ReadWriteLock

多线程读写同一个对象的数据是很普遍的，通常，要避免读写冲突，必须保证任何时候仅有一个线程在写入，有线程正在读取的时候，写入操作就必须等待。

要实现以上的ReadWriteLock，简单的使用synchronized就不行，我们必须自己设计一个ReadWriteLock类，在读之前，必须先获得“读锁”，写之前，必须先获得“写锁”。举例说明：

DataHandler对象保存了一个可读写的char[]数组：

package com.crackj2ee.thread;

public class DataHandler {
// store data:
private char[] buffer = "AAAAAAAAAA".toCharArray();

private char[] doRead() {
char[] ret = new char[buffer.length];
for(int i=0; i ret[i] = buffer[i];
sleep(3);
}
return ret;
}

private void doWrite(char[] data) {
if(data!=null) {
buffer = new char[data.length];
for(int i=0; i buffer[i] = data[i];
sleep(10);
}
}
}

private void sleep(int ms) {
try {
Thread.sleep(ms);
}
catch(InterruptedException ie) {}
}
}

doRead()和doWrite()方法是非线程安全的读写方法。为了演示，加入了sleep()，并设置读的速度大约是写的3倍，这符合通常的情况。

为了让多线程能安全读写，我们设计了一个ReadWriteLock：

package com.crackj2ee.thread;
public class ReadWriteLock {
private int readingThreads = 0;
private int writingThreads = 0;
private int waitingThreads = 0; // waiting for write
private boolean preferWrite = true;

public synchronized void readLock() throws InterruptedException {
while(writingThreads>0 || (preferWrite && waitingThreads>0))
this.wait();
readingThreads++;
}

public synchronized void readUnlock() {
readingThreads--;
preferWrite = true;
notifyAll();
}

public synchronized void writeLock() throws InterruptedException {
waitingThreads++;
try {
while(readingThreads>0 || writingThreads>0)
this.wait();
}
finally {
waitingThreads--;
}
writingThreads++;
}

public synchronized void writeUnlock() {
writingThreads--;