Java thread pool: What happens to idle threads

Question

I am trying to understand multi-threading in Java. I have written the following java program to test thread pool.

public class ThreadPoolTest
{
    public static void main(String[] args)
    {
        ExecutorService executorService = Executors.newFixedThreadPool(5);

        for( int i = 0; i < 3; i++ )
        {
            executorService.submit(new Task(i+1));
        }
        executorService.shutdown();                     

    }

    public static class Task implements Runnable
    {
        private int taskId;

        public Task(int id)
        {
            taskId = id;
        }

        @Override
        public void run() {
            System.out.println("Executing task " + taskId + " performed by " + Thread.currentThread().getName() );
            try
            {
                Thread.sleep(3000);
            }
            catch(InterruptedException interruptEx)
            {
                System.out.println(Thread.currentThread().getName() + " got interrupted ");
            }
            System.out.println("Finished executing task " + taskId );
        }
    }
}

The main thread creates executor which creates 5 threads and I have submitted only 3 tasks. After that I am shutting down the executor. When I run the code, the main thread finishes before the child threads. In this case, does JVM takes care of the child threads? Also I have created a thread pool with 5 threads, but submitted only 3 tasks. Will the remaining 2 threads are terminated when the main thread exits?

What actually happens when the executor service is shutdown?

Answer 1

From the doc of ExecutorService#shutdown():

Initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted.

This means that all the Jobs you submitted to the Executor will finish on their own time without interrupting or "hurrying" them, and the executor will finish up the worker threads properly, but neither will the service accept new Jobs, nor will it terminate instantly.

Compare ExecutorService#shutdownNow(), which will try to terminate as quickly as possible.