The Readers/Writers Problem priority

Question

I developed a solution for the reader/writer problem in java (some information on this http://lass.cs.umass.edu/~shenoy/courses/fall08/lectures/Lec11.pdf ).

However I am not sure how to modify the code to either favor writers or give both reader and writer same priority. What type of this problem is my code and how do I see it?

public class ReaderWriter{


    int numberOfReaders;
    int numberOfWriters;

    public ReaderWriter(){
        this.numberOfReaders = 0;
        this.numberOfWriters = 0;
    }

    public synchronized void requestRead(){
        while(this.numberOfWriters > 0)
            wait();

        this.numberOfReaders++;
    }

    public synchronized void releaseRead(){
        this.numberOfReaders--;
        notifyAll();
    }

    public synchronized void requestWrite(){
        while(this.numberOfReaders > 0 || this.numberOfWriters > 0)
            wait();

        this.numberOfWriters++; 
    }

    public synchronized void releaseWrite(){
        this.numberOfWriters--;
        notifyAll();
    }
}

Answer 1

Your version favors readers (and can starve writers). Here is a modified version that favors writers (and can starve readers):

    int numberOfReaders;
    int numberOfWriters;
    int numberOfRequestedWriters;  // ++

    public ReaderWriter(){
        this.numberOfReaders = 0;
        this.numberOfWriters = 0;
        this.numberOfRequestedWriters = 0; // ++
    }

    public synchronized void requestRead(){
        while(this.numberOfWriters > 0  // --
              || this.numberOfRequestedWriters > 0) // ++
            wait();

        this.numberOfReaders++;
    }

    public synchronized void releaseRead(){
        this.numberOfReaders--;
        notifyAll();
    }

    public synchronized void requestWrite(){
        this.numberOfRequestedWriters++; // ++
        while(this.numberOfReaders > 0 || this.numberOfWriters > 0)
            wait();
        this.numberOfRequestedWriters--; // ++

        this.numberOfWriters++; 
    }

    public synchronized void releaseWrite(){
        this.numberOfWriters--;
        notifyAll();
    }
}

Answer 2

To control how locks are distributed, you need to maintain a queue of waiting threads. Unless the section of code you are protecting is expensive, a simple lock may be better.