多线程 ： 读写锁实现缓存系统

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

public class CacheSystem {

public static void main(String[] args) {

// 测试缓存器
final CacheSystem cache = new CacheSystem();

for (int i = 0; i < 3; i++) {
new Thread(new Runnable() {
public void run() {
while (true) {
int num = new Random().nextInt(10);
String key = num + "";
Object result = cache.get(key);
System.out.println(Thread.currentThread().getName() + "正在查询：" + key);
try {
Thread.sleep(num * 1000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName() + "查询结果为：" + key + "=" + result);
}
}
}).start();
}
}

// 内部存储器
private Map<String, Object> cache = new HashMap<String, Object>();

private ReadWriteLock rwl = new ReentrantReadWriteLock();

public Object get(String key) {
// 上读锁
rwl.readLock().lock();
Object value = cache.get(key);
if (value == null) {
// 释放读锁，加写锁
rwl.readLock().unlock();
rwl.writeLock().lock();
if (value == null) {
value = "value";
cache.put(key, value);
}
// 还原读锁，释放写锁
rwl.readLock().lock();
rwl.writeLock().unlock();
}
// 释放读锁
rwl.readLock().unlock();
return value;
}
}