ThreadGroup in Java

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. Java thread group is implemented by java.lang.ThreadGroup class. A ThreadGroup represents a set of threads. A thread group can also include the other thread group. The thread group creates 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 it cannot access the information about its thread group's parent thread group or any other thread groups. There are only two constructors of ThreadGroup class.

Table 6.3: Threadgroup

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.

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

Table 6.4: Methods for ThreadGroup class

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
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.

$\begin{lstlisting} ThreadGroup tg1 = new ThreadGroup(''Group A''); Thread t1 = ... ...o''); Thread t3 = new Thread(tg1,new MyRunnable(),''three''); \end{lstlisting}$

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.

$\begin{lstlisting} Thread.currentThread().getThreadGroup().interrupt(); \end{lstlisting}$

$\begin{lstlisting} public class ThreadGroupDemo implements Runnable{ public vo... ...read Group Name: ''+tg1.getName()); tg1.list(); \par } } \end{lstlisting}$

$\begin{lstlisting} one two three Thread Group Name: Parent ThreadGroup java.lang... ...d[two,5,Parent ThreadGroup] Thread[three,5,Parent ThreadGroup] \end{lstlisting}$