reader-writer task-queue in Android

Question

I am making an android app with reading tasks and writing tasks . I want to get the following behavior:

• tasks are executed in a queue
• reading tasks can be run concurrently.
• writing tasks are very important and can not run concurrently, if it is running a writing task can not run any other task of any kind

Iam searching a library that solves this problem. I could solve this problem semaphores, but I have fear of making a solution to these problems in android. In my limited experience with android I know it is very difficult to manage the life cycle of these things. And it is very important that writing tasks are executed correctly.

Sorry for my English. Thanks

Answer 1

Use a fixed thread pool executor for your tasks, let all your tasks use the same ReadWriteLock, and let it work itself out. General example:

public class ReadTask implements Runnable {        
    private final ReadWriteLock lock;
    public ReadTask (ReadWriteLock lock) {
        this.lock = lock;
    }
    @Override public void run () {
        lock.readLock().lock();
        // do stuff
        lock.readLock().unlock();
    }
}

public class WriteTask implements Runnable {
    private final ReadWriteLock lock;
    public WriteTask (ReadWriteLock lock) {
        this.lock = lock;
    }
    @Override public void run () {
        lock.writeLock().lock();
        // do stuff
        lock.writeLock().unlock();
    }
}

And then, set up your initial executor and lock:

ExecutorService executor = Executors.newFixedThreadPool(NUMBER_OF_THREADS);
ReadWriteLock lock = new ReentrantReadWriteLock();

And when you want to queue tasks:

// a write task:    
executor.execute(new WriteTask(lock));

// a read task:
executor.execute(new ReadTask(lock));

Just make sure you always unlock the locks; you can use a try ... finally block if necessary but really since run() shouldn't throw any exceptions, you shouldn't be running into issues if you handle everything properly within run() as you should.