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

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.

Answer 2

/* Get an executor service that will run a maximum of 5 threads at a time: */ ExecutorService exec = Executors.newFixedThreadPool(5); /* For all the 100 tasks to be done altogether... */ for (int i = 0; i < 100; i++) {     /* ...execute the task to run concurrently as a runnable: */     exec.execute(new Runnable() {         public void run() {             /* do the work to be done in its own thread */             System.out.println("Running in: " + Thread.currentThread());         }     }); } /* Tell the executor that after these 100 steps above, we will be done: */ exec.shutdown(); try {     /* The tasks are now running concurrently. We wait until all work is done,       * with a timeout of 50 seconds: */     boolean b = exec.awaitTermination(50, TimeUnit.SECONDS);     /* If the execution timed out, false is returned: */     System.out.println("All done: " + b); } catch (InterruptedException e) { e.printStackTrace(); }