Displaying the "system" ThreadGroup Tree

<< ThreadGroup Class and "system" ThreadGroup Tree

<< Java Tutorials - Herong's Tutorial Notes

This section provides a tutorial example on how to display the 'system' ThreadGroup tree by using the enumerate() and list() methods.

As I mentioned in the previous section, there is a single tree structure managed by the JVM to organize all objects of ThreadGroup class and Thread class. Here are the rules of how this tree is maintained:

• The root node of the tree is a ThreadGroup object named "system".
• The "system" thread group contains a number of threads performing JVM system tasks.
• The "system" thread group contains just one ThreadGroup object named "main".
• The "main" thread group contains one thread named "main", which is the starting thread for the application program.
• The "main" thread group contains no other ThreadGroup objects.
• If a new ThreadGroup object is created without a parent ThreadGroup object specified, it will be added into the "main" thread group.
• If a new Thread object is created without a ThreadGroup object specified, it will be added into the "main" thread group.

Here is a sample program showing how to use some of the methods in ThreadGroup class to get information out of the ThreatGroup tree:

/**
 * ThreadGroupTree.java
 * Copyright (c) 2003 by Dr. Herong Yang
 */
class ThreadGroupTree {
   public static void main(String[] args) {
      Thread t = Thread.currentThread();
      ThreadGroup g = t.getThreadGroup();
      g = g.getParent();
      printInfo(g,"");
      g.list();
   }
   public static void printInfo(ThreadGroup g, String p) {
      System.out.println(p+"ThreadGroup name = "+g.getName());
      System.out.println(p+"   Has parent thread group = "
         +(g.getParent()!=null));
      int n = g.activeCount();
      System.out.println(p+"   # of active threads = "+n);
      Thread[] l = new Thread[n];
      n = g.enumerate(l, false);
      for (int i=0; i<n; i++) {
         System.out.println(p+"   Thread name = "+l[i].getName());
      }  
      n = g.activeGroupCount();
      System.out.println(p+"   # of active sub thread groups = "+n);
      ThreadGroup[] s = new ThreadGroup[n];
      n = g.enumerate(s, false);
      for (int i=0; i<n; i++) {
         printInfo(s[i],"   "+p);
      }  
   }   
}

Output:

ThreadGroup name = system
   Has parent thread group = false
   # of active threads = 6
   Thread name = Reference Handler
   Thread name = Finalizer
   Thread name = Signal Dispatcher
   Thread name = CompileThread0
   # of active sub thread groups = 1
   ThreadGroup name = main
      Has parent thread group = true
      # of active threads = 2
      Thread name = main
      # of active sub thread groups = 0
java.lang.ThreadGroup[name=system,maxpri=10]
    Thread[Reference Handler,10,system]
    Thread[Finalizer,8,system]
    Thread[Signal Dispatcher,10,system]
    Thread[CompileThread0,10,system]
    java.lang.ThreadGroup[name=main,maxpri=10]
        Thread[main,5,main]
        Thread[Thread-0,5,main]

Note that:

• There are 4 Thread objects in the "system" thread group. I have no idea what they are for.
• The "main" thread group contains 2 Thread objects. But the enumerate() method copies only one thread object - the "main" thread.
• However the list() method is correctly reporting 2 Thread objects, named "main" and "Thread-0".
• What is the purpose of "Thread-0" thread? How to get a reference of this thread?

Sections in This Chapter

"ThreadGroup" Class - Container of Threads and Thread Groups

Displaying the "system" ThreadGroup Tree

Adding Threads and Thread Groups

JDK 1.3 Bug - Memory Leak With Unstarted Threads