读者写者-【写者优先】

1. 实现原理

首先设计两个线程，一个读者线程，一个写者线程，两个线程

读者写者问题是这样一种情况，就是允许对资源进行两种类型的操作（读和写）。而写操作是排他的（排斥读者，同样排斥其他写者），读操作是共享的（多个读者可读，排斥写者）。这就存在两种策略：1.读者优先（强读者同步Strong reader synchronization）：总是给读者优先权，只要写者当前没有进行写操作，读者就能获得访问权。这种情况存在于读者很多，写者不经常更新的时候使用，如图书馆参考数据库采用读者优先比较好。2.写者优先（强写者同步Strong writer synchronization）：通常把优先权交给写者，而将读者延迟到所有等待的或活动的写者都完成了为止。这种情况存在于经常更新的系统，而读者对最新信息感兴趣，如机票预定系统，写者进行机票价格的更新，读者获取当前机票价。简单的说就是资源没人占用，则谁先到先用，用的时候上锁，读者上读者锁，写者上写者锁。1.假设先来的读者上读者锁，接下来再a）来读者，可以读资源，将读者计数器+1，b）来写者则等待（阻塞或者不断尝试）直到读者释放资源（计数器为0），选取一个写者开始写2.若先来的是写者上写者锁，这时候又来了比如N个读者，M个写者等待（写者锁排他），这时候要是a）读者优先，则写者释放锁后，让等候的N个读者先读（其他写者接着等着等，变为情况1），这时候要是b）写者优先，则写者释放锁后，让等候的M个中的一个写者写，读者接着等待直到没有写者为止（等的的M个写者全写完还有中途再来的写者）。

所以 是讨论读者优先还是写者优先的前提是 ：先来的是写者上写者锁读者写者问题本身就存在着饿死的情况。

2. 上代码：

缓冲区类

package com.bankht.writerReader;/** * @author zhuyong * @创建时间：2012-6-7 下午02:32:02 *  * @类说明 ：缓冲区 */public class CubbyHole {private int readerCount;// 读者数private int writerCount;// 写者数private boolean dbReading;// 读信号量private boolean dbWriting;// 写信号量public CubbyHole() {readerCount = 0;writerCount = 0;dbReading = false;dbWriting = false;}public static void napping() {// 线程睡眠，不消耗CPU资源try {Thread.sleep((int) (Math.random() * 4000));} catch (Exception e) {}}public synchronized int startRead() {// 开始读while (writerCount > 0) {try {System.out.println("reader is waiting");wait();// 等待写者发出notify} catch (Exception e) {}}++readerCount;if (readerCount >= 1) {dbReading = true;}return readerCount;}public synchronized int endReading() {// 结束读--readerCount;if (readerCount == 0) {dbReading = false;}notifyAll();System.out.println("one reader is done reading. Count=" + readerCount);return readerCount;}public synchronized void startWriting() {// 开始写++writerCount;while (dbReading == true || dbWriting == true) {try {System.out.println("Writer is waiting");wait();// 等待读者发出notify} catch (Exception e) {}}dbWriting = true;}public synchronized void endWriting() {// 结束写--writerCount;dbWriting = false;System.out.println("one writer is done writing. Count=" + writerCount);notifyAll();}}

读者类：

package com.bankht.writerReader;/** * @author zhuyong * @创建时间：2012-6-7 下午02:32:51 *  * @类说明 ：读者 */public class Reader extends Thread {// 定义读线程，继承Thread类，重写run方法private CubbyHole C;private int readerNum;public Reader(int r, CubbyHole db) {readerNum = r;C = db;}public void run() {int c;while (true) {System.out.println("reader " + readerNum + " is sleeping");CubbyHole.napping();System.out.println("reader " + readerNum + " wants to read");c = C.startRead();System.out.println("reader " + readerNum + " is reading. Count="+ c);CubbyHole.napping();c = C.endReading();System.out.println("It is reader " + readerNum+ " who has done reading. count=" + c);}}}

写者类：

package com.bankht.writerReader;/** * @author zhuyong * @创建时间：2012-6-7 下午02:34:36 *  * @类说明 ：写者 */public class Writer extends Thread {// 定义写线程private CubbyHole C;private int writerNum;public Writer(int w, CubbyHole db) {writerNum = w;C = db;}public void run() {while (true) {System.out.println("Writer " + writerNum + " is sleeping");CubbyHole.napping();System.out.println("Writer " + writerNum + " wants to write");C.startWriting();System.out.println("Writer " + writerNum + " is writing");CubbyHole.napping();C.endWriting();System.out.println("It is Writer " + writerNum+ " who has done writing .");}}}

测试类：

package com.bankht.writerReader;/** * @author zhuyong * @创建时间：2012-6-7 下午02:31:32 *  * @类说明 ： */public class WriterReader {public static void main(String[] args) {testWriterReader();}public static void testWriterReader() {CubbyHole db = new CubbyHole();// 缓冲区Reader r1 = new Reader(1, db);// 读者Reader r2 = new Reader(2, db);Reader r3 = new Reader(3, db);Reader r4 = new Reader(4, db);Writer w1 = new Writer(1, db);// 写者Writer w2 = new Writer(2, db);r1.start();r2.start();r3.start();w1.start();r4.start();w2.start();}}

运行结果：

reader 1 is sleepingreader 3 is sleepingreader 2 is sleepingreader 4 is sleepingWriter 1 is sleepingWriter 2 is sleepingWriter 1 wants to writeWriter 1 is writingreader 1 wants to readreader is waitingreader 2 wants to readreader is waitingone writer is done writing. Count=0It is Writer 1 who has done writing .reader 1 is reading. Count=2reader 2 is reading. Count=1Writer 1 is sleepingreader 3 wants to readreader 3 is reading. Count=3Writer 1 wants to writeWriter is waitingWriter 2 wants to writeWriter is waitingone reader is done reading. Count=2Writer is waitingIt is reader 1 who has done reading. count=2Writer is waitingreader 1 is sleepingreader 4 wants to readreader is waitingone reader is done reading. Count=1It is reader 2 who has done reading. count=1reader is waitingreader 2 is sleepingWriter is waitingWriter is waitingreader 1 wants to readreader is waitingone reader is done reading. Count=0It is reader 3 who has done reading. count=0reader is waitingreader 3 is sleepingWriter is waitingWriter 2 is writingreader is waitingreader 2 wants to readreader is waitingone writer is done writing. Count=1reader is waitingIt is Writer 2 who has done writing .reader is waitingWriter 2 is sleepingreader is waitingWriter 1 is writingreader 3 wants to readreader is waitingWriter 2 wants to writeWriter is waitingone writer is done writing. Count=1It is Writer 1 who has done writing .reader is waitingWriter 2 is writingreader is waitingWriter 1 is sleepingreader is waitingreader is waiting