IPC之读者写者问题：读者优先

import java.util.concurrent.*;
import java.util.*;

public class ReaderFirst 
{
    public static void main(String[] args)
    {
        ExecutorService executor = Executors.newCachedThreadPool();
        executor.execute(new Reader());    
        executor.execute(new Writer());
        executor.shutdown();
    }

    private static Semaphore mutex = new Semaphore(1);
    private static Semaphore db = new Semaphore(1);

    private static int data;

    private static int readcount = 0;

    private static class Reader implements Runnable 
    {
        public void run()
        {
            try {
                    while ( true )
                    {
                        mutex.acquire();
                        readcount++;
                        if ( readcount == 1 )
                            db.acquire();
                        mutex.release();

                        System.out.println("read " + data);

                        mutex.acquire();
                        readcount--;
                        if ( readcount == 0 )
                            db.release();
                        mutex.release();

                        Thread.sleep((int)(Math.random() * 1000));
                    }
                }

            catch( InterruptedException ex )
            {
                ex.printStackTrace();
            }
        }
    }

    private static class Writer implements Runnable 
    {
        public void run()
        {
            try 
            {
                while ( true )
                {
                    db.acquire();

                    data = (int)(Math.random() * 2312);
                    System.out.println("\t\t\tWrite " + data);

                    db.release();

                    Thread.sleep((int)(Math.random() * 1000));
                }
            }
            
            catch( InterruptedException ex )
            {
                ex.printStackTrace();
            }
        }
    }
}