multithreading - Return values from Java Threads

Question

I have a Java Thread like the following:

   public class MyThread extends Thread {
        MyService service;
        String id;
        public MyThread(String id) {
            this.id = node;
        }
        public void run() {
            User user = service.getUser(id)
        }
    }

I have about 300 ids, and every couple of seconds - I fire up threads to make a call for each of the id. Eg.

for(String id: ids) {
    MyThread thread = new MyThread(id);
    thread.start();
}

Now, I would like to collect the results from each threads, and do a batch insert to the database, instead of making 300 database inserts every 2 seconds.

Any idea how I can accomplish this?

Answer 1

The canonical approach is to use a Callable and an ExecutorService. submitting a Callable to an ExecutorService returns a (typesafe) Future from which you can get the result.

class TaskAsCallable implements Callable<Result> {
    @Override
    public Result call() {
        return a new Result() // this is where the work is done.
    }
}

ExecutorService executor = Executors.newFixedThreadPool(300);
Future<Result> task = executor.submit(new TaskAsCallable());
Result result = task.get(); // this blocks until result is ready

In your case, you probably want to use invokeAll which returns a List of Futures, or create that list yourself as you add tasks to the executor. To collect results, simply call get on each one.