Thread Pool and Thread Group

Java Thread Pool

Java Thread pool represents a group of worker threads that are waiting for the job and reuse many times.

In case of thread pool, a group of fixed size threads are created. A thread from the thread pool is pulled out and assigned a job by the service provider. After completion of the job, thread is contained in the thread pool again.

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Advantage of Java Thread Pool

Better performance It saves time because there is no need to create new thread.

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Real time usage

It is used in Servlet and JSP where container creates a thread pool to process the request.

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Example of Java Thread Pool

Let's see a simple example of java thread pool using ExecutorService and Executors.

File: WorkerThread.java

1. import java.util.concurrent.ExecutorService;  
2. import java.util.concurrent.Executors;  
3. class WorkerThread implements Runnable {  
4.     private String message;  
5.     public WorkerThread(String s){  
6.         this.message=s;  
7.     }  
8.      public void run() {  
9.         System.out.println(Thread.currentThread().getName()+" (Start) message = "+message);  
10.         processmessage();//call processmessage method that sleeps the thread for 2 seconds  
11.         System.out.println(Thread.currentThread().getName()+" (End)");//prints thread name  
12.     }  
13.     private void processmessage() {  
14.         try {  Thread.sleep(2000);  } catch (InterruptedException e) { e.printStackTrace(); }  
15.     }  
16. }  

File: JavaThreadPoolExample.java

1. public class TestThreadPool {  
2.      public static void main(String[] args) {  
3.         ExecutorService executor = Executors.newFixedThreadPool(5);//creating a pool of 5 threads  
4.         for (int i = 0; i < 10; i++) {  
5.             Runnable worker = new WorkerThread("" + i);  
6.             executor.execute(worker);//calling execute method of ExecutorService  
7.           }  
8.         executor.shutdown();  
9.         while (!executor.isTerminated()) {   }  
10.   
11.         System.out.println("Finished all threads");  
12.     }  
13.  }  

Output:

pool-1-thread-1 (Start) message = 0
pool-1-thread-2 (Start) message = 1
pool-1-thread-3 (Start) message = 2
pool-1-thread-5 (Start) message = 4
pool-1-thread-4 (Start) message = 3
pool-1-thread-2 (End)
pool-1-thread-2 (Start) message = 5
pool-1-thread-1 (End)
pool-1-thread-1 (Start) message = 6
pool-1-thread-3 (End)
pool-1-thread-3 (Start) message = 7
pool-1-thread-4 (End)
pool-1-thread-4 (Start) message = 8
pool-1-thread-5 (End)
pool-1-thread-5 (Start) message = 9
pool-1-thread-2 (End)
pool-1-thread-1 (End)
pool-1-thread-4 (End)
pool-1-thread-3 (End)
pool-1-thread-5 (End)
Finished all threads

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.

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]