THREAD GROUP

Java provides a convenient way to group multiple threads in a single object. In such way, we can suspend, resume or interrupt group of threads by a single method call.

Note: Now suspend(), resume() and stop() methods are deprecated.

Java thread group is implemented by java.lang.ThreadGroup class.

Constructors of ThreadGroup class

There are only two constructors of ThreadGroup class.

No.	Constructor	Description
1)	ThreadGroup(String name)	creates a thread group with given name.
2)	ThreadGroup(ThreadGroup parent, String name)	creates a thread group with given parent group and name.

Important methods of ThreadGroup class

There are many methods in ThreadGroup class. A list of important methods are given below.

No.	Method	Description
1)	int activeCount()	returns no. of threads running in current group.
2)	int activeGroupCount()	returns a no. of active group in this thread group.
3)	void destroy()	destroys this thread group and all its sub groups.
4)	String getName()	returns the name of this group.
5)	ThreadGroup getParent()	returns the parent of this group.
6)	void interrupt()	interrupts all threads of this group.
7)	void list()	prints information of this group to standard console.

Let's see a code to group multiple threads.

1. ThreadGroup tg1 = new ThreadGroup("Group A");   
2. Thread t1 = new Thread(tg1,new MyRunnable(),"one");     
3. Thread t2 = new Thread(tg1,new MyRunnable(),"two");     
4. Thread t3 = new Thread(tg1,new MyRunnable(),"three");    

Now all 3 threads belong to one group. Here, tg1 is the thread group name, MyRunnable is the class that implements Runnable interface and "one", "two" and "three" are the thread names.

Now we can interrupt all threads by a single line of code only.

1. Thread.currentThread().getThreadGroup().interrupt();  

ThreadGroup Example

File: ThreadGroupDemo.java

1. public class ThreadGroupDemo implements Runnable{  
2.     public void run() {  
3.           System.out.println(Thread.currentThread().getName());  
4.     }  
5.    public static void main(String[] args) {  
6.       ThreadGroupDemo runnable = new ThreadGroupDemo();  
7.           ThreadGroup tg1 = new ThreadGroup("Parent ThreadGroup");  
8.             
9.           Thread t1 = new Thread(tg1, runnable,"one");  
10.           t1.start();  
11.           Thread t2 = new Thread(tg1, runnable,"two");  
12.           t2.start();  
13.           Thread t3 = new Thread(tg1, runnable,"three");  
14.           t3.start();  
15.                
16.           System.out.println("Thread Group Name: "+tg1.getName());  
17.          tg1.list();  
18.   
19.     }  
20.    }  

Output:

one
two
three
Thread Group Name: Parent ThreadGroup
java.lang.ThreadGroup[name=Parent ThreadGroup,maxpri=10]
    Thread[one,5,Parent ThreadGroup]
    Thread[two,5,Parent ThreadGroup]
    Thread[three,5,Parent ThreadGroup]