how to multithread in Java

Question

I have to multithread a method that runs a code in the batches of 1000 . I need to give these batches to different threads .

Currently i have spawn 3 threads but all 3 are picking the 1st batch of 1000 . I want that the other batches should not pick the same batch instead pick the other batch .

Please help and give suggestions.

Answer 1

I would use an ExecutorService

int numberOfTasks = ....
int batchSize = 1000;
ExecutorService es = Executors.newFixedThreadPool(3);
for (int i = 0; i < numberOfTasks; i += batchSize) {
    final int start = i;
    final int last = Math.min(i + batchSize, numberOfTasks);
    es.submit(new Runnable() {
        @Override
        public void run() {
            for (int j = start; j < last; j++)
                System.out.println(j); // do something with j
        }
    });
}
es.shutdown();

Answer 2

Put the batches in a BlockingQueue and make your worker threads to take the batches from the queue.

Answer 3

Use a lock or a mutex when retrieving the batch. That way, the threads can't access the critical section at the same time and can't accidentally access the same batch.

I'm assuming you're removing a batch once it was picked by a thread.

EDIT: aioobe and jonas' answers are better, use that. This is an alternative. :)

Answer 4

You need to synchronize the access to the list of jobs in the batch. ("Synchronize" essentially means "make sure the threads aware of potential race-conditions". In most scenarios this means "let some method be executed by a single thread at a time".)

This is easiest solved using the java.util.concurrent package. Have a look at the various implementations of BlockingQueue for instance ArrayBlockingQueue or LinkedBlockingQueue.