Java Implement locking mechanism with persistent storage

Question

I have implemented Groovy / Java JAR file with methods to install apps on mobile devices as well as get properties of the apps installed on a mobile device and the properties of a device.

I would like to implement a read and write locking mechanism that allows only a certain number of users access to a device. The write lock would be: 1 request per mobile device and for a read lock 10 requests per mobile device.

Java Semaphore seems like a good way to solve this allowing a number of requests allocated for a read and write lock. But I also need to store the states of each lock acquired against each device persistently.

Any advice on how to do this even without Semaphores would be appreciated. I anticipate less than 50 devices and less than 100 concurrent user requests.

Answer 1

You can use ReentrantReadWriteLock with BlockingQueue. ReentrantReadWriteLock is used to allow one writer, while BlockingQueue limits N readers to read at the same time. Something like this:

import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.locks.ReentrantReadWriteLock;

public class DeviceHandler
{
    private ReentrantReadWriteLock reentrantReadWriteLock = new ReentrantReadWriteLock();
    private BlockingQueue<Object> readersQueue = new ArrayBlockingQueue<>(10);

    public void read() throws InterruptedException
    {
        // if queue is full, this blocks until another thread removes an object
        readersQueue.put(new Object());
        // this blocks if another thread acquires the write lock
        reentrantReadWriteLock.readLock().lock();

        // do read action

        reentrantReadWriteLock.readLock().unlock();
        readersQueue.take();
    }

    public void write()
    {
        // this blocks if the read lock is acquired by other threads
        reentrantReadWriteLock.writeLock().lock();

        // do write action

        reentrantReadWriteLock.writeLock().unlock();
    }
}