12.8.2. Second Level Caching of Object References for Non-mutable Data

Hibernate automatically caches data within memory for improved performance. This is accomplished by an in-memory cache which reduces the number of times that database lookups are required, especially for data that rarely changes.

Hibernate maintains two types of caches. The primary cache (also called the first-level cache) is mandatory. This cache is associated with the current session and all requests must pass through it. The secondary cache (also called the second-level cache) is optional, and is only consulted after the primary cache has been consulted first.

Data is stored in the second-level cache by first disassembling it into a state array. This array is deep copied, and that deep copy is put into the cache. The reverse is done for reading from the cache. This works well for data that does not change (mutable data), but is inefficient for immutable data.

Deep copying data is an expensive operation in terms of memory usage and processing speed. For large data sets, memory and processing speed become a performance-limiting factor. Hibernate allows you to specify that immutable data be referenced rather than copied. Instead of copying entire data sets, Hibernate can now store the reference to the data in the cache.

This can be done by changing the value of the configuration setting hibernate.cache.use_reference_entries to true. By default, hibernate.cache.use_reference_entries is set to false.

When hibernate.cache.use_reference_entries is set to true, an immutable data object that does not have any associations is not copied into the second-level cache, and only a reference to it is stored.