ExecutorService vs Casual Thread Spawner

Question

I have a basic question about how ExecutorService works in Java.

It is quite hard to see the difference between simply creating Threads to perform some tasks in parallel and assigning each tasks to the ThreadPool.

The ExecutorService also looks very simple and efficient to use, so I was wondering why we don't use it all the time.

Is it just a matter of one way executing its job faster than the other ?

Here's two very simple examples to show the difference between the two ways :

Using executor service: Hello World (task)

static class HelloTask implements Runnable {
    String msg;

    public HelloTask(String msg) {
        this.msg = msg; 
    }
    public void run() {
        long id = Thread.currentThread().getId();
        System.out.println(msg + " from thread:" + id);
    }
}

Using executor service: Hello World (creating executor, submitting)

static class HelloTask {
    public static void main(String[] args) {
        int ntasks = 1000;
        ExecutorService exs = Executors.newFixedThreadPool(4);

        for (int i=0; i<ntasks; i++) { 
            HelloTask t = new HelloTask("Hello from task " + i);    
            exs.submit(t);
        }
        exs.shutdown();
    }
}

the following shows a similar example but extending the Callable interface, could you tell me the difference between the two and in which cases one should use a specific one instead of the other ?

Using executor service: Counter (task)

static class HelloTaskRet implements Callable<Long> {
    String msg;

    public HelloTaskRet(String msg) {
        this.msg = msg; }

        public Long call() {
        long tid = Thread.currentThread().getId(); 
        System.out.println(msg + " from thread:" + tid); 
        return tid;
    } 
}

Using executor service: (creating, submitting)

static class HelloTaskRet {
    public static void main(String[] args) {
        int ntasks = 1000;
        ExecutorService exs = Executors.newFixedThreadPool(4);

        Future<Long>[] futures = (Future<Long>[]) new Future[ntasks];

        for (int i=0; i<ntasks; i++) { 
            HelloTaskRet t = new HelloTaskRet("Hello from task " + i);
            futures[i] = exs.submit(t);
        }
        exs.shutdown();
    }
}

Both examples use ExecutorService instead of creating new Threads, so I'm not sure what you're comparing between the 2 examples in that case. Is your confusion about when to use Runnable and when to use Callable? — Dioxin

kaqqao kaqqao · Accepted Answer · 2014-11-14T21:08:37

While the question and the sample code do not correlate, I'll try clarifying both. The advantage of ExecutorService over haphazardly spawning threads is that it behaves predictably and avoids the overhead of thread creation, which is relatively big on the JVM (it needs to reserve memory for each thread, for example). By predictability, at least for the fixedThreadPool, I mean you know the maximum number of concurrent threads, and you know when and how they might get created (so your JVM won't blow up in case of sudden peaks).

By Vince Emigh: ExecutorService also supports cachedThreadPool, which doesn't have a max. The main reason people choose to use ExecutorService is to prevent the overhead of creating multiple threads (by using worker threads). It's mostly used in cases where many small tasks need to be executed on a separate thread. Also, don't forget about singleThreadExecutor.

Now, on the topic of Runnable vs Callable, it is easy to see from your examples. Callables can return a value place-holder (Future) that will eventually be populated by an actual value in the future. Runnables can not return anything.

By Vince Emigh: Runnable also cannot throw exceptions, while Callable can.

ExecutorService vs Casual Thread Spawner

2 Answers