在多线程编程里，CountDownLatch是一个很好的计数器工具。
它所在的包：
package java.util.concurrent
常用的两个方法：
1、计数器减一
public void countDown() {
sync.releaseShared(1);
}
2、线程等待，在计算器未到达0之前会一直等待
public void await() throws InterruptedException {
sync.acquireSharedInterruptibly(1);
}
好，下面来演示两个demo.
1、等待所有子线程执行完成后再执行主线程的情况
2、模拟高并发的情况
代码如下：
package com.figo.study.test;
import java.util.concurrent.CountDownLatch;
/**
* test CountDownLatch
* @author figo
* 20180605.
*/
public class TestSomething {
public static void main(String[] args) {
//testWaitThread();
testConcurrent();
}
/**
* 1、模拟所有子线程都执行完成后再执行主线程
* countdownLatch计数，模拟子线程执行完成之后再执行主线程
* 这个也可以用future来实现
*
*/
public static void testWaitThread()
{
final CountDownLatch latch = new CountDownLatch(2);
new Thread(){
public void run() {
try {
System.out.println("子线程"+Thread.currentThread().getName()+"正在执行");
Thread.sleep(3000);
System.out.println("子线程"+Thread.currentThread().getName()+"执行完毕");
latch.countDown();
} catch (InterruptedException e) {
e.printStackTrace();
}
};
}.start();
new Thread(){
public void run() {
try {
System.out.println("子线程"+Thread.currentThread().getName()+"正在执行");
Thread.sleep(3000);
System.out.println("子线程"+Thread.currentThread().getName()+"执行完毕");
latch.countDown();
} catch (InterruptedException e) {
e.printStackTrace();
}
};
}.start();
try {
System.out.println("等待2个子线程执行完毕...");
latch.await();
System.out.println("2个子线程已经执行完毕");
System.out.println("继续执行主线程");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
/**
* 线程数量
*/
public static final int THREAD_NUM = 100;
/**
* 开始时间
*/
private static long startTime = 0L;
/**
* 2、模拟高并发
*/
public static void testConcurrent()
{
try {
startTime = System.currentTimeMillis();
System.out.println("CountDownLatch started at: " + startTime);
// 初始化计数器为1
CountDownLatch countDownLatch = new CountDownLatch(1);
for (int i = 0; i < THREAD_NUM; i ++) {
new Thread(new Run(countDownLatch)).start();
}
// 启动多个线程
countDownLatch.countDown();
} catch (Exception e) {
System.out.println("Exception: " + e);
}
}
/**
* 线程类
*/
private static class Run implements Runnable {
private final CountDownLatch startLatch;
public Run(CountDownLatch startLatch) {
this.startLatch = startLatch;
}
@Override
public void run() {
try {
// 线程等待
startLatch.await();
// 模拟耗时操作
Thread.sleep(3000);
long endTime = System.currentTimeMillis();
System.out.println(Thread.currentThread().getName() + " ended at: " + endTime + ", cost: " + (endTime - startTime) + " ms.");
} catch (Exception e) {
e.printStackTrace();
}
}
}
}
---------------------
作者：志飞
来源：CSDN
原文：https://blog.csdn.net/figo0423/article/details/80604921
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