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：

方法

说明

await()

使当前线程进入同步队列进行等待，直到latch的值被减到0或者当前线程被中断，当前线程就会被唤醒。

await(long timeout, TimeUnit unit)

带超时时间的await()。

countDown()

使latch的值减1，如果减到了0，则会唤醒所有等待在这个latch上的线程。

getCount()

获得latch的数值。

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