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Exploring Javas Fork Join Framework

Sep 17, 2012

We've been taking a look at concurrency recently, which leaves an excellent opportunity to study Java's new(ish) Fork/Join framework. The idea is, you have a task, and if it can benifit from splitting into a bunch of threads it forks out, and later joins them together when the work is done.

Traditionally, to make a thread we(I) do something like this main():

Thread myThread = new Thread(new Runnable(){
  public void run(){
    System.out.println("Hello from thread 1.");
  }
});

Then, to start the thread:

myThread.start();

Then, sometime later we want to make sure the thread has finished before moving on, so we say:

myThread.join();

And we go about our business as usual. Now, let's take a look at how Fork/Join does it:

My laptop has 2 processors, and so all I needed to do to see this in action was:

1. Download java jdk-7, since I had jdk-6 which doesn't support Fork/Join.
2. Find the ForkBlur.java demo program on javas site.
3. Find a picture of a Sneech.
4. Change line 104 in the code.
5. javac and java using my new jdk-7

The result is a blurred Sneech cover, AWESOME!

So what's going on here? Let's take a look at line 89:

  protected void compute() {
    if (mLength < sThreshold) {
      computeDirectly();
      return;
    }
    
    int split = mLength / 2;
    
    invokeAll(new ForkBlur(mSource, mStart,         split,           mDestination),
              new ForkBlur(mSource, mStart + split, mLength - split, mDestination));
  }

This is performing a single (unthreaded) computation if

mLength < sThreshold

and if not, it splits into 2 threads (one processing the first half of the image, the other processing the second half.

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