java 细粒度的锁

private KeyLock<String> lock = new KeyLock<String>();



lock.lock(request.getSession().getId());


finally {
			lock.unlock(session.getId());
		}

public class KeyLock<K>{  
    // 保存所有锁定的KEY及其信号量  
    private final ConcurrentMap<K, Semaphore> map = new ConcurrentHashMap<K, Semaphore>();  
    // 保存每个线程锁定的KEY及其锁定计数  
    private final ThreadLocal<Map<K, LockInfo>> local = new ThreadLocal<Map<K, LockInfo>>() {  
        @Override  
        protected Map<K, LockInfo> initialValue() {  
            return new HashMap<K, LockInfo>();  
        }  
    };  
  
    /** 
     * 锁定key，其他等待此key的线程将进入等待，直到调用{@link #unlock(K)} 
     * 使用hashcode和equals来判断key是否相同，因此key必须实现{@link #hashCode()}和 
     * {@link #equals(Object)}方法 
     *  
     * @param key 
     */  
    public void lock(K key) {  
        if (key == null)  
            return;  
        LockInfo info = local.get().get(key);  
        if (info == null) {  
            Semaphore current = new Semaphore(1);  
            current.acquireUninterruptibly();  
            Semaphore previous = map.put(key, current);  
            if (previous != null)  
                previous.acquireUninterruptibly();  
            local.get().put(key, new LockInfo(current));  
        } else {  
            info.lockCount++;  
        }  
    }  
      
    /** 
     * 释放key，唤醒其他等待此key的线程 
     * @param key 
     */  
    public void unlock(K key) {  
        if (key == null)  
            return;  
        LockInfo info = local.get().get(key);  
        if (info != null && --info.lockCount == 0) {  
            info.current.release();  
            map.remove(key, info.current);  
            local.get().remove(key);  
        }  
    }  
  
    /** 
     * 锁定多个key 
     * 建议在调用此方法前先对keys进行排序，使用相同的锁定顺序，防止死锁发生 
     * @param keys 
     */  
    public void lock(K[] keys) {  
        if (keys == null)  
            return;  
        for (K key : keys) {  
            lock(key);  
        }  
    }  
  
    /** 
     * 释放多个key 
     * @param keys 
     */  
    public void unlock(K[] keys) {  
        if (keys == null)  
            return;  
        for (K key : keys) {  
            unlock(key);  
        }  
    }  
  
    private static class LockInfo {  
        private final Semaphore current;  
        private int lockCount;  
  
        private LockInfo(Semaphore current) {  
            this.current = current;  
            this.lockCount = 1;  
        }  
    }  
}