multithreading - How to use Thread Pool concept in Java?

Question

I am creating a http proxy server in java. I have a class named Handler which is responsible for processing the requests and responses coming and going from web browser and to web server respectively. I have also another class named Copy which copies the inputStream object to outputStream object . Both these classes implement Runnable interface. I would like to use the concept of Thread pooling in my design, however i don't know how to go about that! Any hint or idea would be highly appreciated.

Answer 1

I suggest you look at Executor and ExecutorService. They add a lot of good stuff to make it easier to use Thread pools.

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@Azad provided some good information and links. You should also buy and read the book Java Concurrency in Practice. (often abbreviated as JCiP) Note to stackoverflow big-wigs - how about some revenue link to Amazon???

Below is my brief summary of how to use and take advantage of ExecutorService with thread pools. Let's say you want 8 threads in the pool.

You can create one using the full featured constructors of ThreadPoolExecutor, e.g.

ExecutorService service = new ThreadPoolExecutor(8,8, more args here...);

or you can use the simpler but less customizable Executors factories, e.g.

ExecutorService service = Executors.newFixedThreadPool(8);

One advantage you immediately get is the ability to shutdown() or shutdownNow() the thread pool, and to check this status via isShutdown() or isTerminated().

If you don't care much about the Runnable you wish to run, or they are very well written, self-contained, never fail or log any errors appropriately, etc... you can call

execute(Runnable r);

If you do care about either the result (say, it calculates pi or downloads an image from a webpage) and/or you care if there was an Exception, you should use one of the submit methods that returns a Future. That allows you, at some time in the future, check if the task isDone() and to retrieve the result via get(). If there was an Exception, get() will throw it (wrapped in an ExecutionException). Note - even of your Future doesn't "return" anything (it is of type Void) it may still be good practice to call get() (ignoring the void result) to test for an Exception.

However, this checking the Future is a bit of chicken and egg problem. The whole point of a thread pool is to submit tasks without blocking. But Future.get() blocks, and Future.isDone() begs the questions of which thread is calling it, and what it does if it isn't done - do you sleep() and block?

If you are submitting a known chunk of related of tasks simultaneously, e.g., you are performing some big mathematical calculation like a matrix multiply that can be done in parallel, and there is no particular advantage to obtaining partial results, you can call invokeAll(). The calling thread will then block until all the tasks are complete, when you can call Future.get() on all the Futures.

What if the tasks are more disjointed, or you really want to use the partial results? Use ExecutorCompletionService, which wraps an ExecutorService. As tasks get completed, they are added to a queue. This makes it easy for a single thread to poll and remove events from the queue. JCiP has a great example of an web page app that downloads all the images in parallel, and renders them as soon as they become available for responsiveness.