Threads - Java Examples in a Nutshell - page 73

simple Swing user interface and uses it to display a listing of its own threads. Fig-
ure 4-1 shows such a listing.
Figure 4−1. The threads and thread groups of a Swing application
The listAllThreads() method uses the static Thread method currentThread() to
obtain the current thread and then calls getThreadGroup() to find the thread
group of that thread. The method then uses the ThreadGroup.getParent() method
to move up through the thread-group hierarchy until it finds the root thread
group, the thread group that contains all other threads and thread groups.
Now listAllThreads() calls the private ThreadLister.printGroupInfo() method
to display the contents of the root thread group and then recursively display the
contents of all the thread groups it contains. printGroupInfo() , and the print-
ThreadInfo() method it calls, use various Thread and ThreadGroup methods to
obtain information about the threads and their groups. Note that the isDaemon()
method returns whether a thread is a daemon thread or not. Daemon threads are
background threads that are not expected to exit. The Java interpreter exits when
all nondaemon threads have quit.
The ThreadLister class has a main() method, so it can be run as a standalone
program. It is more interesting, of course, to invoke the listAllThreads() method
from within another program; it can also help you to diagnose problems you are
having with threads.
Example 4−2: ThreadLister.java
package com.davidflanagan.examples.thread;
import java.io.*;
import java.awt.*; // AWT classes for the demo program
import javax.swing.*; // Swing GUI classes for the demo
/**
* This class contains a useful static method for listing all threads
* and threadgroups in the VM. It also has a simple main() method so it
* can be run as a standalone program.
**/
public class ThreadLister {
/** Display information about a thread. */