主线程，子线程，和join()

在网上看到一个例子

public class JoinTester01 implements Runnable {

    private String name;

    public JoinTester01(String name) {
    this.name = name;
    }

    public void run() {
    System.out.printf("%s begins: %s
", name, new Date());
    try {
        TimeUnit.SECONDS.sleep(4);
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
    System.out.printf("%s has finished: %s
", name, new Date());
    }

    public static void main(String[] args) {
    Thread thread1 = new Thread(new JoinTester01("One"));
    Thread thread2 = new Thread(new JoinTester01("Two"));
    thread1.start();
    thread2.start();
    
    /*try {
        thread1.join();
        thread2.join();
    } catch (InterruptedException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }*/
    
    System.out.println("Main thread is finished");
    }

}

　　输出结果是

Main thread is finished                                                                                                                 

OneWed Mar 01 09:04:52 UTC 2017                                                                                                         

TwoWed Mar 01 09:04:52 UTC 2017                                                                                                         

TwoWed Mar 01 09:04:52 UTC 2017                                                                                                         

OneWed Mar 01 09:04:52 UTC 2017 

开始不相信为什么main（）的主线程都完成了，子线程还能执行，自己试了一下发现真的是这样。

原来，主线程是一个非守护线程，所以主线程执行完了并不影响其他子线程，其他子线程依然该怎么运行就还怎么运行。主线程和子线程优先级一样但是主线程会先抢占CPU，先执行。所以就会出现这样的结果。

如果加上join代码部分，结果就会使真阳：

TwoWed Mar 01 09:10:39 UTC 2017                                                                                                         

OneWed Mar 01 09:10:39 UTC 2017                                                                                                         

TwoWed Mar 01 09:10:40 UTC 2017                                                                                                         

OneWed Mar 01 09:10:40 UTC 2017                                                                                                         

Main thread is finished

thread1.join()的意思就相当于从现在开始，thread1这个线程就接在main()线程后面执行了，只有执行完thread1线程之后才会继续主线程。