Java - Thread Pools

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Thread Pools

A thread pool is a collection of pre-initialized threads. The general plan behind a thread pool is to form variety of threads at method startup and place them into a pool, wherever they sit and expect work. once a server receives a call for participation, it awakens a thread from this pool−if one is available−and passes it the request for service. Once the thread completes its service, it returns to the pool and awaits a lot of work. If the pool contains no accessible thread, the server waits till one becomes free.

Why Use Thread Pools in Java?

• It saves time as a result of there's no need to produce new thread.

• It is utilized in Servlet and JSP wherever instrumentality creates a thread pool to method the request.

Creating Thread Pools in Java

Java provides a java.util.concurrent.Executors class provides couple of methods to create a thread pools.

Executors Class Methods

Following are few important and useful methods this class to create Thread Pools -

Creating a Thread Pool Using newFixedThreadPool() Method

A fixed thread pool obtainted by calling the static newFixedThreadPool() method of Executors class.

Syntax

ExecutorService fixedPool = Executors.newFixedThreadPool(2);

Where,

• Maximum 2 threads will be active to process tasks.

• If more than 2 threads are submitted then they are held in a queue until threads become available.

• A new thread is created to take its place if a thread terminates due to failure during execution shutdown on executor is not yet called.

• Any thread exists till the pool is shutdown.

Example: Creating a Thread Pool Using newFixedThreadPool() Method

The following TestThread program shows usage of Executors newFixedThreadPool() method to create a thread pool of two threads. We're using a ThreadPoolExecutor object and initialized with newFixedThreadPool(2), a fix thread pool of size 2. Then we're printing various attributes of the threadpool. Then we're adding few threads to the executor and then same attributes of threadpool are printed to reflect the changes.

package com.tutorialspoint;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

public class TestThread {
	
   public static void main(final String[] arguments) throws InterruptedException {
      ExecutorService executor = Executors.newFixedThreadPool(2);

      // Cast the object to its class type
      ThreadPoolExecutor pool = (ThreadPoolExecutor) executor;

      //Stats before tasks execution
      System.out.println("Largest executions: "
         + pool.getLargestPoolSize());
      System.out.println("Maximum allowed threads: "
         + pool.getMaximumPoolSize());
      System.out.println("Current threads in pool: "
         + pool.getPoolSize());
      System.out.println("Currently executing threads: "
         + pool.getActiveCount());
      System.out.println("Total number of threads(ever scheduled): "
         + pool.getTaskCount());

      executor.submit(new Task());
      executor.submit(new Task());
	  executor.submit(new Task());
      executor.submit(new Task());

      //Stats after tasks execution
      System.out.println("Core threads: " + pool.getCorePoolSize());
      System.out.println("Largest executions: "
         + pool.getLargestPoolSize());
      System.out.println("Maximum allowed threads: "
         + pool.getMaximumPoolSize());
      System.out.println("Current threads in pool: "
         + pool.getPoolSize());
      System.out.println("Currently executing threads: "
         + pool.getActiveCount());
      System.out.println("Total number of threads(ever scheduled): "
         + pool.getTaskCount());

      executor.shutdown();
   }  

   static class Task implements Runnable {

      public void run() {
         
         try {
            Long duration = (long) (Math.random() * 5);
            System.out.println("Running Task! Thread Name: " +
               Thread.currentThread().getName());
               TimeUnit.SECONDS.sleep(duration);
            
            System.out.println("Task Completed! Thread Name: " +
               Thread.currentThread().getName());
         } catch (InterruptedException e) {
            e.printStackTrace();
         }
      }
   }
}

Output

Largest executions: 0
Maximum allowed threads: 2
Current threads in pool: 0
Currently executing threads: 0
Total number of threads(ever scheduled): 0
Core threads: 2
Largest executions: 2
Maximum allowed threads: 2
Current threads in pool: 2
Currently executing threads: 2
Total number of threads(ever scheduled): 4
Running Task! Thread Name: pool-1-thread-2
Running Task! Thread Name: pool-1-thread-1
Task Completed! Thread Name: pool-1-thread-2
Running Task! Thread Name: pool-1-thread-2
Task Completed! Thread Name: pool-1-thread-1
Running Task! Thread Name: pool-1-thread-1
Task Completed! Thread Name: pool-1-thread-2
Task Completed! Thread Name: pool-1-thread-1

Here although, we've submitted four threads but only two threads are executed as ThreadPool is fixed to accept only two threads.

Creating a Thread Pool Using newCachedThreadPool() Method

A cached thread pool obtainted by calling the static newCachedThreadPool() method of Executors class.

Syntax

ExecutorService executor = Executors.newCachedThreadPool();

Where,

• newCachedThreadPool method creates an executor having an expandable thread pool.

• Such an executor is suitable for applications that launch many short-lived tasks.

Example: Creating a Thread Pool Using newCachedThreadPool() Method

The following TestThread program shows usage of Executors newCachedThreadPool() method to create a expandable thread pool of threads. We're using a ThreadPoolExecutor object and initialized with newCachedThreadPool(). Then we're printing various attributes of the threadpool. Then we're adding few threads to the executor and then same attributes of threadpool are printed to reflect the changes.

package com.tutorialspoint;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

public class TestThread {
	
   public static void main(final String[] arguments) throws InterruptedException {
      ExecutorService executor = Executors.newCachedThreadPool();

      // Cast the object to its class type
      ThreadPoolExecutor pool = (ThreadPoolExecutor) executor;

      //Stats before tasks execution
      System.out.println("Largest executions: "
         + pool.getLargestPoolSize());
      System.out.println("Maximum allowed threads: "
         + pool.getMaximumPoolSize());
      System.out.println("Current threads in pool: "
         + pool.getPoolSize());
      System.out.println("Currently executing threads: "
         + pool.getActiveCount());
      System.out.println("Total number of threads(ever scheduled): "
         + pool.getTaskCount());

      executor.submit(new Task());
      executor.submit(new Task());
	  executor.submit(new Task());
      executor.submit(new Task());

      //Stats after tasks execution
      System.out.println("Core threads: " + pool.getCorePoolSize());
      System.out.println("Largest executions: "
         + pool.getLargestPoolSize());
      System.out.println("Maximum allowed threads: "
         + pool.getMaximumPoolSize());
      System.out.println("Current threads in pool: "
         + pool.getPoolSize());
      System.out.println("Currently executing threads: "
         + pool.getActiveCount());
      System.out.println("Total number of threads(ever scheduled): "
         + pool.getTaskCount());

      executor.shutdown();
   }  

   static class Task implements Runnable {

      public void run() {
         
         try {
            Long duration = (long) (Math.random() * 5);
            System.out.println("Running Task! Thread Name: " +
               Thread.currentThread().getName());
               TimeUnit.SECONDS.sleep(duration);
            
            System.out.println("Task Completed! Thread Name: " +
               Thread.currentThread().getName());
         } catch (InterruptedException e) {
            e.printStackTrace();
         }
      }
   }
}

Output

Largest executions: 0
Maximum allowed threads: 2147483647
Current threads in pool: 0
Currently executing threads: 0
Total number of threads(ever scheduled): 0
Core threads: 0
Largest executions: 4
Maximum allowed threads: 2147483647
Current threads in pool: 4
Currently executing threads: 4
Total number of threads(ever scheduled): 4
Running Task! Thread Name: pool-1-thread-2
Running Task! Thread Name: pool-1-thread-4
Running Task! Thread Name: pool-1-thread-3
Running Task! Thread Name: pool-1-thread-1
Task Completed! Thread Name: pool-1-thread-3
Task Completed! Thread Name: pool-1-thread-4
Task Completed! Thread Name: pool-1-thread-2
Task Completed! Thread Name: pool-1-thread-1

Creating a Thread Pool Using newScheduledThreadPool() Method

A scheduled thread pool obtainted by calling the static newScheduledThreadPool() method of Executors class.

Syntax

ExecutorService executor = Executors.newScheduledThreadPool(1);

Example: Creating a Thread Pool Using newScheduledThreadPool() Method

The following TestThread program shows usage of Executors newScheduledThreadPool() method to create a thread pool of a thread. We're using a ScheduledExecutorService object as scheduler and initialized with newScheduledThreadPool(). We've created a ScheduledFuture object to schedule a task to execute every two seconds after an initial delay of two seconds. Using scheduler, we scheduled the task to run for ten seconds.

package com.tutorialspoint;

import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.ScheduledFuture;
import java.util.concurrent.TimeUnit;

public class TestThread {

   public static void main(final String[] arguments) throws InterruptedException {
      final ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(1);

      final ScheduledFuture<?> beepHandler = 
         scheduler.scheduleAtFixedRate(new BeepTask(), 2, 2, TimeUnit.SECONDS);

      scheduler.schedule(new Runnable() {

         @Override
         public void run() {
            beepHandler.cancel(true);
            scheduler.shutdown();			
         }
      }, 10, TimeUnit.SECONDS);
   }  

   static class BeepTask implements Runnable {

      public void run() {
         System.out.println("beep");      
      }
   }
}

Output

beep
beep
beep
beep
beep