介绍
CountDownLatch是一个同步辅助类，它允许一个或多个线程一直等待直到其他线程执行完毕才开始执行。
用给定的计数初始化CountDownLatch，其含义是要被等待执行完的线程个数。
每次调用CountDown()，计数减1
主程序执行到await()函数会阻塞等待线程的执行，直到计数为0
实现原理
计数器通过使用锁（共享锁、排它锁）实现
实例1
场景：模拟10人赛跑。10人跑完后才喊"Game Over."
package com.jihite;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class CountDownLatchTest {
private static final int RUNNER_COUNT = 10;
public static void main(String[] args) throws InterruptedException {
final CountDownLatch begin = new CountDownLatch(1);
final CountDownLatch end = new CountDownLatch(RUNNER_COUNT);
final ExecutorService exec = Executors.newFixedThreadPool(10);
for (int i = 0; i < RUNNER_COUNT; i++) {
final int NO = i + 1;
Runnable run = new Runnable() {
@Override
public void run() {
try {
begin.await();
Thread.sleep((long)(Math.random() * 10000));
System.out.println("No." + NO + " arrived");
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
end.countDown();
}
}
};
exec.submit(run);
}
System.out.println("Game Start ...");
begin.countDown();
end.await();
// end.await(30, TimeUnit.SECONDS);
System.out.println("Game Over.");
exec.shutdown();
}
}
分析：代码中定义了2个计数器，个数分别为1和10。
如果不执行begin.countDown()，进程会一致阻塞在begin.await()
主进程执行到end.awit()阻塞等待end计数器清0，进程中每执行一次CountDown()减1，所有执行完后主进程继续往下执行
输出
Game Start ... No.6 arrived No.4 arrived No.10 arrived No.3 arrived No.9 arrived No.5 arrived No.8 arrived No.7 arrived No.1 arrived No.2 arrived Game Over.
注：countDown()一定要执行到（考虑异常及线程与开始计数设置不一致），否则会一直卡在await()（可以设置时间，超过一定时间就不等了）
实例2（和join的相似处）
场景：流水线上有3个worker: worker1、worker2、worker3,只有当worker1和worker2执行完时才可以执行worker3
WorkerCount.java
package com.jihite;
import java.util.concurrent.CountDownLatch;
public class WorkerCount extends Thread {
private String name;
private long time;
private CountDownLatch countDownLatch;
public WorkerCount(String name, long time, CountDownLatch countDownLatch) {
this.name = name;
this.time = time;
this.countDownLatch = countDownLatch;
}
@Override
public void run() {
try {
System.out.println(name + "开始工作");
Thread.sleep(time);
System.out.println(name + "工作完成, 耗时："+ time);
countDownLatch.countDown();
System.out.println("countDownLatch.getCount():" + countDownLatch.getCount());
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
CountDownLatch实现：
@Test
public void CountDownLatchTest() throws InterruptedException {
int COUNT = 2;
final CountDownLatch countDownLatch = new CountDownLatch(COUNT);
WorkerCount worker0 = new WorkerCount("lilei-0", (long)(Math.random() * 10000), countDownLatch);
WorkerCount worker1 = new WorkerCount("lilei-1", (long)(Math.random() * 10000), countDownLatch);
worker0.start();
worker1.start();
countDownLatch.await();
System.out.println("准备工作就绪");
WorkerCount worker2 = new WorkerCount("lilei-2", (long)(Math.random() * 10000), countDownLatch);
worker2.start();
Thread.sleep(10000);
}
输出：
lilei-0开始工作 lilei-1开始工作 lilei-1工作完成, 耗时：4039 countDownLatch.getCount():1 lilei-0工作完成, 耗时：9933 countDownLatch.getCount():0 准备工作就绪 lilei-2开始工作 lilei-2工作完成, 耗时：6402 countDownLatch.getCount():0
该场景join也可以完成
Worker.java
package com.jihite;
public class Worker extends Thread{
private String name;
private long time;
public Worker(String name, long time) {
this.name = name;
this.time = time;
}
@Override
public void run() {
try {
System.out.println(name + "开始工作");
Thread.sleep(time);
System.out.println(name + "工作完成, 耗时："+ time);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
join实现
@Test
public void JoinTest() throws InterruptedException {
Worker worker0 = new Worker("lilei-0", (long)(Math.random() * 10000));
Worker worker1 = new Worker("lilei-1", (long)(Math.random() * 10000));
Worker worker2 = new Worker("lilei-2", (long)(Math.random() * 10000));
worker0.start();
worker1.start();
worker0.join();
worker1.join();
System.out.println("准备工作就绪");
worker2.start();
Thread.sleep(10000);
}
输出
lilei-0开始工作 lilei-1开始工作 lilei-1工作完成, 耗时：4483 lilei-0工作完成, 耗时：6301 准备工作就绪 lilei-2开始工作 lilei-2工作完成, 耗时：6126
既然这样，那CountDownLatch和join的区别在哪？通过下面的场景三就可以看出
实例3（和join的不同处）
场景：流水线上有3个worker: worker1、worker2、worker3,只有当worker1和worker2两者的阶段一都执行完后才可以执行worker3
WorkerCount2.java
package com.jihite;
import java.util.concurrent.CountDownLatch;
public class WorkerCount2 extends Thread {
private String name;
private long time;
private CountDownLatch countDownLatch;
public WorkerCount2(String name, long time, CountDownLatch countDownLatch) {
this.name = name;
this.time = time;
this.countDownLatch = countDownLatch;
}
@Override
public void run() {
try {
System.out.println(name + "开始阶段1工作");
Thread.sleep(time);
System.out.println(name + "阶段1完成, 耗时："+ time);
countDownLatch.countDown();
System.out.println(name + "开始阶段2工作");
Thread.sleep(time);
System.out.println(name + "阶段2完成, 耗时："+ time);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
此时用join无法实现，只能用CountDownLatch
@Test
public void CountDownLatchTest2() throws InterruptedException {
int COUNT = 2;
final CountDownLatch countDownLatch = new CountDownLatch(COUNT);
WorkerCount2 worker0 = new WorkerCount2("lilei-0", (long)(Math.random() * 10000), countDownLatch);
WorkerCount2 worker1 = new WorkerCount2("lilei-1", (long)(Math.random() * 10000), countDownLatch);
worker0.start();
worker1.start();
countDownLatch.await();
System.out.println("准备工作就绪");
WorkerCount2 worker2 = new WorkerCount2("lilei-2", (long)(Math.random() * 10000), countDownLatch);
worker2.start();
Thread.sleep(10000);
}
输出
lilei-0开始阶段1工作 lilei-1开始阶段1工作 lilei-0阶段1完成, 耗时：3938 lilei-0开始阶段2工作 lilei-1阶段1完成, 耗时：6259 lilei-1开始阶段2工作 准备工作就绪 lilei-2开始阶段1工作 lilei-0阶段2完成, 耗时：3938 lilei-1阶段2完成, 耗时：6259 lilei-2阶段1完成, 耗时：7775 lilei-2开始阶段2工作