How to initialize all threads of a fixed thread pool before submitting any tasks? (JAVA)

Question

I am trying to speed up a program that uses ExecutorService to run tasks in parallel. It basically works like this:

1. Initialize a fixed size thread pool of size n

2. Read a bunch (about 2500 files) of XML files containing input for the tasks

3. Process the XML files using the worker threads from the pool

Everything works as expected, but the problem I have is that each worker thread has an instance of a class that does calculations on the input data. This instance is stored in a ThreadLocal. Now, all these thread local instances are created when the correspondig worker thread is started, that means after all XML input files are read.

Since initialization of the calculation objects takes quite some time, I'd rather have the thread pool initialize all worker threads right from the start, so that the initializations of the calculation objects can run parallel to the reading of the input files.

Here is some code to help you get an idea about how it currently works (I stripped code not related to the problem).

Initializing the thread pool and the thread local:

  private final ExecutorService executor = Executors.newFixedThreadPool(Math.max(1, Runtime
      .getRuntime().availableProcessors() - 1));

  private ThreadLocal<Calculator> calculator = new ThreadLocal<Calculator>() {
    @Override
    protected Calculator initialValue() {
      try {
        Calculator instance = createCalculator();
        return instance;
      } catch (Throwable e) {
        throw new RuntimeException(e);
      }
    }
  };

Submitting a new calculation:

  @Override
  public FutureTask<Output> calc(Input input) {
    FutureTask<Output> task = new FutureTask<>(
        new Callable<Rueckgabe>() {
          @Override
          public Output call() throws Exception {
            try {
              return calculator.get().calc(input);
            } catch (Throwable e) {
              System.err.println("Exception: " + e.getMessage());
              e.printStackTrace(System.err);
              return null;
            }
          }
        });
    executor.execute(task);
    return task;
  }

What is the right way to have the ExecutorService start all worker threads for the pool right from the start? Or do I have to something like n dummy tasks to force initialization?

PS: I have to use Java 7 for the foreseeable future due to IT restrictions.

Answer 1

Add a Threadfactory to the Executor which would do the initialization.

executor = Executors.newFixedThreadPool(numprocrssors, new ThreadFactory ( ){
    public Thread newThread(Runnable r) {
        return new Thread(r){
            {calculator.get();} // this is an initialization statement, added to all constructors.
        };
    }});