CountDownLatch

概念

countDownLatch这个类使一个线程等待其他线程各自执行完毕后再执行。

是通过一个计数器来实现的，计数器的初始值是线程的数量。每当一个线程执行完毕后，计数器的值就-1，当计数器的值为0时，表示所有线程都执行完毕，然后在闭锁上等待的线程就可以恢复工作了。

核心方法

//构造方法,初始化一个CountDownLatch实例,指定计数count,一般指定为多线程的数量
public CountDownLatch(int count);
//阻塞线程,等待所有线程执行完成释放,继续执行后续代码
public void await() throws InterruptedException;
//阻塞线程,指定阻塞时间,阻塞时间超过后,无论所有线程是否执行完毕,都将继续执行后续代码
public boolean await(long timeout, TimeUnit unit) throws InterruptedException;
//计数器减1
public void countDown();
//获取当前计数器的值
public long getCount();

示例

不指定阻塞时间

import lombok.extern.slf4j.Slf4j;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

/**
 * @author scottxuan
 */
@Slf4j
public class CountDownLatchExample1 {

    private final static int threadCount = 20;

    public static void main(String[] args) throws InterruptedException {
        //初始化计数器,一般和线程数目相同
        CountDownLatch countDownLatch = new CountDownLatch(threadCount);
        ExecutorService executor = Executors.newCachedThreadPool();
        for (int i = 0; i < threadCount; i++) {
            final int num = i;
            executor.execute(()->{
                count(num);
                //多线程每执行一次,计数器减1
                countDownLatch.countDown();
            });
        }
        //主线程阻塞,等待所有线程执行完毕,才会执行后续的代码
        countDownLatch.await();
        log.info("all thread finish");
        executor.shutdown();
    }

    private static void count(int num){
        log.info("current num {}",num);
    }
}

//执行结果,所有线程执行完毕后,日志输出  all thread finish
//22:17:56.095 [pool-1-thread-1] INFO com.aqs.CountDownLatchExample1 - current num 0
//22:17:56.095 [pool-1-thread-9] INFO com.aqs.CountDownLatchExample1 - current num 8
//22:17:56.096 [pool-1-thread-7] INFO com.aqs.CountDownLatchExample1 - current num 6
//22:17:56.095 [pool-1-thread-4] INFO com.aqs.CountDownLatchExample1 - current num 3
//22:17:56.095 [pool-1-thread-10] INFO com.aqs.CountDownLatchExample1 - current num 9
//22:17:56.095 [pool-1-thread-8] INFO com.aqs.CountDownLatchExample1 - current num 7
//22:17:56.095 [pool-1-thread-2] INFO com.aqs.CountDownLatchExample1 - current num 1
//22:17:56.095 [pool-1-thread-3] INFO com.aqs.CountDownLatchExample1 - current num 2
//22:17:56.095 [pool-1-thread-6] INFO com.aqs.CountDownLatchExample1 - current num 5
//22:17:56.095 [pool-1-thread-5] INFO com.aqs.CountDownLatchExample1 - current num 4
//22:17:56.100 [main] INFO com.aqs.CountDownLatchExample1 - all thread finish

指定阻塞时间

import lombok.extern.slf4j.Slf4j;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

/**
 * @author scottxuan
 */
@Slf4j
public class CountDownLatchExample2 {

    private final static int threadCount = 10;

    public static void main(String[] args) throws InterruptedException {
        //初始化计数器,一般和线程数目相同
        CountDownLatch countDownLatch = new CountDownLatch(threadCount);
        ExecutorService executor = Executors.newCachedThreadPool();
        for (int i = 0; i < threadCount; i++) {
            final int num = i;
            executor.execute(()->{
                count(num);
                //多线程每执行一次,计数器减1
                countDownLatch.countDown();
            });
        }
        //主线程阻塞指定的时间,改时间超过后继续执行后续的代码,无论所有线程是否执行完毕
        countDownLatch.await(1000, TimeUnit.MILLISECONDS);
        log.info("await time out");
        executor.shutdown();
    }

    private static void count(int num){
        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        log.info("current num {}",num);
    }
}

//输出结果 线程的执行每个需要2000ms,但1000ms之后阻塞释放,先输出
//22:24:10.016 [main] INFO com.aqs.CountDownLatchExample1 - await time out
//22:24:11.014 [pool-1-thread-10] INFO com.aqs.CountDownLatchExample1 - current num 9
//22:24:11.014 [pool-1-thread-2] INFO com.aqs.CountDownLatchExample1 - current num 1
//22:24:11.015 [pool-1-thread-4] INFO com.aqs.CountDownLatchExample1 - current num 3
//22:24:11.014 [pool-1-thread-9] INFO com.aqs.CountDownLatchExample1 - current num 8
//22:24:11.014 [pool-1-thread-1] INFO com.aqs.CountDownLatchExample1 - current num 0
//22:24:11.015 [pool-1-thread-3] INFO com.aqs.CountDownLatchExample1 - current num 2
//22:24:11.015 [pool-1-thread-8] INFO com.aqs.CountDownLatchExample1 - current num 7
//22:24:11.014 [pool-1-thread-6] INFO com.aqs.CountDownLatchExample1 - current num 5
//22:24:11.015 [pool-1-thread-7] INFO com.aqs.CountDownLatchExample1 - current num 6
//22:24:11.014 [pool-1-thread-5] INFO com.aqs.CountDownLatchExample1 - current num 4