Java Callable Future Example

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

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