We start with the database.

/**
 * This class represents a database.  There are many competing
 * threads wishing to read and write.  It is acceptable to have
 * multiple processes reading at the same time, but if one thread
 * is writing then no other process may either read or write.
 */
public class Database
{
    private int readers; // number of active readers
 
    /**
       Initializes this database.
    */
    public Database()
    {
        this.readers = 0;
    }
 
    /**
       Read from this database.
 
       @param number Number of the reader.
    */
    public void read(int number)
    {
        synchronized(this)
        {
            this.readers++;
            System.out.println("Reader " + number + " starts reading.");
        }
 
        final int DELAY = 5000;
        try
        {
            Thread.sleep((int) (Math.random() * DELAY));
        }
        catch (InterruptedException e) {}
 
        synchronized(this)
        {
            System.out.println("Reader " + number + " stops reading.");
            this.readers--;
            if (this.readers == 0)
            {
                this.notifyAll();
            }
        }
    }
 
    /**
       Writes to this database.
 
       @param number Number of the writer.
    */
    public synchronized void write(int number)
    {
        while (this.readers != 0)
        {
            try
            {
                this.wait();
            }
            catch (InterruptedException e) {}
        }
        System.out.println("Writer " + number + " starts writing.");
 
        final int DELAY = 5000;
        try
        {
            Thread.sleep((int) (Math.random() * DELAY));
        }
        catch (InterruptedException e) {}
 
        System.out.println("Writer " + number + " stops writing.");
        this.notifyAll();
    }
}

Next, we present the reader.

Next, we present the writer.

Finally, we present the simulator.