Question

I’m using this code to divide up a few hundred tasks between different CPU cores.

    final List<Throwable> errors = Collections.synchronizedList(Lists.<Throwable>newArrayList());

    final ExecutorService pool = Executors.newFixedThreadPool(Runtime.getRuntime().availableProcessors());

    for (...) {

        pool.execute(new Runnable() { @Override public void run() {

            try {

                // TASK HERE

            } catch (Throwable e) {
                errors.add(e);
            }

        }});

    }

    pool.shutdown();
    try {
        pool.awaitTermination(1000, TimeUnit.DAYS); // wait "indefinitely"
    } catch (InterruptedException e) {
        throw new RuntimeException(e);
    }

    if (!errors.isEmpty()) throw Exceptions.wrap(errors.get(0)); // TODO multi-exception

It works, but it’s not nice.

• There is no version of awaitTermination without timeout, which is what I want.
• I need to do my own error collecting.

What is the proper/common way to do this?

Answer 1

The point of a thread pool is to reuse threads. You should create it on application startup, outside of your code that creates tasks, and inject it. There is no need to shut down the pool after adding tasks. You do that when your application is shutting down.

To run a collection of tasks, use ExecutorService.invokeAll. To get the results afterwards, call get on each of the returned Futures. It will rethrow any exception that the task threw, so you can collect it afterwards.