多线程工具CountDownLatch使用

桃扇骨 2023-05-30 13:00 3阅读 0赞

# **一：CountDownLatch概念** #

CountDownLatch是一个同步工具类，用来协调多个线程之间的同步，或者说起到线程之间的通信（而不是用作互斥的作用）。

# 二：**CountDownLatch的用法** #

CountDownLatch典型用法：

某一线程在开始运行前等待n个线程执行完毕。将CountDownLatch的计数器初始化为new CountDownLatch(n)，每当一个任务线程执行完毕，就将计数器减1 countdownLatch.countDown()，当计数器的值变为0时，在CountDownLatch上await()的线程就会被唤醒。一个典型应用场景就是启动一个服务时，主线程需要等待多个组件加载完毕，之后再继续执行。

如下案例：xxx业务逻辑，需要等待线程1、线程2、线程3执行完成后，然后执行 countDownLatch.await()才会执行。

# **三：CountDownLatch的不足** #

CountDownLatch是一次性的，计算器的值只能在构造方法中初始化一次，之后没有任何机制再次对其设置值，当CountDownLatch使用完毕后，它不能再次被使用。

package test.latch;
    
    import java.util.concurrent.CountDownLatch;
    
    /**
     * @author
     */
    public class CountDownLatchTest {
    
        public static void main(String[] args) {
    
            CountDownLatch countDownLatch = new CountDownLatch(3);
    
            System.out.println("程序开始运行");
            new ThreadTest1(countDownLatch).start();
            new ThreadTest2(countDownLatch).start();
            new ThreadTest3(countDownLatch).start();
    
            try {
                System.out.println("等待执行countDownLatch.await()");
                countDownLatch.await();
                System.out.println("countDownLatch.await()执行完成");
                //xxx业务逻辑
    
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("程序运行结束");
    
        }
    
        public static class ThreadTest1 extends Thread {
    
            private CountDownLatch countDownLatch;
    
            public ThreadTest1(CountDownLatch countDownLatch) {
                this.countDownLatch = countDownLatch;
            }
    
            @Override
            public void run() {
                System.out.println("线程1运行");
                countDownLatch.countDown();
                super.run();
            }
        }
    
        public static class ThreadTest2 extends Thread {
    
            private CountDownLatch countDownLatch;
    
            public ThreadTest2(CountDownLatch countDownLatch) {
                this.countDownLatch = countDownLatch;
            }
    
            @Override
            public void run() {
                System.out.println("线程2运行");
                countDownLatch.countDown();
                super.run();
            }
        }
    
        public static class ThreadTest3 extends Thread {
    
            private CountDownLatch countDownLatch;
    
            public ThreadTest3(CountDownLatch countDownLatch) {
                this.countDownLatch = countDownLatch;
            }
    
            @Override
            public void run() {
                System.out.println("线程3开始运行");
                try {
                    System.out.println("线程3开始休眠");
    
                    Thread.sleep(5000);
                    System.out.println("线程3休眠完成");
    
                    countDownLatch.countDown();
    
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                super.run();
            }
        }
    }

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