调用会被阻塞,直至计数值到达0。
二、完成信号,允许司机等到所有工人完成为止
package com.linux.huhx.concurreny; import java.util.Random; import java.util.concurrent.CountDownLatch; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; public class Driver2 { public static void main(String[] args) throws Exception { CountDownLatch doneSignal = new CountDownLatch(5); ExecutorService service = Executors.newFixedThreadPool(3); for (int i = 0; i < 5; ++i) // create and start threads service.execute(new WorkerRunnable(doneSignal, i)); System.out.println("await before"); doneSignal.await(); System.out.println("await after"); service.shutdown(); } private static class WorkerRunnable implements Runnable { private final CountDownLatch doneSignal; private final int i; public WorkerRunnable(CountDownLatch doneSignal, int i) { this.doneSignal = doneSignal; this.i = i; } @Override public void run() { doWork(i); try { Thread.sleep(new Random().nextInt(1000)); doneSignal.countDown(); } catch (InterruptedException e) { e.printStackTrace(); } } private void doWork(int i) { System.out.println("in run method: " + i + " " + Thread.currentThread().getName()); } } }
运行的结果如下:子线程的执行顺序不固定
await before in run method: 0 pool-1-thread-1 in run method: 2 pool-1-thread-3 in run method: 1 pool-1-thread-2 in run method: 3 pool-1-thread-1 in run method: 4 pool-1-thread-2 await after