How to multithread a computationally intensive code segment in Java?

Question

I hava a java program, a section of it is compute intensive, like this

for i = 1 :512
   COMPUTE INTENSIVE SECTION
end

I want to split it into multithread, make it faster when running.

COMPUTE INTENSIVE SECTION is not sequential-wise. It means running i=1 first or i=5 fist are the same...

Can anybody give me a grand guide about this. How to do it? Thanks indeed! Happy Thanksgiving!

Answer 1

You should read the Concurrency Trail of the Java Tutorial. Especially Executors and Thread Pools should be relevant for you.

Basically, you create a thread pool (which is an Executor) through one of the factory methods in the Executors class and submit Runnable instances to it:

for(int i = 0; i < 512; i++){
    executor.execute(new Runnable(){public void run(){
        // your heavy code goes here
    }});
}

Answer 2

Sounds like a thread pool would be good. Basically, you whip up a collection of N different threads, then request them in a loop. The request blocks until a thread is available.

ThreadPool pool = Executors.newFixedThreadPool(10); // 10 threads in the pool
ArrayList<Callable> collectionOfCallables = new ArrayList<Callable>( );
for (...) {
  Callable callable = new Callable<Foo>() { public Foo call() { COMPUTE INTENSIVE SECTION } }
  collectionOfCallables.add(callable);
}

ArrayList<Future<Foo>> results = pool.invokeAll( collectionOfCallables );

pool.awaitTermination(5, TimeUnit.MINUTES ); // blocks till everything is done or 5 minutes have passed.

With the Future's you really don't need to await termination. get()ing the result from a future will block until the corresponding thread is done (or canceled).