CountDownLatch的使用

作用

CountDownLatch，闭锁。能让线程等待其它线程完成个子的工作后再执行。

使用方法

CountDownLatch通过一个计数器来实现，计数器的初始值为初始任务的数量，每当完成一个任务后，计数器的值会减去1。当计数器到达0时候，表示所有任务完成，调用了CountDownLatch.await()方法的线程就可以恢复执行。

应用场景

/**
 * 演示CountDownLatch用法，
 * 共5个初始化子线程，6个闭锁扣除点，扣除完毕后，主线程和业务线程才能继续执行
 * Created by linshujie on 2022/3/26.
 */
public class UseCountDownLatch {
    static CountDownLatch latch = new CountDownLatch(6);

    /**
     * 初始化线程
     */
    private static class InitRunnable implements Runnable {

        public void run() {
            System.out.println("Thread_" + Thread.currentThread().getId()
                    + " ready init work......");
            latch.countDown();
            for (int i = 0; i < 2; i++) {
                System.out.println("Thread_" + Thread.currentThread().getId()
                        + " ........continue do its work");
            }
        }
    }

    /**
     * 业务线程等待latch的计数器为0完成
     */
    private static class BusiRunnable implements Runnable {

        public void run() {
            try {
                //业务线程工作
                latch.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            for (int i = 0; i < 3; i++) {
                System.out.println("BusiRunnable_" + Thread.currentThread().getId()
                        + " do business-----");
            }
        }
    }

    public static void main(String[] args) throws InterruptedException {
        //start一个初始化线程
        new Thread(new Runnable() {
            public void run() {
                SleepTools.ms(1);
                System.out.println("Thread_" + Thread.currentThread().getId()
                        + " ready init work step 1st......");
                latch.countDown();
                System.out.println("Thread_" + Thread.currentThread().getId()
                        + " begin step 2nd.......");
                SleepTools.ms(1);
                System.out.println("Thread_" + Thread.currentThread().getId()
                        + " ready init work step 2nd......");
                latch.countDown();
            }
        }).start();
        //start一个业务线程
        new Thread(new BusiRunnable()).start();
        //start 4个初始化线程
        for (int i = 0; i < 4; i++) {
            Thread thread = new Thread(new InitRunnable());
            thread.start();
        }
        //主线程工作
        latch.await();
        System.out.println("Main do ites work........");
    }
}

执行结果：

Thread_29 ready init work......
Thread_26 ready init work......
Thread_28 ready init work......
Thread_27 ready init work......
Thread_24 ready init work step 1st......
Thread_27 ........continue do its work
Thread_29 ........continue do its work
Thread_26 ........continue do its work
Thread_29 ........continue do its work
Thread_28 ........continue do its work
Thread_27 ........continue do its work
Thread_28 ........continue do its work
Thread_24 begin step 2nd.......
Thread_26 ........continue do its work
Thread_24 ready init work step 2nd......
Main do ites work........
BusiRunnable_25 do business-----
BusiRunnable_25 do business-----
BusiRunnable_25 do business-----
Class transformation time: 0.019353s for 148 classes or 1.307635135135135E-4s per class

Process finished with exit code 0

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