读写锁接口实例-通过ReadWriteLock实现一个简单的缓存

读写锁的特征是写锁必须以独占的方式获取，读锁可以共享

利用读写锁实现的缓存实例，代码如下

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

public class Cache {
    private static final Map    map = new HashMap();
    private static final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();
    private static final Lock                   r   = rwl.readLock();
    private static final Lock                   w   = rwl.writeLock();

    public static final Object get(String key) {
        r.lock();
        try {
            return map.get(key);
        } finally {
            r.unlock();
        }
    }

    public static final Object put(String key, Object value) {
        w.lock();
        try {
            return map.put(key, value);
        } finally {
            w.unlock();
        }
    }

    public static final void clear() {
        w.lock();
        try {
            map.clear();
        } finally {
            w.unlock();
        }
    }
}