Displaying the "system" ThreadGroup Tree

Java Tutorials - Herong's Tutorial Examples

∟ThreadGroup Class and "system" ThreadGroup Tree

∟Displaying the "system" ThreadGroup Tree

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) HerongYang.com. All Rights Reserved.
 */
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 = 5
   Thread name = Reference Handler
   Thread name = Finalizer
   Thread name = Signal Dispatcher
   Thread name = Attach Listener
   # of active sub thread groups = 1
   ThreadGroup name = main
      Has parent thread group = true
      # of active threads = 1
      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,9,system]
    Thread[Attach Listener,5,system]
    java.lang.ThreadGroup[name=main,maxpri=10]
        Thread[main,5,main]

Note that:

There are 5 Thread objects in the "system" thread group: 4 threads (Reference Handler, Finalizer, Signal Dispatcher, Attach Listener) and 1 group (main).
The "main" thread is located in the "main" group under the "system" group.