My program is not returning/ending with Executor

Question

Given the following Java example, which uses multithreading:

 import java.util.concurrent.*;

 public class SquareCalculator {
     private ExecutorService executor = Executors.newSingleThreadExecutor();

     public Future<Integer> calculate(Integer input) {
         return executor.submit( () -> {
             Thread.sleep(1000);
             return input * input;
         });
     }

     public static void main(String[] args) {
         try {
             Future<Integer> future = new SquareCalculator().calculate(10);
             while (!future.isDone()){
                 System.out.println("Calculating...");
                 Thread.sleep(300);
             }

             Integer result = future.get();
             System.out.println("we got: " + result);
         } catch(InterruptedException | ExecutionException e) {
             System.out.println("had exception");
         }
     }
 }

It produces:

java SquareCalculator
Calculating...
Calculating...
Calculating...
Calculating...
we got: 100

But the application is never terminating.

Am I suppose to join the thread or something?

What do you mean by "it's not returning the prompt"? Can please explain a bit more. — Amit Bera, Feb 04 '19 at 18:20
@AmitBera my shell is not returning to me as if program is still running. i have to ctrl+c to get it back and to be able to interact with shell again — ealeon, Feb 04 '19 at 18:22

score 3 · Answer 1 · answered Feb 04 '19 at 18:26

3

Should be in comment but not enough reputation.

You should call shutdown on executor. You can get more details from below link: Reason for calling shutdown() on ExecutorService

answered Feb 04 '19 at 18:26

Shesh Nath Mishra

71
1

score 3 · Answer 2 · answered Feb 04 '19 at 18:26

3

I bet you want to add something like this:

   finally {
        if (executor != null) {
            executor.shutdown();
        }
    }

answered Feb 04 '19 at 18:26

automatictester

2,436
1
19
34

score 2 · Accepted Answer · answered Feb 04 '19 at 18:32

You need to shut down the executor framework towards the end of the program and wait until it gracefully terminates..

executor.shutdown();
try {
    executor.awaitTermination(4 * 3600, TimeUnit.SECONDS);
} catch (Exception e) {
    e.printStackTrace();
}

My program is not returning/ending with Executor

3 Answers3