java - How to implement simple threading with a fixed number of worker threads

I'm looking for the simplest, most straightforward way to implement the following:

• The main program instantiates worker
  
  threads to do a task.


• Only n tasks can be running at once.


• When n is reached, no more workers
  are started until the count of
  running threads drops back below n.

Answer

I think that Executors.newFixedThreadPool fits your requirements. There are a number of different ways to use the resulting ExecutorService, depending on whether you want a result returned to the main thread, or whether the task is totally self-contained, and whether you have a collection of tasks to perform up front, or whether tasks are queued in response to some event.

  Collection tasks = new ArrayList();

  YourTask yt1 = new YourTask();
  ...
  tasks.add(yt1);
  ...
  ExecutorService exec = Executors.newFixedThreadPool(5);
  List> results = exec.invokeAll(tasks);

Alternatively, if you have a new asynchronous task to perform in response to some event, you probably just want to use the ExecutorService's simple execute(Runnable) method.