[Bluej-discuss] Thread Question

[Davin McCall]

Hi Mark,

The crux of the problem is that the thread that is running is not the 
one you are calling getThreadGroup() on. Your program creates a total of 
6 thread objects (two of them are instances of EventOne, and one is an 
an instance of EventTwo; the others are just plain old Threads). It only 
actually starts 3 of them (the plain old Threads).

For example:

	EventOne e1 = new EventOne();
        // Create a new Thread running under the 'threadGroup' ThreadGroup
        new Thread(threadGroup, e1).start(); 


... actually creates an EventOne object (e1) whose thread group will be 
the same thread group as myTest1() is called on. It then creates a 
Thread (in another thread group) which calls the "run" method of e1, 
which does this:

	System.out.println("EventOne: " + getThreadGroup()

... However, the getThreadGroup() call retrieves the thread group of e1 
(which as I mentioned before is the same thread group as that which 
myTest1() is run on) rather than the thread group of the currently 
running thread. To re-iterate: e1 is NOT the currently running thread 
and in fact e1 is NEVER actually started as a thread in its own right 
(yes, it is a Thread object, but it never actually represents a 
thread-of-execution - a real thread - because the "start" method is 
never called).

To get the behaviour which (I think) you want, you should add a 
constructor to EventOne and EventTwo which takes a thread group as a 
parameter and which passes that parameter to the superclass constructor, 
something like:

    public EventOne(ThreadGroup group)
    {
        super(group, null);
    }


... and then change the code which creates the thread to:

	EventOne e1 = new EventOne(threadGroup);
	e1.start();

- Davin


Mark Dutchuk (MALA) wrote:
> Hello folks.  I'm puzzled by a problem I am having with Threads and ThreadGroups.  I've distilled the problem down to the following program to demonstrate the problem:
>
> public class TGTest {
>
>     private static ThreadGroup threadGroup = new ThreadGroup("Some Thread Group");
>     
>     public static void myTest1() {
>         
>         EventOne e1 = new EventOne();
>         // Create a new Thread running under the 'threadGroup' ThreadGroup
>         new Thread(threadGroup, e1).start(); 
>         
>         EventTwo e2 = new EventTwo();
>         new Thread(threadGroup, e2).start();
>     }
>     public static void myTest2() {
>         EventOne e1 = new EventOne();
>         new Thread(threadGroup, e1).start(); 
>     }
> }
>
> class EventOne extends Thread {
>     public void run() {
>         System.out.println("EventOne: " + getThreadGroup());
>     }
> }
>
> class EventTwo extends Thread {
>     public void run() {
>         TGTest.myTest2();
>     }
> }
>
> Here's the output when I run TGTest.test1():
>
> EventOne: java.lang.ThreadGroup[name=main,maxpri=10]
> EventOne: java.lang.ThreadGroup[name=Some Thread Group,maxpri=10]
>
>
> Why does EventOne run under the 'main' ThreadGroup when I call myTest1() directly from BlueJ, but under 'Some Thread Group' when EventTwo calls myTest1()?  I want it to run under 'Some Thread Group' both times, but I can't see the problem.
>
> btw:  The problem has nothing to do with static methods and/or variables.  I've rewritten the program both ways (setting everything non-static and creating an instance of TGTest), the outcome is the same.  Any help or suggestions would be much appreciated.
>
>
> dmd.
>