How to execute tasks in ExecutorService sequentially?

Question

I have three threads that are joined, i.e. the second thread executes after the first dies.

This is the code I have:

public class Main {
    public s         


        

Answer 1

If you already use a thread pool in your application, you can reuse it by means of zero-threaded proxy serial executor, without creating special single threaded thread pool. One implementation is described in the javadoc section of Executor interface, another, optimized implementation, at my Github repository CodeSamples.

Answer 2

You can control sequential execution of threads using SingleThread ExecutorService with future.get() method.

DummyTask Class

import java.util.concurrent.Callable;

public class DummyTask implements Callable<Integer>
{

int taskId;

public DummyTask(int taskId) {
    this.taskId = taskId;
}

@Override
public Integer call() throws Exception
{
    System.out.println("excuting task... Task Id: " + taskId);
    return taskId;
}

}

SequentialExecution Class

package com.amit.executorservice;

import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;

public class SequentialExecution 
{
public static void main(String[] args) throws InterruptedException, ExecutionException {

    DummyTask task1 = new DummyTask(1);
    DummyTask task2 = new DummyTask(2);
    DummyTask task3 = new DummyTask(3);
    Future<Integer> result = null;
    ExecutorService executor = Executors.newSingleThreadExecutor();

    result = executor.submit( task1 );
    // future.get() Waits for the task to complete, and then retrieves its result.
    result.get();

    result = executor.submit( task2 );
    // future.get() Waits for the task to complete, and then retrieves its result.
    result.get();

    result = executor.submit( task3 );
    // future.get() Waits for the task to complete, and then retrieves its result.
    result.get();

    executor.shutdown();

}
}

Output

excuting task... Task Id: 1
excuting task... Task Id: 2
excuting task... Task Id: 3

Output will always be same and all task will get executed in sequence.

Answer 3

If what you want/need is to execute a group of jobs one after another but in a single thread different that the main app thread, then use Executors#newSingleThreadExecutor.

ExecutorService es = Executors.newSingleThreadExecutor();
es.submit(() -> System.out.println("Message 1"));
es.submit(() -> System.out.println("Message 2"));
es.submit(() -> System.out.println("Message 3"));
es.shutdown();