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: WorkerThrad.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