Part 4: java.util.concurrent - Submit Runnable via ExecutorService

Future<?> submit( Runnable r )

Today in part 4 of the series we will talk about submission of runnable task via executor service. This method can be very beneficial if someone wants to upgrade legacy code from 1.4 to 1.5 onwards.
As per JAVA version 6.0, ExecutorService Interface has following method -

Future< ? >  submit( Runnable r ) 

  > Submits a Runnable task for execution and returns a Future representing that task.
  > This method will return NULL on successful completeion of the process, otherwise result will not be null.


Lets start with Runnable task - As we  know that Runnable interface has following method - public void run()
So when we say - ExecutorService.submit(Runnable Task) --> It starts a new stack starting with run method of runnable task, but as run method can not return any value so does future object. Due to this reason the signature of submit(Runnable r) is Future< ? >.
? --> means anything that extends Object, has no lower limit

/**
*Copyright (C) 2010  Jovial Java Admin [email: jovialjava.blogspot@gmail.com]
*
*This program is free software: you can redistribute it and/or modify
*it under the terms of the GNU General Public License as published by
*the Free Software Foundation, version 3 of the License.
*
*This program is distributed in the hope that it will be useful,
*but WITHOUT ANY WARRANTY; without even the implied warranty of
*MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
*GNU General Public License for more details.
*
*You should have received a copy of the GNU General Public License
*along with this program.  If not, see .
*@date Jan 21, 2011
*@author JovialJava
*/
package com.jovialjava.blog.threads;

import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;

public class RunnableExample {
 
 private static final ExecutorService executorPool=Executors.newFixedThreadPool(2);
 
 public static void main(String[] args) {
  RunnableTask_1 task_1 = new RunnableTask_1();
  RunnableTask_2 task_2 = new RunnableTask_2();
  
  /**
   * Submit the first task
   */
  Future< ? > fut_1 = executorPool.submit(task_1);
  Future< ? > fut_2 = executorPool.submit(task_2);
  
  try{
   if(fut_1.get() == null){
    System.out.println("TASK 1 completed SUCCESSFULLY");
     }
   if(fut_2.get() == null){
    System.out.println("TASK 2 completed SUCCESSFULLY");
     }
  }catch(ExecutionException e){   
   System.out.println(e.getMessage());
  }catch(InterruptedException e){
   System.out.println(e.getMessage());
  }finally{
   /**====VERY IMPORTANT===
    * This is required to stop the executor pool to
    * stop accepting new request.
    */
   executorPool.shutdown();
  }

 }
 /**
  * This task will complete successfully
  */
 public static class RunnableTask_1 implements Runnable{
  public void run()throws NullPointerException{
   System.out.println("Hi, Inside Runnable Task 1");   
  }  

 }
 /**
  * This task will result in error.
  */
 public static class RunnableTask_2 implements Runnable{
  public void run()throws NullPointerException{
   System.out.println("Hi, Inside Runnable Task 2");
   throw new IllegalStateException("Runnable Task Exception");
  }  

 }
}