Threads, executors and ending a "hanging" main method

[simon fletcher]

The following code is from a tutorial on multithreading (meant to introduce the CountDownLatch and incorporating a thread pool). When it's run, it just "hangs" at the end. If it wasn't using the thread pool it would use thread objects and the join() method could be called on each thread from the main thread and, presumably, the main thread would end when the other threads end. But what do you do to end the main thread when a thread pool is used and why does the main thread not just end after the last line of the main method anyway?

import java.util.concurrent.CountDownLatch; 
import java.util.concurrent.ExecutorService; 
import java.util.concurrent.Executors;

public class App {       
 public static void main(String[] args) {                   
  CountDownLatch latch = new CountDownLatch(3);                   
  ExecutorService executor = Executors.newFixedThreadPool(3);                   
  for(int i=0; i < 3; i++) {             
    executor.submit(new LatchTest(latch));         
  }                   
  try {             
    latch.await();         
  } catch (InterruptedException e) {             
    e.printStackTrace();         
  }                   
  System.out.println("Completed.");
 }

 static class LatchTest implements Runnable {     
  private CountDownLatch latch;           
  public LatchTest(CountDownLatch latch) {         
   this.latch = latch;     
  }           
  public void run() {         
   System.out.println("Started.");                   
   try {             
     Thread.sleep(3000);         
   } catch (InterruptedException e) {             
     e.printStackTrace();         
   }                   
   latch.countDown(); 
  }  
 }

}

The ouptput is:
Started.
Started.
Started.
Completed.
...And cursor just sits at the end blinking and unresponsive.

Thanks.

[Paul Clapham]

You have an ExecutorService which is still available to process more Runnables or Callables. So it's waiting to be told to do something. That's why your application doesn't shut down.

(Hint: there's a way to tell an ExecutorService to shut down.)

[simon fletcher]

Got it. Thanks.