concurrent 包里面的 CountDownLatch 应用

应用场景

主线程想要往下执行，但必须要等到子线程的任务执行完毕后才可以继续往下执行。其中主线程调用一个 CountDownLatch 对象的 await()方法，其他子线程的任务执行完自己的任务后调用同一个 CountDownLatch 对象上的 countDown()方法，直到这个 CountDownLatch 对象的计数值减到 0 为止，此时主线程才能继续往下执行；否则主线程调用 await()方法的任务将一直阻塞等待；

Example

• 有三个工人在为老板干活，这个老板有一个习惯，就是当三个工人把一天的活都干完了的时候，他就来检查所有工人所干的活。记住这个条件：三个工人先全部干完活，老板才检查。所以在这里用 Java 代码设计两个类，Worker 代表工人，Boss 代表老板，具体的代码实现如下:

• Boss code

public class Boss implements Runnable {

    private CountDownLatch downLatch;

    public Boss(CountDownLatch downLatch){
        this.downLatch = downLatch;
    }

    public void run() {
        System.out.println("老板正在等所有的工人干完活......");
        try {
            this.downLatch.await();
        } catch (InterruptedException e) {
        }
        System.out.println("工人活都干完了，老板开始检查了！");
    }

}

• Worker code

public class Worker implements Runnable{

    private CountDownLatch downLatch;
    private String name;

    public Worker(CountDownLatch downLatch, String name){
        this.downLatch = downLatch;
        this.name = name;
    }

    public void run() {
        this.doWork();
        try{
            TimeUnit.SECONDS.sleep(new Random().nextInt(10));
        }catch(InterruptedException ie){
        }
        System.out.println(this.name + "活干完了！");
        this.downLatch.countDown();

    }

    private void doWork(){
        System.out.println(this.name + "正在干活!");
    }

}

• Main code

public class CountDownLatchDemo {
    public static void main(String[] args) {
        ExecutorService executor = Executors.newCachedThreadPool();

        CountDownLatch latch = new CountDownLatch(3);

        Worker w1 = new Worker(latch,"张三");
        Worker w2 = new Worker(latch,"李四");
        Worker w3 = new Worker(latch,"王二");

        Boss boss = new Boss(latch);

        executor.execute(w3);
        executor.execute(w2);
        executor.execute(w1);
        executor.execute(boss);

        executor.shutdown();
    }

}

• 输出结果

王二正在干活!
张三正在干活!
李四正在干活!
老板正在等所有的工人干完活......
王二活干完了！
张三活干完了！
李四活干完了！
工人活都干完了，老板开始检查了！

串行执行也可以用到

		for (final String st : stArray) {

            CommonThreadPool.getThreadPool("getThread").execute(new Runnable() {
                @Override
                public void run() {
                    try {
                       System.out.printIn(st);
                    } catch (RuntimeException e) {
                       logger.error(LOGGER,, e));
                    } finally {
                        overLatch.countDown();
                    }
                }
            });
        }

        try {
            overLatch.await();
        } catch (InterruptedException e) {
            logger.error(LOGGER, , e));
        }