How to lazily create tasks for use by a Java thread-pool

Question

I'm writing a load-testing application in Java, and have a thread pool that executes tasks against the server under test. So to make 1000 jobs and run them in 5 threads I do something like this:

    ExecutorService pool = Executors.newFixedThreadPool(5);
    List<Runnable> jobs = makeJobs(1000);
    for(Runnable job : jobs){
        pool.execute(job);
    }

However I don't think this approach will scale very well, because I have to make all the 'job' objects ahead of time and have them sitting in memory until they are needed.

I'm looking for a way to have the threads in the pool go to some kind of 'JobFactory' class each time they need a new job, and for the factory to build Runnables on request until the required number of jobs have been run. The factory could maybe start returning 'null' to signal to the threads that there is no more work to do.

I could code something like this up by hand, but it seems like a common enough use-case and was wondering if there was anything in the wonderful but complex 'java.util.concurrent' package that I could use instead?

Answer 1

You can do all the work in the executing threads of the thread pools using an AtomicInteger to monitor the number of runnables executed

 int numberOfParties = 5;
 AtomicInteger numberOfJobsToExecute = new AtomicInteger(1000);
 ExecutorService pool = Executors.newFixedThreadPool(numberOfParties );
 for(int i =0; i < numberOfParties; i++){
     pool.submit(new Runnable(){
        public void run(){
            while(numberOfJobsToExecute.decrementAndGet() >= 0){
                makeJobs(1).get(0).run();
            }
        }
     });
 }

You can also store the returned Future's in a List and get() on them to await completion (among other mechanisms)