Java ExecutorService Example – Tutorial

Katerina ZamaniJanuary 28th, 2014Last Updated: January 15th, 2021

2 625 5 minutes read

Java ExecutorService is an interface that extends Executor class and represents an asynchronous execution. Executor service provides us mechanisms to manage the end and detect progress of the asynchronous tasks.

In this example, we are going to see some basic functionalities of ExecutorService, as well as handle the Future object, the result of asynchronous computation.

1. Runnable vs Callable

The Callable interface is similar to Runnable, in that both are designed for classes whose instances are potentially executed by another thread. However Runnable interface cannot do everything that Callable does:

Callable instance returns a result, whereas a Runnable instance does not.
Callable instance may throw checked exceptions, whereas as Runnable instance can not.

2. Create the Runnable

We are going to create a Runnable that is intended to be executed by the ExecutorService. Create a java class named myThread and paste the following code.

myThread.java

package com.javacodegeeks.core.concurrency.executorservicetest;
 
public class MyThread implements Runnable {
     
    private String myName;
    private int count;
    private final long timeSleep;
 
    MyThread(String name, int newcount, long newtimeSleep) {
        this.myName = name;
        this.count = newcount;
        this.timeSleep = newtimeSleep;
    }
     
    @Override
    public void run() {
        // TODO Auto-generated method stub
 
        int sum = 0;
        for (int i = 1; i <= this.count; i++) {
            sum = sum + i;
        }
        System.out.println(myName + " thread has sum = " + sum + 
                " and is going to sleep for " + timeSleep);
        try {
            Thread.sleep(this.timeSleep);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
 
}

The functionality of the Runnable is very simple. It computes a sum from the giving argument and it sleeps for a specified time.

3. Code the ExecutorService in Java

In this example we will use a factor method of ExecutorService that creates a thread pool of fixed number of threads. For this reason, newFixedThreadPool() method is used where we specify the number of threads in the pool. To execute the thread, we can use either execute() method or submit(), where both of them take Runnable as a parameter. execute() method is depending on the implementation of the Executor class and may perform the Runnable in a new thread, in a pooled thread, or in the calling thread. submit() method extends execute(), by returning a Future that represents the submitting task.

The Future can be used to indicate the termination of execution of the thread. For instance, get() method waits for the completion of the computation. If the returning value is null, the task has finished correctly. Otherwise, cancel() method can be called in order to end the execution of this task. It is worth to mention that for bulk or a collection of thread execution, invokeAll() and invokeAny() are used respectively, although there are not used in this example.

To close down the ExecutorService, there are many methods that can be used. In our example we use shutdown() method, in which the submitted tasks are executed before the shutting down but new tasks can not be accepted. Another approach is shutdownNow() method, which stops the executing tasks, pause the waiting ones and returns the list of the awaiting ones. Moreover, awaitTermination() can be used in order to wait until all threads are terminated.

For further understanding of the main functionality of ExecutorService, have a look at the code below. Create ExecutorServiceTest.java file and paste the following.

ExecutorServiceTest.java

package com.javacodegeeks.core.concurrency.executorservicetest;
 
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.TimeUnit;
 
public class ExecutorServiceTest {
 
    private static Future taskTwo = null;
    private static Future taskThree = null;
     
    public static void main(String[] args) throws InterruptedException, ExecutionException {
        ExecutorService executor = Executors.newFixedThreadPool(2);
        
        // execute the Runnable
        Runnable taskOne = new MyThread("TaskOne", 2, 100);
        executor.execute(taskOne);
        for(int i = 0; i < 2; i++) {
            // if this task is not created or is canceled or is completed
            if ((taskTwo == null) || (taskTwo.isDone()) || (taskTwo.isCancelled())) {
                // submit a task and return a Future
                taskTwo = executor.submit(new MyThread("TaskTwo", 4, 200));
            }
     
            if ((taskThree == null) || (taskThree.isDone()) || (taskThree.isCancelled())) {
                taskThree = executor.submit(new MyThread("TaskThree", 5, 100));
            }
            // if null the task has finished
            if(taskTwo.get() == null) {
                System.out.println(i+1 + ") TaskTwo terminated successfully");
            } else {
                // if it doesn't finished, cancel it
                taskTwo.cancel(true);
            }
            if(taskThree.get() == null) {
                System.out.println(i+1 + ") TaskThree terminated successfully");
            } else {
                taskThree.cancel(true);
            }
        }
        executor.shutdown();
        System.out.println("-----------------------");
        // wait until all tasks are finished
        executor.awaitTermination(1, TimeUnit.SECONDS);
        System.out.println("All tasks are finished!");
     
    }
 
}

Now you can see the output of the execution.

Output

TaskOne thread has sum = 3 and is going to sleep for 100
TaskTwo thread has sum = 10 and is going to sleep for 200
TaskThree thread has sum = 15 and is going to sleep for 100
1) TaskTwo terminated successfully
1) TaskThree terminated successfully
TaskTwo thread has sum = 10 and is going to sleep for 200
TaskThree thread has sum = 15 and is going to sleep for 100
2) TaskTwo terminated successfully
2) TaskThree terminated successfully
-----------------------
All tasks are finished!

4. The ScheduledExecutorService Interface

ScheduledExecutorService is java.util.concurrent.ExecutorService thus it inherits all the functional abstractions from ExecutorService interface (and its super interfaces!); nevertheless, the difference being, ScheduledExecutorService can “schedule commands to run after a given delay, or to execute periodically” (Source).

The public API to use ScheduledExecutorService is pretty straightforward. We can “schedule” a repetitive or delayed task encapsulated in a java.lang.Runnable or java.util.concurrent.Callable instance with ScheduledExecutorService configuring the delays. It is obvious that a Runnable task would be non result bearing one and Callable would produce some result.

An instance of ScheduledThreadPoolExecutor can be retrieved from the convenient factory API, java.util.concurrent.Executors.newScheduledThreadPool(int corePoolSize) or its overloaded version Executors.newScheduledThreadPool(int corePoolSize, ThreadFactory threadFactory). In the subsequent sections, we will demonstrate these APIs through the way of an example.

5. ExecutorService vs Fork/Join Pool

Each of these APIs are targeted to fulfil respective business needs of your application.

ExecutorService

ExecutorService is an interface that extends Executor class and represents an asynchronous execution. It provides us mechanisms to manage the end and detect progress of the asynchronous tasks.

Executor service manage thread in asynchronous way
Use callable to get the return result after thread completion
Manage allocation of work to free thread and resale completed work from thread for assigning new work automatically
Better communication between threads
invokeAll and invokeAny give more control to run any or all thread at once
shutdown provide capability for completion of all thread assigned work
Scheduled Executor Services provide methods for producing repeating invocations of runnables and callables Hope it will help you

ForkJoinPool

Fork-Join framework is an extension to Executor service framework to particularly address ‘waiting’ issues in recursive multi-threaded programs. In fact, the new Fork-Join framework classes all extend from the existing classes of the Executor framework. ForkJoinPool is for many, dependent, task-generated, short, hardly ever blocking (i.e. compute-intensive) tasks

parallelism – the parallelism level. For default value, use Runtime.availableProcessors()
factory – the factory for creating new threads. For default value, use defaultForkJoinWorkerThreadFactory
handler – the handler for internal worker threads that terminate due to unrecoverable errors
asyncMode – if true, establishes local first-in-first-out scheduling mode for forked tasks that are never joined.