Java等待多个线程执行完毕

1、使用join

public class TestCountDownLatch2
{

    public static void main(String[] args) throws InterruptedException
    {    
        Thread[] threadArr = new Thread[10];
        for(int i = 0; i < 10; i++){
            threadArr[i] = new Thread(new WorkThread());
        }

        for(int i=0;i<10;i++){
            threadArr[i].start();
        }

        for(int i=0;i<10;i++){
            threadArr[i].join();
        }

        System.out.println("main done!");        
    }
}

class WorkThread implements Runnable {

    @Override
    public void run()
    {
        System.out.println(Thread.currentThread().getName() + "  worked");
    }
}

2、使用ExecutorService.isTerminated死循环判断

public class TestCountDownLatch
{

    public static void main(String[] args) throws InterruptedException
    {
        ExecutorService executorService = Executors.newCachedThreadPool();
        for(int i = 0; i < 10; i++){
            executorService.execute(new Thread(new WorkThread()));
        }

        System.out.println("submit all task");
        executorService.shutdown();  // stop accept new task

        while(!executorService.isTerminated()){  // must call shutdown first, when before tasks finished, then the flag is set true
            ; // do nothing
        }

        System.out.println("main thread done!");        
    }
}

class WorkThread implements Runnable {

    @Override
    public void run()
    {
        System.out.println(Thread.currentThread().getName() + "  worked");
        try
        {
            Thread.sleep(2000);
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }
    }
}

3、使用CountDownLatch

特点：CountDown方法可以用在任何地方，可以是N个线程，也可以是1个线程里的N个执行步骤

public class TestCountDownLatch2
{

    public static void main(String[] args) throws InterruptedException
    {
        CountDownLatch countDownLatch = new CountDownLatch(10);

        ExecutorService executorService = Executors.newCachedThreadPool();
        for(int i = 0; i < 10; i++){
            executorService.execute(new Thread(new WorkThread(countDownLatch)));
        }

        System.out.println("submit all task");
        executorService.shutdown();  // stop accept new task

        countDownLatch.await();  // wait until the latch has count down to zero

        System.out.println("main done!");        
    }
}

class WorkThread implements Runnable {

    private CountDownLatch countDownLatch;

    public WorkThread(CountDownLatch countDownLatch){
        this.countDownLatch = countDownLatch;
    }

    @Override
    public void run()
    {
        System.out.println(Thread.currentThread().getName() + "  worked");
        countDownLatch.countDown();
    }
}

比较总结：

join用于让当前执行线程等待join线程执行结束。其实现原理是不停检查join线程是否存活，如果join线程存活则让当前线程永远wait,直到join线程中止后，线程的this.notifyAll会被调用

JDK不推荐在线程实例上使用wait，notify和notifyAll方法

推荐使用CountDownLatch