主线程可以在子线程之前死掉

Question

我相信主线程不能在子线程之前死掉.但有什么方法可以检查吗？我在下面写了一个简单的程序.任何人都可以证明它几乎将理论抛在一边吗？

class childre extends Thread
{   
    public void run()
    {   
        for( int i=0 ; i<10 ;i++)
        {
            System.out.println( " child " + i);

            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }   
        }
    }
}

public class ChildThreadb4main
{

/**
 * @param args
 */
    public static void main(String[] args)
    {
    // TODO Auto-generated method stub

        System.out.println("main");

        childre c1 = new childre();

        c1.start();
        for(int i=0;i<5;i++)
        {
            try {
                Thread.sleep(500);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
        System.out.println( " child thread alive ? " + c1.isAlive());
    }
}

---

根据詹姆斯的建议.我尝试了以下程序.

public class MainChildDie {

    public static void main(String ar[]){

        final Thread mainThread = Thread.currentThread();
        System.out.println("main run ");

        new Thread(){           

            public void run(){

                Thread childThread= Thread.currentThread();
                for(int i=0; i<10;i++){
                    System.out.println( "child"+i);

                    try {
                        Thread.sleep(1000);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                System.out.println("main alive  " + mainThread.isAlive());
            }
        }.start();      
    }
}

Answer 1

代码正在执行时,执行完整线程转储并查看所有线程处于活动状态.

class AnotherClass {
    public static void main(String arrp[]) throws Exception {
        Thread t = new Thread() {
            public void run() {
                while (true) {
                        // do nothing
                }
            }
        };
        t.start();
            //Sleep for 15 seconds
        Thread.sleep(15000);
    }
}

编译并执行它:

$ javac AnotherClass.java
$ java AnotherClass

找到过程:

$ ps -ef | grep AnotherClass

nikunj <<10720>> 10681   2 12:01:02 pts/9       0:04 java AnotherClass
nikunj 10722 10693   0 12:01:05 pts/6       0:00 grep Another

采取线程转储:

$ kill -3 <<10720>> 

输出(摘录):

"main" prio=10 tid=0x00039330 nid=0x1 waiting on condition [0xffbfe000..0xffbfe2a8]
    at java.lang.Thread.sleep(Native Method)
    at AnotherClass.main(AnotherClass.java:12)

"Thread-0" prio=10 tid=0x00a1b770 nid=0x12 runnable [0xadc7f000..0xadc7f970]
    at AnotherClass$1.run(AnotherClass.java:7)

进行另一个线程转储(15秒后):

$ kill -3 <<10720>> 

新产出(摘录):

"Thread-0" prio=10 tid=0x00a1b770 nid=0x12 runnable [0xadc7f000..0xadc7f970]
    at AnotherClass$1.run(AnotherClass.java:7)

结论: 主要消失了.

Answer 2

来自http://docs.oracle.com/javase/6/docs/api/java/lang/Thread.html:

Java虚拟机继续执行线程,直到发生以下任一情况:

1. 已调用类Runtime的exit方法,并且安全管理器已允许执行退出操作.

2. 所有非守护程序线程的线程都已死亡,无论是通过从run方法调用返回还是抛出传播超出run方法的异常.

在您的情况下,当主线程死亡时,JVM不会退出,因为您仍然在运行已创建的线程,并且默认情况下它们是守护进程,因为:

当且仅当创建它的线程当前标记为守护程序线程时,新创建的线程最初被标记为守护程序线程.方法setDaemon可用于更改线程是否是守护程序.

引用:http://docs.oracle.com/javase/6/docs/api/java/lang/Thread.html#setDaemon(boolean)