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?
