原子替换Java中List中所有内容的最佳方法是什么？

Question

Suppose that I have a List with role of cache. Most time list is read-only buy every few seconds I want to do atomic replacement of all List contents.

In atomic I mean that I don't want to allow cache clients to hit read between for example clear() and addAll() .

What list implementation to use and how to perform replacement for best performance?

It is better to replace list contents or to replace reference value itself?

Answer 1

Java has a ReadWriteLock which supports reads concurrently and writes exclusively. As mentioned in the JavaDoc, it is a good choice if updates occur not very frequently and reads occur often. The faster your writer updates the List the better the performance you get.

The methods readLock() has to be called by readers and writeLock() by writers. Then you have to call lock() on the Lock obtained. If it is available, the tread will continue working, otherwise it will block, until the lock is available.

Use fairness when constructing the ReadWriteLock to enable reader and writer threads to obtain their locks in the order they requested it. Otherwise some thread could wait forever (in a worst case scenario).

The benefit of a ReadWriteLock is that many reader may share the same lock without obtaining it, which is an expansive operation. This benefit is only observable if the ratio between reading and writing is heavily in favor of reading.

Answer 2

If you really want to use the List<> type for a cache, you can use a lock like:

private static final Objet cacheLock  = new Object();


 public void updateListCache()
 {
    synchronized(lock)
    {
       //update your list cache
    }

 }

Your read operation would also need to implement this lock, or it wouldn't be much use.

But for caching purposes I would personally go with ConcurrentHashMap since it's thread safe.