How and Why Thread is interrupted under overloaded traffic

Question

I have implemented a connection check thread. When I have created a heavy traffic I have realized that there are many "ConnWatchdogThread" instances. I think "continue" statement triggers the interrupted thread live and in that case new "ConnWatchdogThread" is created. If so how this happen? How thread is interrupted?

 private class ConnWatchdog extends Thread {

 public ConnWatchdog(){
 setName("ConnWatchdogThread");
 }
      private void checkConnections() {

   //connection check 
      }

      @Override
      public void run() {

         while (true) {
            try {
               Thread.sleep(checkPeriod);
            } catch (InterruptedException e) {
//              e.prinstackTrace()

               continue;
            }

            try {

               this.checkConnections();

            } catch (Throwable t) {

            }

         }

      }
   }

score 1 · Accepted Answer · edited May 23 '17 at 11:49

Interruption happens when the interrupt method on the thread is called. It's not shown in your example what is doing the interrupting. Here's an example of using interrupt.

The continue advances control to the next iteration of the loop. So when something calls interrupt on your thread while it's sleeping, it bails out of the sleep (clearing the interrupt flag), then the continue sends it back to the top of the loop and it goes to sleep again, which seems pointless.

Nothing in the code shown causes a new thread to be created.

It would be more normal to use the interruption to exit the while loop and cause the thread to terminate. Using interruption for things other than thread-cancellation is not recommended (Java Concurrency in Practice, 7.1, page 138):

There is nothing in the API or language specification that ties interruption to any specific cancellation semantics, but in practice, using interruption for anything but cancellation is fragile and difficult to sustain in larger applications.

score 0 · Answer 2 · answered Nov 04 '14 at 13:55

The only way to create a new Thread is with new Thread(...). Your continue is causing interrupt to be ignored and for it to sleep a little longer.

Interrupt is a special flag (but basically still just a flag) When you interrupt() a Thread it will cause some methods to throw an InterruptedException even if they are blocking. Once triggered the flag is reset.

It is generally a bad idea to ignore exception/error thrown esp in code which is not working perfectly.

score 0 · Answer 3 · answered Nov 04 '14 at 14:28

Your not showing the code that uses this class. No way to say what is making the instances. Continue wont do it. In fact can remove continue from your code in the ConnWatchdog first try-catch and it will work just fine.

Post your code that is calling ConnWatchdog / instantiating it to see full picture. Is that in a loop?

How and Why Thread is interrupted under overloaded traffic

3 Answers3