Read Write Lock

        读的时候，实例的状态不会改变，“写”会改变线程的状态。

        这种模式，把读写分开，读之前获取锁定，不给写；写之前获取锁定，不给读。

        示例：WriteThread和ReadThread不断的读写公共的data。类Data中，有读写操作，他有一个lock，读写之前都会进行锁定。这个锁ReadWriteLock是按照如下实现的：

         如果真正写的数量大于0或者等待优先的等待写入者大于0，那么想读的线程进入等待状态，直到可以读为止，然后读入，这时读入人数加1，读完后如如人数减去1，使优先写入为真，并通知在这个对象中的所有等待线程。写的过程与这个类似。

public class ReadWriteLock extends Thread{
	private int readingReaders=0;  //实际在读取的线程数
	private int writingWriters=0;    //实际写入的线程数
	private int waitingWriters=0;  //等待的线程数
	private boolean preferWriter=true; //是否写入优先
	
	public synchronized void readLock() throws InterruptedException{
		while(writingWriters>0||(preferWriter &&waitingWriters>0)){
			wait();
		}
		readingReaders++;
	}
	public synchronized void readUnlock(){
		readingReaders--;
		preferWriter=true;
		notifyAll();
	}
	public synchronized void writeLock() throws InterruptedException{
		waitingWriters++;
		try{
			while(readingReaders>0||writingWriters>0){
				wait();
			}
		}finally{
			waitingWriters--;
		}
		writingWriters++;
	}
	public synchronized void writeUnlock(){
		writingWriters--;
		preferWriter=false;
		notifyAll();
	}
}

 

public class ReadThread extends Thread {
	private final Data data;
	public ReadThread(Data data){
		this.data=data;
	}
	public void run(){
		try{
			while(true){
				char[] readbuf=data.read();
				System.out.println(Thread.currentThread().getName()+" reads "+String.valueOf(readbuf));
			}
		}catch(InterruptedException e){
			e.printStackTrace();
		}
	}

}

 

import java.util.Random;

public class WriteThread extends Thread{
	private static final Random random=new Random();
	private final Data data;
	private String filler;
	private int index;
	public WriteThread(Data data,String filler){
		this.data=data;
		this.filler=filler;
	}
	public void run(){
		try{
			while(true){
				char c=nextChar();
				data.write(c);
				Thread.sleep(3000);
			}
		}catch(InterruptedException e){
			e.printStackTrace();
		}
	}
	private char nextChar() {
		char c=filler.charAt(index);
		index++;
		if(index>=filler.length()){
			index=0;
		}
		return c;
	}

}

 

public class Main {
    public static void main(String[] arg){
    	Data data=new Data(10);
    	new ReadThread(data).start();
    	new ReadThread(data).start();
    	new ReadThread(data).start();
    	new ReadThread(data).start();
    	new ReadThread(data).start();
    	new ReadThread(data).start();
    	new WriteThread(data,"ABCDEFGHIJKLMNOPQRSTUVWXYZ").start();
    	new WriteThread(data,"abcdefghijklmnopqrstuvwxyz").start();
    }
}