Java program does not exit

Question

I am exploring multi threading in Java. I have come across this program to compute primes. Primes are computed in the most inefficient way possible. This is done in purpose to maximize the execution time.

Here is the code

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

import static java.lang.System.exit;

public class Part19_CalculatePrimeNumbersUsingFutureAndCallable {

    public static void main(String[] args) {
        ExecutorService executorService = Executors.newCachedThreadPool();
        List<Future<Integer>> futures = new ArrayList<>();
        System.out.println(String.format("The main thread : %s", Thread.currentThread().getName()));
        while (true) {
            Scanner sc = new Scanner(System.in);
            System.out.println("\nI can tell you the nth prime number, Enter n(0 to exit): ");
            int n = sc.nextInt();
            if (n == 0)
                break;

            Callable<Integer> primeCalculator = new Callable<Integer>() {
                @Override
                public Integer call() throws Exception {
                    return calculatePrime(n);
                }
            };

            Future<Integer> primeNumberFuture = executorService.submit(primeCalculator);

            futures.add(primeNumberFuture);

        }

        System.out.println("EXITED");

        // check if any of the futures are completed

        futures.forEach(f -> {
            try {
                System.out.println(String.format("\tComputed prime %s", f.get()));
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            } catch (ExecutionException e) {
                throw new RuntimeException(e);
            }
        });

    }

    private static int calculatePrime(int n) {
        int num = 1, count = 0, i;

        while (count < n) {
            num = num + 1;
            for (i = 2; i <= num; i++) {
                // determines the modulo and compare it with 0
                if (num % i == 0) {
                    // breaks the loop if the above condition returns true
                    break;
                }
            }
            if (i == num) {
                // increments the count variable by 1 if the number is prime
                count = count + 1;
            }
        }
        return num;
    }
}

OUTPUT 1

The main thread : main

I can tell you the nth prime number, Enter n(0 to exit): 

0

EXITED

Process finished with exit code 0

This outputs as expected

OUTPUT 2

The main thread : main

I can tell you the nth prime number, Enter n(0 to exit): 
1

I can tell you the nth prime number, Enter n(0 to exit): 
2

I can tell you the nth prime number, Enter n(0 to exit): 
3

I can tell you the nth prime number, Enter n(0 to exit): 
0
EXITED
  Computed prime 2
  Computed prime 3
  Computed prime 5

Here the program does not seem to terminate and I cannot understand why.

Answer 1

Minimal fix

Add a:

executorService.shutdown(); // alternatively try(executorService.awaitTermination(...)) ...

as the last line of your main().

The executor service (seems to, surely implemented also "as a thread") "reserves resources" and prevents the main thread from exiting.

In the 0 scenario we don't submit any tasks...

Alternatives/Documentation (17):

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

shutdownNow() Attempts to stop all actively executing tasks, halts the processing of waiting tasks, and returns a list of the tasks that were awaiting execution. This method does not wait for actively executing tasks to terminate. Use awaitTermination to do that...

boolean awaitTermination(long timeout, TimeUnit unit) throws InterruptedException Blocks until all tasks have completed execution after a shutdown request, or the timeout occurs, or the current thread is interrupted, whichever happens first...

• System.exit(/*code*/); ;(

Thx, @tgkprog:

• How to exit ExecutorService after all the threads have done their execution?
• How to properly shutdown java ExecutorService
• https://www.google.com/search?q=java+daemon+vs+non+daemon+threads