Java Callable Future Example

Filed Under: Java

Java Callable and Future are used a lot in multithreaded programming. In last few posts, we learned a lot about java threads but sometimes we wish that a thread could return some value that we can use. Java 5 introduced java.util.concurrent.Callable interface in concurrency package that is similar to Runnable interface but it can return any Object and able to throw Exception.

Java Callable

java Callable, java Future, java callable example, java executorservice callable
Java Callable interface use Generic to define the return type of Object. Executors class provide useful methods to execute Java Callable in a thread pool. Since callable tasks run in parallel, we have to wait for the returned Object.

Java Future

Java Callable tasks return java.util.concurrent.Future object. Using Java Future object, we can find out the status of the Callable task and get the returned Object. It provides get() method that can wait for the Callable to finish and then return the result.

Java Future provides cancel() method to cancel the associated Callable task. There is an overloaded version of get() method where we can specify the time to wait for the result, it’s useful to avoid current thread getting blocked for longer time. There are isDone() and isCancelled() methods to find out the current status of associated Callable task.

Here is a simple example of Java Callable task that returns the name of thread executing the task after one second. We are using Executor framework to execute 100 tasks in parallel and use Java Future to get the result of the submitted tasks.


package com.journaldev.threads;

import java.util.ArrayList;
import java.util.Date;
import java.util.List;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;

public class MyCallable implements Callable<String> {

    @Override
    public String call() throws Exception {
        Thread.sleep(1000);
        //return the thread name executing this callable task
        return Thread.currentThread().getName();
    }
    
    public static void main(String args[]){
        //Get ExecutorService from Executors utility class, thread pool size is 10
        ExecutorService executor = Executors.newFixedThreadPool(10);
        //create a list to hold the Future object associated with Callable
        List<Future<String>> list = new ArrayList<Future<String>>();
        //Create MyCallable instance
        Callable<String> callable = new MyCallable();
        for(int i=0; i< 100; i++){
            //submit Callable tasks to be executed by thread pool
            Future<String> future = executor.submit(callable);
            //add Future to the list, we can get return value using Future
            list.add(future);
        }
        for(Future<String> fut : list){
            try {
                //print the return value of Future, notice the output delay in console
                // because Future.get() waits for task to get completed
                System.out.println(new Date()+ "::"+fut.get());
            } catch (InterruptedException | ExecutionException e) {
                e.printStackTrace();
            }
        }
        //shut down the executor service now
        executor.shutdown();
    }

}

Once we execute the above program, you will notice the delay in output because java Future get() method waits for the java callable task to complete. Also notice that there are only 10 threads executing these tasks.

Here is snippet of the output of above program.


Mon Dec 31 20:40:15 PST 2012::pool-1-thread-1
Mon Dec 31 20:40:16 PST 2012::pool-1-thread-2
Mon Dec 31 20:40:16 PST 2012::pool-1-thread-3
Mon Dec 31 20:40:16 PST 2012::pool-1-thread-4
Mon Dec 31 20:40:16 PST 2012::pool-1-thread-5
Mon Dec 31 20:40:16 PST 2012::pool-1-thread-6
Mon Dec 31 20:40:16 PST 2012::pool-1-thread-7
Mon Dec 31 20:40:16 PST 2012::pool-1-thread-8
Mon Dec 31 20:40:16 PST 2012::pool-1-thread-9
Mon Dec 31 20:40:16 PST 2012::pool-1-thread-10
Mon Dec 31 20:40:16 PST 2012::pool-1-thread-2
...

Tip: What if we want to override some of the methods of Java Future interface, for example overriding get() method to timeout after some default time rather than waiting indefinitely, in this case Java FutureTask class comes handy that is the base implementation of Future interface. Check out Java FutureTask Example to learn more about this class.

Comments

  1. Nisha says:

    hi,

    I have a query does Future creates new thread.
    Thanks!

  2. BGRRRRR says:

    Thanks a lot,it really helped me 🙂

  3. Alex says:

    Place
    Callable callable = new MyCallable();
    into “fori”

    It will be right

  4. urvi says:

    hi,
    thanks for nice explanation of threadpool.

    I have some question. please clear my understanding.
    1.Here the thread pool size is 10. So it creates 10 threads right? that means whatever the size we give to executor creates that many threads and all threads execute same method call() .
    2.Here in output why we have output for thred-2 again as per method once it prints method finish then why its executing thread again ?

  5. Aakshi says:

    Thanks for the example.

  6. Shane says:

    Thanks for this tutorial! I did have to make one modification.

    I changed (A) to (B) where:

    (A) Future future = executor.submit(callable);

    (B) Future future = executor.submit(new MyCallable());

  7. Utpal says:

    Thanks for sharing simple and understandable example.

    I have a question, it may be very silly but i want to clarify it with you.
    In above example we will always get output as : pool-1-thread-(Number), where pool-1 is common, here my question is
    since we have created pool of size 5. Output should also change accordingly like pool-1, pool-2 etc.

    1. Pankaj says:

      We are creating 1 pool with 10 threads, that’s why the output.

  8. Veranga Sooriyabandara says:

    Hi Pankaj,

    This is very useful. Thanks fro sharing your knowledge with us.

  9. shgy says:

    Thanks ! , useful…!

  10. Chaitanya says:

    Super note.

    Thanks for sharing the stuff

  11. aditi says:

    Hi…it is really easy explanation .thanks for that…do you have any good material in angularjs?

  12. Russ Ray says:

    First of all, THANK YOU! A well written article and you explained the concepts so clearly. The code examples for marvelous! When is your book coming out? 😉

    Russ

    1. Pankaj says:

      Thanks for the appreciation Russ, there are few eBooks I have written that you can download after subscribing to newsletter.

  13. laudukang says:

    thanks a lot.

  14. ramesh says:

    Hi Pankaj,

    In my project we have one batch running more time.
    The scenario is one record getting from one database and inserting into another database.
    It is happening record by record. I want to execute these parallarly.

    In this case can I implement collable?

    1. Vishal says:

      Yes You can do the same by adding an extra flag. Set the flag value when any of your thread pick the row to insert.

  15. Binh Thanh Nguyen says:

    Thanks, nice post

  16. Hemanth says:

    Nice examples you have given pankaj.

    Great Job 🙂

    Thanks.

  17. Praveen says:

    You are the best and my favorite in Threading concepts!!

  18. Sayantan says:

    Hi Pankaj. One doubt regarding Callable interface. The compiler creates a synthetic method to invoke the call() method. Why is it required?

    1. Pankaj says:

      I didn’t understood what you mean here?

  19. HIMANSU NAYAK says:

    Hi Pankaj,
    We normally pass a “User/Worker Thread instance” to the ThreadPoolExecutor/Scheduler but here the Callable Interface is not extending Runnable interface neither we are overriding any run() method. We are creating 100 callable instance but not thread, does it mean we cannot use callable for multi-threading purpose

    1. Pankaj says:

      We are creating only 1 instance of Callable implementation. When we submit it to the Executor, it creates the thread for us and execute it. This is multithreading.

    2. X says:

      e.execute(r);

      is the same that…

      (new Thread(new Runnable() {…})).start();

  20. Agha Khan says:

    All the tutorial are very Good and helpful. I want to run more than 1000 threads (Tasks) but not at a time may be 20 at a time using one of thread pools
    But I also want to cancel the task if it is blocked for more than 30 minutes. But that 30 minutes are not including submit times of the task. No more than 30 minutes should be used for execution.
    Is there any function for any class using that can put the maximum execution time? If yes will that function interrupt the thread that is executing the task? If yes then do we need to handle “ InterruptedException” for that thread?

    Please give your best suggestion and thanks for this good articles

  21. Silviu Burcea says:

    Hi,

    I believe that your implementation is not the best you can do in this scenario. Your callable waits for exactly 1 second, but if there was a little bit of random, you will probably notice that the get() method, a blocking one, will slow down the execution in the for loop(because some of the results are not ready yet). Rewrite it using a while(!list.isEmpty()) block, every time you have a Future ready(use isDone()) you can safely get the result without delaying the execution and remove it from the list.

    HTH

    1. Pankaj says:

      Hi Silviu,

      This is just an example to show how can we use it, in real life scenarios most of the times we will start a callable and then we get the response later on. Obviously we can modify for better performance but if I will add too much implementation, we will loose focus on learning the Callable-Future usage.

      Thanks,
      Pankaj

    2. Rvindranath says:

      If we use while(!list.isEmpty()) { future.iosDone() … }, don’t we consume waste CPU cycles checking for isDone continuously ?

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