JAVA线程锁-读写锁应用，简单的缓存系统

在JAVA1.5版本以后，JAVA API中提供了ReadWriteLock，此类是一个接口，在它的实现类中ReentrantReadWriteLock中有这样一段代码

class CachedData {
Object data;
volatile boolean cacheValid;
ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();

void processCachedData() {
rwl.readLock().lock();
if (!cacheValid) {
// Must release read lock before acquiring write lock
rwl.readLock().unlock();
rwl.writeLock().lock();
// Recheck state because another thread might have acquired
//   write lock and changed state before we did.
if (!cacheValid) {
data = ...
cacheValid = true;
}
// Downgrade by acquiring read lock before releasing write lock
rwl.readLock().lock();
rwl.writeLock().unlock(); // Unlock write, still hold read
}

use(data);
rwl.readLock().unlock();
}
}

以上代码是一个简单的缓存实现方式，根据以上思路写出如下代码

import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

public class CacheSystemTest {

private Map<String, Object> cacheMap = new HashMap<String, Object>();

private ReadWriteLock rwl = new ReentrantReadWriteLock();

/**
* 为了防止在多线程情景下数据安全问题，需要线程互斥，实现方式是用锁
* @param key
* @return
*/
public Object get(String key){
rwl.readLock().lock();    //任何一个线程进来后，第一时间加上读锁
Object obj = null;
try{
obj = cacheMap.get(key);
if(obj == null){
rwl.readLock().unlock();    //在赋值前关闭读锁，并在此检查对象
if(obj == null){
rwl.writeLock().lock();    //打开一个写锁
try{
obj = "read write lock";
}finally{
rwl.writeLock().unlock();    //关闭写锁
rwl.readLock().lock();    //恢复正常读锁
}
}
}
}finally{
rwl.readLock().unlock();
}
return obj;
}
}

注：此锁最多支持65535个递归写入锁和读锁，如果试图超出则抛出Error错误