Question

I am new to Java/threads and I inherited something like the following code. It is a command line program that main() only starts 5-6 different kind of threads and exits with ^C. I want to add a shutdown hook to close all threads properly and adapted it the following way.

I added a Shutdown hook and a stopThread() method in all threads (like the one in MyWorker class)

The problem is that when I press ^C I don’t see the end message from the Thread’s run method. Is this done in the background or is there something wrong with my method. Also, Is there a better pattern I should follow?

Thanks

 public class Main {
     public static MyWorker worker1 = new MyWorker();
     // .. various other threads here

     public static void startThreads() {
         worker1.start();
         // .. start other threads
     }

     public static void stopThreads() {
         worker1.stopThread();
         // .. stop other threads
     }

     public static void main(String[] args)
             throws Exception {

         startThreads();

         // TODO this needs more work (later)

         Runtime.getRuntime().addShutdownHook(new Thread() {
             @Override
             public void run() {
                 try {
                     stopThreads();
                 } catch (Exception exp) {

                 }
             }
         });
     } }

 public class MyWorker extends Thread {
     private volatile boolean stop = false;

     public void stopThread() {
         stop = true;
     }

     public void run() {
         while (!stop) {
             // Do stuff here
         }
         // Print exit message with logger
     } 
}

Answer 1

When you call System.exit() or terminate via a signal, it stop all the existing threads and starts all the shutdown hooks. i.e. all your threads could be dead by the time you hook starts.

Instead of trying to stop threads cleanly, you should ensure resources are closed cleanly.