Java parallel work iterator?

Question

I\'m looking for a class where I can override a method to do the work, and return the results like an iterator. Something like this:

ParallelWorkIterator<         


        

Answer 1

The closest thing I can think of is to use a CompletionService to accumulate results as they complete.

Simple example:

ExecutorService executor = Executors.newSingleThreadExecutor(); // Create vanilla executor service.
CompletionService completionService = new ExecutorCompletionService(executor); // Completion service wraps executor and is notified of results as they complete.
Callable callable = new MyCallable();

executor.submit(callable); // Do not store handle to Future here but rather obtain from CompletionService when we *know* the result is complete.

Future fut = completionService.take(); // Will block until a completed result is available.
Result result = fut.get(); // Will not block as we know this future represents a completed result.

I would not recommend wrapping this behind an Iterator interface as the Future get() method can throw two possible checked exceptions: ExecutionException and InterruptedException, and you would therefore need to catch and either swallow these or rethrow them as RuntimeExceptions, neither of which is a very nice thing to do. In addition, your Iterator's hasNext() or next() methods would potentially need to block if there was a task in progress, which could be considered counterintuitive for clients using an Iterator. Instead I would implement my own more descriptive interface; e.g.

public interface BlockingResultSet {
  /**
   * Returns next result when it is ready, blocking is required.
   * Returns null if no more results are available.
   */  
  Result take() throws InterruptedException, ExecutionException;
}

(Methods called take() typically represent a blocking call in the java.util.concurrent package).