在一个主线程中，要求有大量子线程执行完之后，主线程才执行完成？多种方式，考虑效率。

1、在主函数中使用join()方法。

    t1.start();
    t2.start();
    t3.start();

    t1.join();//不会导致t1和t2和t3的顺序执行
    t2.join();
    t3.join();
System.out.println("Main finished");

2、CountDownLatch，一个同步辅助类，在完成一组正在其他线程中执行的操作之前，它允许一个或多个线程一直等待。

public class WithLatch{
    public static void main(String[] args){
        CountDownLatch latch = new CountDownLatch(3);
        for(int i=0;i<3;i++){
            new ChildThread(i,latch).start();
        }
        try{
            latch.await();
        }catch(InterruptedException e){
            e.printStackTrace();
        }
        System.out.println("Main finished");
    }
    static calss ChildThread extends Thread{
        private int id = -1;
        private CountDownLatch latch = null;

        public ChildThread(int id, CountDownLatch latch){
            this.id = id;
            this.latch = latch;
        }
        public void run(){
            try{
                Thread.sleep(Math.abs(new Random().nextInt(5000)));
                System.out.println(String.format("Child Thread %d finished",id));
            }catch(InterruptedExcepion e){
                e.printStackTrace();
            }finally{
                latch.countDown();
            }
        }
    }
}

3、使用线程池

public class WithExecutor{
    public static void main(String[] args) throws InterruptedExcepion{
        ExecutorService pool = Executors.newFixedThreadPool(3);
        List<Callable<Void>> list = new ArrayList<Callable<Void>>();
        for(int i=0;i<3;i++){
            list.add(new ChildThread(i));
        }
        try{
            pool.invokeAll(list);
        }finally{
            pool.shutdown();
        }
        System.out.println("Main finished");
    }

    static class ChildThread implements Callable<Void>{
        private int id =  -1;
    
        public ChildThread (int id){
            this.id = id;
        }
        public Void call() throws Exception{
            try{
                Thread.sleep(Math.abs(new Random().nextInt(5000)));
                System.out.println(String.format("Child Thread %d finished",id));
            }catch(InterruptedException e){
                e.printStackTrace();
            }
            return null;
        }
    }
}