Java ExecutorService examples

In Java, we can use ExecutorService to create a thread pool, and tracks the progress of the asynchronous tasks with Future.

The ExecutorService accept both Runnable and Callable tasks.

1. Runnable – Return void, nothing.
2. Callable – Return a Future.

1. ExecutorService

1.1 A classic ExecutorService example to create a thread pool with 5 threads, submit two tasks, get the result from Future and also how to handle the exception.

package com.mkyong.concurrency.executor;
import java.util.concurrent.*;
public class ExecutorExample1 {
    public static void main(String[] args) {
        ExecutorService executor = Executors.newFixedThreadPool(5);
        // Runnable, return void, nothing, submit and run the task async
        executor.submit(() -> System.out.println("I'm Runnable task."));
        // Callable, return a future, submit and run the task async
        Future<Integer> futureTask1 = executor.submit(() -> {
            System.out.println("I'm Callable task.");
            return 1 + 1;
        });
        /* Before Java 8
        executor.submit(new Runnable() {
            @Override
            public void run() {
                System.out.println("I'm Runnable task.");
        });
        Future<Integer> futureTask1 = executor.submit(new Callable<Integer>() {
            @Override
            public Integer call() {
                System.out.println("I'm Callable task.");
                return 1 + 1;
        });*/
        try {
            otherTask("Before Future Result");
            // block until future returned a result, 
			// timeout if the future takes more than 5 seconds to return the result
            Integer result = futureTask1.get(5, TimeUnit.SECONDS);
            System.out.println("Get future result : " + result);
            otherTask("After Future Result");
        } catch (InterruptedException e) {// thread was interrupted
            e.printStackTrace();
        } catch (ExecutionException e) {// thread threw an exception
            e.printStackTrace();
        } catch (TimeoutException e) {// timeout before the future task is complete
            e.printStackTrace();
        } finally {
            // shut down the executor manually
            executor.shutdown();
    private static void otherTask(String name) {
        System.out.println("I'm other task! " + name);

Output

I'm Runnable task.
I'm Callable task.
I'm other task! Before Future Result
Get future result : 2
I'm other task! After Future Result

2. invokeAll

2.1 We can also create a list of Callable tasks, and run them all with invokeAll

package com.mkyong.concurrency.executor;
import java.util.Arrays;
import java.util.List;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
public class ExecutorExample2 {
    public static void main(String[] args) {
        ExecutorService executor = Executors.newCachedThreadPool();
        List<Callable<Integer>> listOfCallable = Arrays.asList(
                () -> 1,
                () -> 2,
                () -> 3);
        try {
            List<Future<Integer>> futures = executor.invokeAll(listOfCallable);
            int sum = futures.stream().map(f -> {
                try {
                    return f.get();
                } catch (Exception e) {
                    throw new IllegalStateException(e);
            }).mapToInt(Integer::intValue).sum();
            System.out.println(sum);
        } catch (InterruptedException e) {// thread was interrupted
            e.printStackTrace();
        } finally {
            // shut down the executor manually
            executor.shutdown();

Output

//… To be update

References

1. ExecutorService JavaDoc
2. Wikipedia – Thread Pool