How does one stop a thread without a stop() method?

Question

I have question about the Java threads. Here is my scenario:

I have a thread calling a method that could take while. The thread keeps itself on that method until I get the result. If I send another request to that method in the same way, now there are two threads running (provided the first did not return the result yet). But I want to give the priority to the last thread and don't want to get the results from the previously started threads. So how could I get rid of earlier threads when I do not have a stop method?

Answer 1

The standard design pattern is to use a local variable in the thread that can be set to stop it:

public class MyThread extends Thread {
   private volatile boolean running = true;

   public void stop() {
      running = false;
   }

   public void run() {
      while (running) {
         // do your things
      }    
   }
}

This way you can greacefully terminate the thread, i.e. without throwing an InterruptedException.

Answer 2

The best way really depends on what that method does. If it waits on something, chances are an interrupt will result in an InterruptedException which you handle and cleanly exit. If it's doing something busy, it won't:

class Scratchpad {
    public static void main(String[] a) {
        Thread t = new Thread(new Runnable() {
            public void run() {doWork();}
        });
        t.start();

        try {
            Thread.sleep(50);
        } catch (InterruptedException ie) {}

        t.interrupt();
    }

    private static void doWork() {
        for ( long i = 1; i != 0; i *=5 );
    }
}

In the case above, the only viable solution really is a flag variable to break out of the loop early on a cancel, ala @inflagranti.

Another option for event-driven architectures is the poison-pill: if your method is waiting on a blocking queue for a new item, then you can have a global constant item called the "poison-pill" that when consumed (dequeued) you kill the thread:

try {
   while(true) {
      SomeType next = queue.take();
      if ( next == POISON_PILL ) {
          return;
      }
      consume(next);
   }
} catch //...

EDIT:

It looks like what you really want is an executor service. When you submit a job to an executor service, you get back a Future which you can use to track results and cancel the job.