Question

I am trying to listen to RabbitMQ queue by polling it. But somehow due to network issues if once connection to queue is lost then thread silently dies off and connection and everything is closed. But this is a background task and we wont know untill queue really grows huge and start send out notification.

Can some one please help me with graceful shutdown of thread (which I guess I am already doing by dealing carefully with exception in catch clause). But I don’t know how to re-start a stopped thread.

Is there a way through which I can restart a new instance of stopped thread.

PS: I am instantiating the thread using @postconstruct and calling init thread soon after container loads all beans.

Answer 1

This sounds to me that you are not doing proper exception handling. You say “thread silently dies off” but that doesn’t happen in Java. I would audit your exceptions and watch for the following problems:

• Watch out for throws Exception on a method. This hides all sorts of evils. A method should usually enumerate the exceptions that it throws.
• If an Exception throws too many different types of exceptions then that is a signal that it is too large. Consider splitting it into multiple smaller methods. Or handle the specific exceptions inside of the method and throw one exception out.
• Try to have small try/catch blocks which catch a single exception if possible. Don’t inclose huge blocks of code with try { ... } catch (Exception e) { ... }. That, again, hides evils.
• If you catch an exception, make sure you aren’t just blindly continuing. If this is a background thread then maybe it should exit or restart the socket or…
• Make sure you are properly reporting all exceptions. Every catch block should do something with the exception. e.printStackTrace() may work but providing more information about the problem is usually in order.

But I don’t know how to re-start a stopped thread.

You do not re-start a stopped thread, you start another one. If the thread should not be shutting down at all then it needs to re-open a socket or maybe re-start its RabbitMQ connection. Again, it’s about proper exception handling. I don’t know RabbitMQ but something like the following pseudo code might help:

public void run() {
    while (!shutdown) {
       Connection conn = null;
       try {
           conn = rabbitMq.start();
           processQueue(conn);
       } catch (IOException e) {
           // TODO: log the exception here
       } finally {
           // make sure we close the connection
           if (conn != null) { conn.close(); }
       }
       try {
          // we sleep here to not spin if the RabbitMQ host goes down
          Thread.sleep(1000);
       } catch (InterruptedException e) {
          Thread.currentThread.interrupt();
          // bail if someone interrupts us
          return;
       }
}

Best of luck.