Question

我知道线程可以是守护进程或非守护进程。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

此处System也是非守护程序线程组。线程组如何成为守护进程？我的意思是守护程序线程组的属性是什么？系统线程组是如何非守护进程的？

Answer 1

与线程相同：java.lang.ThreadGroup#setDaemon。创建线程组时，可以将其标记为守护程序。

根据javadoc：

守护程序线程组在其最后一个线程时自动销毁停止或其最后一个线程组被销毁。

Answer 2

是的，您可以将Thread group设置为守护程序线程。

/**
 * Changes the daemon status of this thread group.
 * <p>
 * First, the <code>checkAccess</code> method of this thread group is
 * called with no arguments; this may result in a security exception.
 * <p>
 * A daemon thread group is automatically destroyed when its last
 * thread is stopped or its last thread group is destroyed.
 *
 * @param      daemon   if <code>true</code>, marks this thread group as
 *                      a daemon thread group; otherwise, marks this
 *                      thread group as normal.
 * @exception  SecurityException  if the current thread cannot modify
 *               this thread group.
 * @see        java.lang.SecurityException
 * @see        java.lang.ThreadGroup#checkAccess()
 * @since      JDK1.0
 */

什么是JAVA中的守护程序线程组？

2 个答案: