JPA cache

 PersistenceContext  Level2 Cache Extend PersistenceContext .
 二级缓存优缺点The advantages of L2 caching are:

    * avoids database access for already loaded entities
    * faster for reading frequently accessed  unmodified entities

The disadvantages of L2 caching are:

    * memory consumption for large amount of objects
    * Stale data for updated objects
    * Concurrency for write (optimistic lock exception, or pessimistic lock)
    * Bad scalability for frequent or concurrently updated entities


You should configure L2 caching for entities that are:

    * read often
    * modified infrequently
    * Not critical if stale

You should protect any data that can be concurrently modified with a locking strategy:

    * Must handle optimistic lock failures on flush/commit
    * configure expiration, refresh policy to minimize lock failures

The Query cache is useful for queries that are run frequently with the same parameters, for not modified table

具体参考：[url]http://weblogs.java.net/blog/archive/2009/08/21/jpa-caching[/url]