java - JAVA 中的守护进程线程组是什么？

Question

我知道线程可以是守护进程也可以是非守护进程。我们可以使用 isDaemon() 方法来检查线程是否是守护进程。 isDaemon() 方法也适用于线程组。

class MyThread extends Thread
{
 MyThread(ThreadGroup g, String name)
 {
  super(g,name);
 }
 public void run()
 {
  long i = 0;
  for(long l=0; l<999999999; l++)
  {
   i=i+3;
  }
 }
}

class Check
{
 public static void main(String[] args)
 {
  ThreadGroup sys = Thread.currentThread().getThreadGroup().getParent();
  ThreadGroup parent = new ThreadGroup("parent");
  MyThread t1 = new MyThread(parent, "t1");
  ThreadGroup child = new ThreadGroup(parent,"child");
  Thread t2 = new Thread(child, "t2");
  t1.start();
  t2.start();
  ThreadGroup[] t = new ThreadGroup[sys.activeGroupCount()];
  sys.enumerate(t);
  for(ThreadGroup ti: t)
  {
    System.out.println(ti.getName()+"  "+ti.isDaemon());
  }
    System.out.println(sys.getName()+"  "+sys.isDaemon());
}

输出:

main  false
parent  false
child  false
system  false

这里System也是一个非守护线程组。线程组如何成为守护进程？我的意思是守护线程组的属性是什么？系统线程组如何非守护？

Answer 1

与Thread方法相同:java.lang.ThreadGroup#setDaemon。当您创建线程组时，您可以将其标记为守护进程。

根据 javadoc:

A daemon thread group is automatically destroyed when its last thread is stopped or its last thread group is destroyed.