本文介绍两种主线程等待子线程的实现方式，以5个子线程来说明：

1、使用Thread的join()方法，join()方法会阻塞主线程继续向下执行。

2、使用.util.concurrent中的CountDownLatch，是一个倒数计数器。初始化时先设置一个倒数计数初始值，每调用一次countDown()方法，倒数值减一，他的await()方法会阻塞当前进程，直到倒数至0。

join方式代码如下：

 

package com.test.thread;

import java.util.ArrayList;
import java.util.List;

public class MyThread extends Thread
{

    public MyThread(String name)
    {
        this.setName(name);
    }

    @Override
    public void run()
    {
        System.out.println(this.getName() + " staring...");

        System.out.println(this.getName() + " end...");
    }

    /**
     * @param args
     */
    public static void main(String[] args)
    {
        System.out.println("main thread starting...");

        List<MyThread> list = new ArrayList<MyThread>();

        for (int i = 1; i <= 5; i++)
        {
            MyThread my = new MyThread("Thrad " + i);
            my.start();
            list.add(my);
        }

        try
        {
            for (MyThread my : list)
            {
                my.join();
            }
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }

        System.out.println("main thread end...");

    }

}

运行结果如下：

 

main thread starting...

Thrad 2 staring...

Thrad 2 end...

Thrad 4 staring...

Thrad 4 end...

Thrad 1 staring...

Thrad 1 end...

Thrad 3 staring...

Thrad 3 end...

Thrad 5 staring...

Thrad 5 end...

main thread end...

CountDownLatch方式代码如下：

 

package com.test.thread;

import java.util.concurrent.CountDownLatch;

public class MyThread2 extends Thread
{

    private CountDownLatch count;

    public MyThread2(CountDownLatch count, String name)
    {
        this.count = count;
        this.setName(name);
    }

    @Override
    public void run()
    {
        System.out.println(this.getName() + " staring...");

        System.out.println(this.getName() + " end...");
        this.count.countDown();
    }

    /**
     * @param args
     */
    public static void main(String[] args)
    {
        System.out.println("main thread starting...");

        CountDownLatch count = new CountDownLatch(5);

        for (int i = 1; i <= 5; i++)
        {
            MyThread2 my = new MyThread2(count, "Thread " + i);
            my.start();
        }

        try
        {
            count.await();
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }

        System.out.println("main thread end...");

    }

}

运行结果如下：

 

main thread starting...

Thread 2 staring...

Thread 2 end...

Thread 4 staring...

Thread 4 end...

Thread 1 staring...

Thread 1 end...

Thread 3 staring...

Thread 3 end...

Thread 5 staring...

Thread 5 end...

main thread end...