IPC之读者写者问题：写者优先

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

public class WriterFirst {
    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 rmutex = new Semaphore(1);
    private static Semaphore wmutex = new Semaphore(1);
    private static Semaphore readable = new Semaphore(1);

    private static int data = 0;
    private static int wcount = 0;
    private static int rcount = 0;

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

                    rmutex.acquire();
                    rcount++;
                    rmutex.release();

                    readable.release();

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

                    rmutex.acquire();
                    rcount--;
                    rmutex.release();
                    
                    Thread.sleep((int)(Math.random() * 1000));
                }
            }

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

    private static class Writer implements Runnable 
    {
        public void run()
        {
            try 
            {
                while ( true )
                {
                    mutex.acquire();
                    
                    wmutex.acquire();
                    wcount++;
                    if ( wcount == 1 )
                        readable.acquire();
                    wmutex.release();

                    data = (int)(Math.random() * 1231);
                    System.out.println("\t\t\twrite " + data);

                    wmutex.acquire();
                    wcount--;
                    if ( wcount == 0 )
                        readable.release();
                    wmutex.release();

                    mutex.release();

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

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