Question

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 1

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<YourTask> tasks = new ArrayList<YourTask>();   YourTask yt1 = new YourTask();   ...   tasks.add(yt1);   ...   ExecutorService exec = Executors.newFixedThreadPool(5);   List<Future<YourResultType>> 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.