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Threadgroup in java

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ThreadGroup in Java

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

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.

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 a given parent group and name.

Methods of ThreadGroup class

There are many methods in ThreadGroup class. A list of ThreadGroup methods is 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.

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.

ThreadGroup Example

File: ThreadGroupDemo.java

Output:

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

Thread Pool Methods Example: int activeCount()

Let’s see how one can use the method activeCount().

FileName: ActiveCountExample.java

Output:

Starting the first  Starting the second  The total number of active threads are: 2  

Thread Pool Methods Example: int activeGroupCount()

Now, we will learn how one can use the activeGroupCount() method in the code.

FileName: ActiveGroupCountExample.java

Output:

Starting the first  Starting the second  The total number of active thread groups are: 1  the second thread has finished executing  the first thread has finished executing  

Thread Pool Methods Example: void destroy()

Now, we will learn how one can use the destroy() method in the code.

FileName: DestroyExample.java

Output:

Starting the first  Starting the second  the first thread has finished executing  the second thread has finished executing  the child group is destroyed.  the parent group is destroyed.  

Thread Pool Methods Example: int enumerate()

Now, we will learn how one can use the enumerate() method in the code.

FileName: EnumerateExample.java

Output:

Starting the first  Starting the second  Thread the first is found.  Thread the second is found.  the first thread has finished executing  the second thread has finished executing  

Thread Pool Methods Example: int getMaxPriority()

The following code shows the working of the getMaxPriority() method.

FileName: GetMaxPriorityExample.java

Output:

Starting the first  Starting the second  The maximum priority of the parent ThreadGroup: 10  the first thread has finished executing  the second thread has finished executing  

Thread Pool Methods Example: ThreadGroup getParent()

Now, we will learn how one can use the getParent() method in the code.

FileName: GetParentExample.java

Output:

Starting the first  Starting the second  The ParentThreadGroup for the parent group is main  The ParentThreadGroup for the child group is the parent group  the first thread has finished executing  the second thread has finished executing  

Thread Pool Methods Example: void interrupt()

The following program illustrates how one can use the interrupt() method.

FileName: InterruptExample.java

Output:

Starting the first  Starting the second  The exception has been encountered java.lang.InterruptedException: sleep interrupted  The exception has been encountered java.lang.InterruptedException: sleep interrupted  the second thread has finished executing  the first thread has finished executing  

Thread Pool Methods Example: boolean isDaemon()

The following program illustrates how one can use the isDaemon() method.

FileName: IsDaemonExample.java

Output:

Starting the first  Starting the second  The group is not a daemon group.  the second thread has finished executing  the first thread has finished executing  

Thread Pool Methods Example: boolean isDestroyed()

The following program illustrates how one can use the isDestroyed() method.

FileName: IsDestroyedExample.java

Output:

Starting the first  Starting the second  The group has not been destroyed.  the first thread has finished executing  the second thread has finished executing  

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