Question

I have the following code:

public void run() 
{
     try 
     {
         logger.info("Looking for new tasks to fetch...  ");
         // definitions ..

         for(Task t: tasks)
         {
             logger.info(" Task " + t.getId() + " is being fetched ");
            // processing ... fetching task info from db using some methods
         }
         Thread.sleep(FREQUENCY);
         //t.start();
     } catch (Exception e) 
     {
         logger.info("FetcherThread interrupted: "+e.getMessage());
     }
}

I’m trying to make the thread to sleep for a specific time “FREQUENCY” and then work again. when I execute this code in eclipse, the thread works only once and then nothing happens and process terminates. If I remove the comment from the statement: t.start(), I get “FetcherThread interrupted: null”.
Can anyone tell me where I’m going wrong?

N.B.: I want the thread to be working all the time, but fetching on periods (say every 5 minutes)

Answer 1

You’re missing any sort of loop in that code.

It seems that the thread is actually doing what you tell it to do: it runs all the tasks, then sleeps for a bit – then it has no more work to do, and so exits. There are several ways to address this, in ascending order of complexity and correctness:

1. The simple (and naive) way to address this is to wrap the try-catch block in an infinite loop (while(true) { ... }). This way after the thread finishes sleeping, it will loop back to the top and process all the tasks again.

2. However this isn’t ideal, as it’s basically impossible to stop the thread. A better approach is to declare a boolean field (e.g. boolean running = true;), and change the loop to while(running). This way, you have a way to make the thread terminate (e.g. expose a method that sets running to false.) See Sun’s Why is Thread.stop() deprecated article for a longer explanation of this.

3. And taking a step further back, you may be trying to do this at too low a level. Sleeping and scheduling isn’t really part of the job of your Runnable. The actual solution I would adopt is to strip out the sleeping, so that you have a Runnable implementation that processes all the tasks and then terminates. Then I would create a ScheduledExecutorService, and submit the “vanilla” runnable to the executor – this way it’s the job of the executor to run the task periodically.

The last solution is ideal from an engineering perspective. You have a class that simply runs the job once and exits – this can be used in other contexts whenever you want to run the job, and composes very well. You have an executor service whose job is the scheduling of arbitrary tasks – again, you can pass different types of Runnable or Callable to this in future, and it will do the scheduling bit just as well. And possibly the best part of all, is that you don’t have to write any of the scheduling stuff yourself, but can use a class in the standard library which specifically does this all for you (and hence is likely to have the majority of bugs already ironed out, unlike home-grown concurrency code).