我们先看一个例子，在这个例子中你会发现主线程结束后，过了一段时间两个子线程才结束。

定义实现Runnable接口的线程类，模拟执行一定时间后结束。

public class MyRunnable implements Runnable{    @Override    public void run() {        System.out.printf("%s: I am start working.\n", Thread.currentThread().getName());        try {            TimeUnit.SECONDS.sleep((long) (Math.random() * 10 + 1));        } catch (InterruptedException e) {            e.printStackTrace();        }        System.out.printf("%s: I am end working.\n", Thread.currentThread().getName());    }}

定义主方法，启动两个子线程：

public class Main {    public static void main(String[] args) {        Thread thread1 = new Thread(new MyRunnable());        Thread thread2 = new Thread(new MyRunnable());        System.out.printf("%s: start two threads.\n", Thread.currentThread().getName());        thread1.start();        thread2.start();        System.out.printf("%s: end.\n", Thread.currentThread().getName());    }}

查看控制台日志，你会发现主线程先于两个子线程而结束。

main: start two threads.main: end.Thread-1: I am start working.Thread-0: I am start working.Thread-1: I am end working.Thread-0: I am end working.

当你想让主线程等待所有子线程执行结束后，再执行一段代码才结束，应该怎么做呢。Java提供了join()方法。我们重写主方法，在主方法中调用子线程的join()方法，即可让主线程进入WAITING状态并等待子线程终止后继续执行。

public class Main {    public static void main(String[] args) {        Thread thread1 = new Thread(new MyRunnable());        Thread thread2 = new Thread(new MyRunnable());        System.out.printf("%s: start two threads.\n", Thread.currentThread().getName());        thread1.start();        thread2.start();        try {            thread2.join();            thread1.join();            System.out.printf("%s: child runnable both ends.\n", Thread.currentThread().getName());        } catch (InterruptedException e) {            e.printStackTrace();        }        System.out.printf("%s: end.\n", Thread.currentThread().getName());    }}

查看控制台日志，你会发现主线程在等待两个子线程终止之后才继续执行。

main: start two threads.Thread-0: I am start working.Thread-1: I am start working.Thread-0: I am end working.Thread-1: I am end working.main: child runnable both ends.main: end.