ThreadGroup in Java

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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.

ThreadGroup creates a group of threads. It offers a convenient way to manage groups of threads as a unit. This is particularly valuable in situation in which you want to suspend and resume a number of related threads.

  • The thread group form a tree in which every thread group except the initial thread group has a parent.

  • A thread is allowed to access information about its own thread group but not to access information about its thread group’s parent thread group or any other thread group.

Constructors:

public ThreadGroup(String name): Constructs a new thread group. The parent of this new group is the thread group of the currently running thread.


Throws: SecurityException - if the current thread cannot
create a thread in the specified thread group.

public ThreadGroup(ThreadGroup parent, String name): Creates a new thread group. The parent of this new group is the specified thread group.


Throws: 
NullPointerException - if the thread group argument is null.
SecurityException - if the current thread cannot create a thread in the 
specified thread group.
Constructors of ThreadGroup class

There are only two constructors of ThreadGroup class.

No.ConstructorDescription
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.
Methods of ThreadGroup class

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

S.N. Modifier and Type Method Description
1) void checkAccess() This method determines if the currently running thread has permission to modify the thread group.
2) int activeCount() This method returns an estimate of the number of active threads in the thread group and its subgroups.
3) int activeGroupCount() This method returns an estimate of the number of active groups in the thread group and its subgroups.
4) void destroy() This method destroys the thread group and all of its subgroups.
5) int enumerate(Thread[] list) This method copies into the specified array every active thread in the thread group and its subgroups.
6) int getMaxPriority() This method returns the maximum priority of the thread group.
7) String getName() This method returns the name of the thread group.
8) ThreadGroup getParent() This method returns the parent of the thread group.
9) void interrupt() This method interrupts all threads in the thread group.
10) boolean isDaemon() This method tests if the thread group is a daemon thread group.
11) void setDaemon(boolean daemon) This method changes the daemon status of the thread group.
12) boolean isDestroyed() This method tests if this thread group has been destroyed.
13) void list() This method prints information about the thread group to the standard output.
14) boolean parentOf(ThreadGroup g This method tests if the thread group is either the thread group argument or one of its ancestor thread groups.
15) void suspend() This method is used to suspend all threads in the thread group.
16) void resume() This method is used to resume all threads in the thread group which was suspended using suspend() method.
17) void setMaxPriority(int pri) This method sets the maximum priority of the group.
18) void stop() This method is used to stop all threads in the thread group.
19) String toString() This method returns a string representation of the Thread group.

Let's see a code to group multiple threads.


ThreadGroup tg1 = new ThreadGroup("Group A");   
Thread t1 = new Thread(tg1,new MyRunnable(),"one");     
Thread t2 = new Thread(tg1,new MyRunnable(),"two");     
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.


Thread.currentThread().getThreadGroup().interrupt();
ThreadGroup Example

public class ThreadGroupDemo implements Runnable{  
    public void run() {  
          System.out.println(Thread.currentThread().getName());  
    }  
   public static void main(String[] args) {  
      ThreadGroupDemo runnable = new ThreadGroupDemo();  
          ThreadGroup tg1 = new ThreadGroup("Parent ThreadGroup");  
            
          Thread t1 = new Thread(tg1, runnable,"one");  
          t1.start();  
          Thread t2 = new Thread(tg1, runnable,"two");  
          t2.start();  
          Thread t3 = new Thread(tg1, runnable,"three");  
          t3.start();  
               
          System.out.println("Thread Group Name: "+tg1.getName());  
         tg1.list();  
  
    }  
   }


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]



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