91 Java - ThreadGroup Class

ThreadGroup Class

The Java ThreadGroup class represents a set of threads. It can also include other thread groups. The thread groups form a tree in which every thread group except the initial thread group has a parent.

ThreadGroup Class Declaration

Following is the declaration for java.lang.ThreadGroup class −

public class ThreadGroup
   extends Object
      implements Thread.UncaughtExceptionHandler

ThreadGroup Class Constructors

Sr.No.	Constructor & Description
1	ThreadGroup(String name) This constructs a new thread group.
2	ThreadGroup(ThreadGroup parent, String name) This creates a new thread group.

ThreadGroup Class Methods

Sr.No.	Method & Description
1	int activeCount() This method returns an estimate of the number of active threads in this thread group.
2	int activeGroupCount() This method returns an estimate of the number of active groups in this thread group.
3	void checkAccess() This method determines if the currently running thread has permission to modify this thread group.
4	void destroy() This method Destroys this thread group and all of its subgroups.
5	int enumerate(Thread[] list) This method copies into the specified array every active thread in this thread group and its subgroups.
6	int enumerate(Thread[] list, boolean recurse) This method copies into the specified array every active thread in this thread group.
7	int enumerate(ThreadGroup[] list) This method copies into the specified array references to every active subgroup in this thread group.
8	int enumerate(ThreadGroup[] list, boolean recurse) This method copies into the specified array references to every active subgroup in this thread group.
9	int getMaxPriority() This method returns the maximum priority of this thread group.
10	String getName() This method returns the name of this thread group.
11	ThreadGroup getParent() This method returns the parent of this thread group.
12	void interrupt() This method interrupts all threads in this thread group.
13	boolean isDaemon() This method Tests if this thread group is a daemon thread group.
14	boolean isDestroyed() This method tests if this thread group has been destroyed.
15	void list() This method prints information about this thread group to the standard output.
16	boolean parentOf(ThreadGroup g) This method tests if this thread group is either the thread group argument or one of its ancestor thread groups.
17	void setDaemon(boolean daemon) This method changes the daemon status of this thread group.
18	void setMaxPriority(int pri) This method sets the maximum priority of the group.
19	String toString() This method returns a string representation of this Thread group.
20	void uncaughtException(Thread t, Throwable e) This method called by the Java Virtual Machine when a thread in this thread group stops because of an uncaught exception, and the thread does not have a specific Thread.UncaughtExceptionHandler installed.

Methods Inherited

This class inherits methods from the following classes −

• java.lang.Object

Example of ThreadGroup Class in Java

Following example showcases the usage of ThreadGroup in a multithreaded program.

package com.codeheap;

public class ThreadGroupDemo implements Runnable {
   public static void main(String[] args) {
      ThreadGroupDemo tg = new ThreadGroupDemo();
      tg.func();
   }

   public void func() {
      try {     
         // create a parent ThreadGroup
         ThreadGroup pGroup = new ThreadGroup("Parent ThreadGroup");
    
         // create a child ThreadGroup for parent ThreadGroup
         ThreadGroup cGroup = new ThreadGroup(pGroup, "Child ThreadGroup");

         // create a thread
         Thread t1 = new Thread(pGroup, this);
         System.out.println ("Starting " + t1.getName() + "...");
         t1.start();
            
         // create another thread
         Thread t2 = new Thread(cGroup, this);
         System.out.println ("Starting " + t2.getName() + "...");
         t2.start();
            
         // display the number of active threads 
         System.out.println    ("Active threads in \"" + pGroup.getName() 
            + "\" = " + pGroup.activeCount());

         // block until the other threads finish
         t1.join();
         t2.join();
      } catch (InterruptedException ex) {
         System.out.println(ex.toString());
      }
   }

   // implements run()
   public void run() {

      for(int i = 0;i < 1000;i++) {
         i++;
      }
      System.out.println (Thread.currentThread().getName() + " finished executing.");
   }
} 

Output

Let us compile and run the above program, this will produce the following result −

Starting Thread-0...
Starting Thread-1...
Thread-0 finished executing.
Active threads in "Parent ThreadGroup" = 1
Thread-1 finished executing.