# Executor Service : ScheduledThreadPool

Paul Tomoiu

●

Examples,

Threads,

Spring,

Executors

●

Published 2022-12-03

This tutorial explains how to use the Executor Service in Java.

The Java ExecutorService is the interface which allows us to execute tasks on threads asynchronously. It defines a pool of threads to be used.

How you create an ExecutorService depends on the implementation you use.

Examples:
// It create a single thread pool  
ExecutorService es1 = Executors.newSingleThreadExecutor();

// It create a thread pool with maximum 10 threads
ExecutorService es2 = Executors.newFixedThreadPool(10);

// It create a thread pool with maximum 10 threads. The tasks could be scheduled using a pattern.
ScheduledExecutorService es3 = Executors.newScheduledThreadPool(10);

ScheduledExecutorService could be used for:

Run a task one time after a number of seconds (you can receive or not a value of the end of the task, function of the implementation)
Repeating the task every n second, with an initial delay

Take a look at the following example and read carefully the comment in the code.

package com.exampe.java;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.*;

@SpringBootApplication
public class DemoApplication {

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

		SpringApplication.run(DemoApplication.class, args);

		System.out.println("--------------------------------------------");
		Runnable task1 = () -> {
           System.out.println("Task #1 is running");
		};

		Callable<Integer> task2 = () -> {
		  {
			System.out.println("Task #2 is running");
		  };
		  return 2;
		};

		Runnable task3 = () -> {
			{
				System.out.println("Task #3 is running");
			};
		};

		ScheduledExecutorService es1 = Executors.newScheduledThreadPool(10);

		//Run task1 after 5 seconds (nonblocking) - using Runnable
		es1.schedule(task1, 5, TimeUnit.SECONDS);

		// Run a task. The task has a result kept in the future object
		Future<Integer> schedule = es1.submit(task2);

        // block and get the result
		System.out.println("1st get = "+schedule.get());

		// initial Delay = 5, repeat the task every 1 second
		ScheduledFuture<?> scheduledFuture = es1.scheduleAtFixedRate(task3, 5, 1, TimeUnit.SECONDS);

		// Stop the Executor Service
		es1.shutdown();
		System.out.println("--------------------------------------------");
	}
}

Info

shutdown() - shutdowns the Executor Service - no new tasks are accepted, it waits for previously submitted tasks to complete execution.
shutdownNow() - forcibly, attempts to stop all actively executing tasks, and returns a list of the tasks that were awaiting execution.