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JavaWorld.com > Java Standard Edition >

Java 101: Understanding Java threads, Part 4: Thread groups, volatility, and thread-local variables

Final concepts for improving Java application programming with Java threads

By Jeff Friesen, JavaWorld.com, 08/02/02

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Page 2 of 8

public static void main (String [] args)
{
   ThreadGroup tg1 = new ThreadGroup ("A");
   ThreadGroup tg2 = new ThreadGroup (tg1, "B");
}

In the code above, the main thread creates two thread groups: A and B. First, the main thread creates A by calling ThreadGroup(String name). The tg1-referenced thread group's parent is main because main is the main thread's thread group. Second, the main thread creates B by calling ThreadGroup(ThreadGroup parent, String name). The tg2-referenced thread group's parent is A because tg1's reference passes as an argument to ThreadGroup (tg1, "B") and A associates with tg1.

Tip: Once you no longer need a hierarchy of ThreadGroup objects, call ThreadGroup's void destroy() method via a reference to the ThreadGroup object at the top of that hierarchy. If the top ThreadGroup object and all subgroup objects lack thread objects, destroy() prepares those thread group objects for garbage collection. Otherwise, destroy() throws an IllegalThreadStateException object. However, until you nullify the reference to the top ThreadGroup object (assuming a field variable contains that reference), the garbage collector cannot collect that object. Referencing the top object, you can determine if a previous call was made to the destroy() method by calling ThreadGroup's boolean isDestroyed() method. That method returns true if the thread group hierarchy was destroyed.

By themselves, thread groups are useless. To be of any use, they must group threads. You group threads into thread groups by passing ThreadGroup references to appropriate Thread constructors: 

ThreadGroup tg = new ThreadGroup ("subgroup 2");
Thread t = new Thread (tg, "my thread");

The code above first creates a subgroup 2 group with main as the parent group. (I assume the main thread executes the code.) The code next creates a my thread Thread object in the subgroup 2 group.

Now, let's create an application that produces our figure's hierarchical thread-group structure:

Listing 1. ThreadGroupDemo.java

// ThreadGroupDemo.java
class ThreadGroupDemo
{
   public static void main (String [] args)
   {
      ThreadGroup tg = new ThreadGroup ("subgroup 1");
      Thread t1 = new Thread (tg, "thread 1");
      Thread t2 = new Thread (tg, "thread 2");
      Thread t3 = new Thread (tg, "thread 3");
      tg = new ThreadGroup ("subgroup 2");
      Thread t4 = new Thread (tg, "my thread");
      tg = Thread.currentThread ().getThreadGroup ();
      int agc = tg.activeGroupCount ();
      System.out.println ("Active thread groups in " + tg.getName () +
                          " thread group: " + agc);
      tg.list ();
   }
}

ThreadGroupDemo creates the appropriate thread group and thread objects to mirror what you see in the figure above. To prove that the subgroup 1 and subgroup 2 groups are main's only subgroups, ThreadGroupDemo does the following:

  1. Retrieves a reference to the main thread's ThreadGroup object by calling Thread's static currentThread() method (which returns a reference to the main thread's Thread object) followed by Thread's ThreadGroup getThreadGroup() method.
  2. Calls ThreadGroup's int activeGroupCount() method on the just-returned ThreadGroup reference to return an estimate of active groups within the main thread's thread group.
  3. Calls ThreadGroup's String getName () method to return the main thread's thread group name.
  4. Calls ThreadGroup's void list () method to print on the standard output device details on the main thread's thread group and all subgroups.

When run, ThreadGroupDemo displays the following output:

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Resources
  • Learn more about Java: See the complete listing for Jeff Friesen's Java 101 series -- archived on JavaWorld.
  • Also see the Java Tips series: More than five years of compiled tips from JavaWorld's expert readers.