Persistent Variables in ExecutorService (Java)

Question

I\'ve created a pool of 4 worker threads to process some files. In the test there\'s about 200 of them. The thread pool version is already about 3 times faster than doing it

Answer 1

You can use an object pool for your message digest objects. And set the pool size to 4 in your case.

apache commons provides excellent pool api: http://commons.apache.org/pool/

Answer 2

Extend ThreadLocal for your class and override the initialValue() method. By default, it returns null.

private static class ThreadLocalGenerateSHA1 extends
  ThreadLocal<GenerateSHA1> {

  @Override
  protected GenerateSHA1 initialValue() {
    return new GenerateSHA1();
  }

}

private static final ThreadLocalGenerateSHA1 generateSHA1 = new ThreadLocalGenerateSHA1();

...

On the tasks, simply call the get() method of generateSHA1. You do not need to call set().

Answer 3

You can take help of beforeExecute and afterExecute methods of the ThreadPoolExecutor by extending it. In your extended class, create 4 MessageDigest objects and assign them to each task in beforeExecute(...) in a round-robin fashion.

private static final Executor executor = new MyThreadPoolExecutor(10, 10, 50000L,   TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>(100));

EDIT

I feel the object pool option mentioned by Ravi Bhatt as the simple and elegant idea. Create a pool and let the task ask for the MessageDigest from the pool.