[concurrency-interest] Access to pool size

[Tim Peierls]

Claude Hussenet wrote:
> Is there a way to have access to the pool size of an
> ExecutorService created through the factory methods
> of the Executors class?
> 
> It looks like that I would have to instantiate
> directly
> ThreadPoolExecutor if I want to monitor the pool size
> of an ExecutorService.
> 
> Please confirm...

It's true: the Executors factory methods aren't specified to return a 
ThreadPoolExecutor, even though they do in practice.

If all you want is to retrieve the count of active threads in the pool, 
however, you don't have to use a TPE constructor:

   int getActiveThreadCount(ExecutorService exec) {
       if (exec instanceof ThreadPoolExecutor) {
           return ((ThreadPoolExecutor) exec).getActiveCount();
       else // probably obtained with newSingleThreadExecutor
           return 1;
   }

That else-clause is a bit of a gamble; you might prefer to return some 
distinguished value to indicate "unknown".

--tim