Isolation Level Cache
Summary: This cache is sometimes not really called a cache. However, in order to implement certain isolation levels the database itself may be caching some query results.
Lifecycle/Scope: This cache is scoped to a single Session/EntityManager. The lifecycle is bound to the transaction lifecycle.
Clearing the cache: No way I know of other than starting a new transaction
What gets cached: Queries and result (if isolation is at repeatable read or serializable level)
On by default: Depends on the default isolation level which comes from the database. By default, MySQL ships with repeatable read isolation and so yes, this is on by default for MySQL.
Turning it on/off: Can be specified when creating the transaction. Can also be changed by changing the default on the database.
Useful Information: Hibernate/JPA doesn't really have any control over the operation of this cache other than specifying which isolation level is desired.
Session Level (1st-Level) Cache
Summary: This cache is the EntityManager/Session cache. I believe this is also what is referred to as the persistence context.
Lifecycle/Scope: This cache is scoped to a single Session/EntityManager. The lifecycle is bound to the transaction lifecycle.
Clearing the cache: Calling clear() on the EntityManager or Session clears the entire cache. Calling evict() on the Session clears a single object from the cache.
What gets cached: Everything
On by default: Yes
Turning it on/off: Can't be turned off
Useful Information: This cache gets merged with the database whenever flush() is called. Unless that happens other transactions will not be able to see things in this cache. The best way to guarantee a flush() is to commit the transaction.
2nd-Level Cache
Summary: This is a secondary cache that can be enabled (usually to try and improve performance).
Lifecycle/Scope: I believe this is bound to the EntityManagerFactory/SessionFactory. Automatic eviction of this cache depends on the cache strategy. In a read-only strategy data is never evicted automatically. In a read-write or nostrict read-write strategy data will be evicted when the session closes. Not 100% certain of this.
Clearing the cache: You can call getCache().evict(class) to evict a specific class and getCache().evictAll() to evict the entire cache. These methods are on the EntityManagerFactory.
What gets cached: You explicitly configure which entities should be cached.
On by default: No
Turning it on/off: Turned on/off in the Hibernate configuration
Useful Information:
Query Cache
Summary: Query Cache is a cache which stores queries, query parameters and results. If the query and query parameters are the same, you can expect the result to be the same.
Lifecycle/Scope: I have no idea when data in this cache is determined to be stale. I believe the scope is at the EntityManagerFactory/SessionFactory level. In addition, Hibernate keeps a list of "last update by Hibernate" timestamps for each of the tables. Hibernate uses these timestamps to determine if query results are stale and evict stale queries automatically.
Clearing the cache: The evictQueries() method on the SessionFactory can be used to manually evict the query cache.
What gets cached: Queries and their results
On by default: No
Turning it on/off: Turned on/off in the Hibernate configuration
Useful Information: The query cache only caches entity IDs. It must be used in conjunction with a 2nd-level cache to achieve a true (no DB access) cache.
