How to use Java Single Thread Executor? In this post we’ll show how the ExecutorService nicely executes jobs in order.

As usually, java.util.concurrent.Excutors has two methods to create Single Threaded Executor:

• ExecutorService newSingleThreadExecutor()
  Creates an executor with single worker thread.
• ExecutorService newSingleThreadExecutor(ThreadFactory threadFactory) Creates an executor with single worker thread and allows to specify own ThreadFactory.

In the following example we’ll use the Executor to run a number of print jobs. As we’ve got only one printer and the driver is not thread-safe, we’re going to run jobs sequentially thanks to the executor:

package com.farenda.java.util.concurrent;

import java.util.Random;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

import static java.util.concurrent.TimeUnit.MILLISECONDS;

public class SingleThreadExecutorExample {

    private static class PrinterJob implements Runnable {

        private static int nth = 0;

        private final int id = ++nth;
        private final int speed;

        public PrinterJob() {
            // draw 0-99 and give +1 to progress at least by 1%:
            this.speed = new Random().nextInt(99) + 1;
        }

        @Override
        public void run() {
            System.out.println("Starting printer work: " + id);
            for (int i = 0; i <= 100; i += speed) {
                try {
                    MILLISECONDS.sleep(300);
                } catch (InterruptedException e) {
                    // ignore
                }
                System.out.printf("worker %d, done %d%%%n", id, i);
            }
            System.out.println("Done PrinterJob: " + id);
        }
    }

    public static void main(String&#91;&#93; args) {
        System.out.println("Starting Single Thread Executor");
        ExecutorService executor = Executors.newSingleThreadExecutor();
        for (int i = 0; i < 5; ++i) {
            executor.execute(new PrinterJob());
        }
        executor.shutdown();
    }
}
&#91;/sourcecode&#93;

<p> And here's sample output of running the above program: </p>


Starting Single Thread Executor
Starting printer work: 1
worker 1, done 0%
worker 1, done 59%
Done PrinterJob: 1
Starting printer work: 2
worker 2, done 0%
worker 2, done 24%
worker 2, done 48%
worker 2, done 72%
worker 2, done 96%
Done PrinterJob: 2
Starting printer work: 3
worker 3, done 0%
worker 3, done 10%
worker 3, done 20%
worker 3, done 30%
worker 3, done 40%
worker 3, done 50%
worker 3, done 60%
worker 3, done 70%
worker 3, done 80%
worker 3, done 90%
worker 3, done 100%
Done PrinterJob: 3
Starting printer work: 4
worker 4, done 0%
worker 4, done 62%
Done PrinterJob: 4
Starting printer work: 5
worker 5, done 0%
worker 5, done 25%
worker 5, done 50%
worker 5, done 75%
worker 5, done 100%
Done PrinterJob: 5

The executor runs only one task at a time, in one worker thread. All tasks are run in the order of submission. Not blazing fast, but does the work in a separate thread and we don’t have to worry about synchronization!