Which ThreadPool in Java should I use?

Question

There are a huge amount of tasks. Each task is belong to a single group. The requirement is each group of tasks should executed serially just like executed in a single thread an

Answer 1

I had a problem similar to your, and I used an ExecutorCompletionService that works with an Executor to complete collections of tasks. Here is an extract from java.util.concurrent API, since Java7:

Suppose you have a set of solvers for a certain problem, each returning a value of some type Result, and would like to run them concurrently, processing the results of each of them that return a non-null value, in some method use(Result r). You could write this as:

void solve(Executor e, Collection> solvers)
        throws InterruptedException, ExecutionException {
    CompletionService ecs = new ExecutorCompletionService(e);
    for (Callable s : solvers)
        ecs.submit(s);
    int n = solvers.size();
    for (int i = 0; i < n; ++i) {
        Result r = ecs.take().get();
        if (r != null)
            use(r);
    }
}

So, in your scenario, every task will be a single Callable, and tasks will be grouped in a Collection>.

Reference: http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/ExecutorCompletionService.html