CountDownLatch详解以及用法示例

CountDownLatch详解以及用法示例

一、什么是​​CountDownLatch​​

CountDownLatch是一个同步工具类，它通过一个计数器来实现的,初始值为线程的数量。

每当一个线程完成了自己的任务，计数器的值就相应得减1。

当计数器到达0时，表示所有的线程都已执行完毕,然后在等待的线程就可以恢复执行任务。

二、方法详解

​​CountDownLatch​​ 提供了一些方法：

三、CountDownLatch应用场景

1. 某个线程需要在其他n个线程执行完毕后再向下执行2. 多个线程并行执行同一个任务，提高响应速度

四、CountDownLatch代码示例

下面代码演示​​2​​​个等待线程通过​​CountDownLatch​​​去等待​​3​​个工作线程完成操作：

public class CountDownLatchTest { public static void main(String[] args) throws InterruptedException { // 让2个线程去等待3个三个工作线程执行完成 CountDownLatch c = new CountDownLatch(3); // 2 个等待线程 WaitThread waitThread1 = new WaitThread("wait-thread-1", c); WaitThread waitThread2 = new WaitThread("wait-thread-2", c); // 3个工作线程 Worker worker1 = new Worker("worker-thread-1", c); Worker worker2 = new Worker("worker-thread-2", c); Worker worker3 = new Worker("worker-thread-3", c); // 启动所有线程 waitThread1.start(); waitThread2.start(); Thread.sleep(1000); worker1.start(); worker2.start(); worker3.start(); }}/** * 等待线程 */class WaitThread extends Thread { private String name; private CountDownLatch c; public WaitThread(String name, CountDownLatch c) { this.name = name; this.c = c; } @Override public void run() { try { // 等待 System.out.println(this.name + " wait..."); c.await(); System.out.println(this.name + " continue running..."); } catch (InterruptedException e) { e.printStackTrace(); } }}/** * 工作线程 */class Worker extends Thread { private String name; private CountDownLatch c; public Worker(String name, CountDownLatch c) { this.name = name; this.c = c; } @Override public void run() { System.out.println(this.name + " is running..."); try { Thread.sleep(2); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println(this.name + " is end."); c.countDown(); }}

运行结果

wait-thread-1 wait...wait-thread-2 wait...worker-thread-3 is running...worker-thread-2 is running...worker-thread-1 is running...worker-thread-1 is end.worker-thread-3 is end.worker-thread-2 is end.wait-thread-1 continue running...wait-thread-2 continue running...Process finished with exit code 0

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