Java读写锁问题

前几天在对HBase Client进行优过程中，需要扩展HTable，需要加一个Timer来对Client buffer进行定时的flush。由于HTable不是线程安全的，所以为扩展的HTable使用了Java ReentrantReadWriteLock来进行读写锁。在不同的方法中加了读锁或者写锁，从而导致了死锁的问题。

看了一下文档，Java的读写锁只能downgrading不能upgrading，导致死锁的问题就是upgrading导致的。

下面是一个锁upgrading导致死锁的例子：

 ReentrantReadWriteLock lock = new ReentrantReadWriteLock();        lock.readLock().lock();        try{            System.out.println("Got a read lock");            lock.writeLock().lock();//want to upgrade from read lock to write, here is the deadlock            try{                System.out.println("Got a write lock");            } finally {                lock.writeLock().unlock();                System.out.println("Unlock a write lock");            }        } finally {            lock.readLock().unlock();            System.out.println("Unlock a read lock");        }

下面是一个锁downgrading的例子：

ReentrantReadWriteLock lock = new ReentrantReadWriteLock();        lock.writeLock().lock();        try{            System.out.println("Got a write lock");            lock.readLock().lock();//Downgrade from write lock to read lock, it's ok.            try{                System.out.println("Got a read lock");            } finally {                lock.readLock().unlock();                System.out.println("Unlock a read lock");            }        } finally {            lock.writeLock().unlock();            System.out.println("Unlock a write lock");        }