Overview of the Second-Level Cache

A second-level cache is a local store of entity data managed by the persistence provider to improve application performance. A second-level cache helps improve performance by avoiding expensive database calls, keeping the entity data local to the application. A second-level cache is typically transparent to the application, as it is managed by the persistence provider and underlies the persistence context of an application. That is, the application reads and commits data through the normal entity manager operations without knowing about the cache.

Note:

Persistence providers are not required to support a second-level cache. Portable applications should not rely on support by persistence providers for a second-level cache.

The second-level cache for a persistence unit may be configured to one of several second-level cache modes. The following cache mode settings are defined by the Java Persistence API.

Table 47-1 Cache Mode Settings for the Second-Level Cache

Cache Mode Setting

Description

ALL

All entity data is stored in the second-level cache for this persistence unit.

NONE

No data is cached in the persistence unit. The persistence provider must not cache any data.

ENABLE_SELECTIVE

Enable caching for entities that have been explicitly set with the @Cacheable annotation.

DISABLE_SELECTIVE

Enable caching for all entities except those that have been explicitly set with the @Cacheable(false) annotation.

UNSPECIFIED

The caching behavior for the persistence unit is undefined. The persistence provider’s default caching behavior will be used.

One consequence of using a second-level cache in an application is that the underlying data may have changed in the database tables, while the value in the cache has not, a circumstance called a stale read. To avoid stale reads, use any of these strategies:

Change the second-level cache to one of the cache mode settings
Control which entities may be cached (see Controlling whether Entities May Be Cached)
Change the cache’s retrieval or store modes (see Setting the Cache Retrieval and Store Modes)

Which of these strategies works best to avoid stale reads depends upon the application.

Controlling whether Entities May Be Cached

The javax.persistence.Cacheable annotation is used to specify that an entity class, and any subclasses, may be cached when using the ENABLE_SELECTIVE or DISABLE_SELECTIVE cache modes. Subclasses may override the @Cacheable setting by adding a @Cacheable annotation and changing the value.

To specify that an entity may be cached, add a @Cacheable annotation at the class level:

@Cacheable
@Entity
public class Person { ... }

By default, the @Cacheable annotation is true. The following example is equivalent:

@Cacheable(true)
@Entity
public class Person{ ... }

To specify that an entity must not be cached, add a @Cacheable annotation and set it to false:

@Cacheable(false)
@Entity
public class OrderStatus { ... }

When the ENABLE_SELECTIVE cache mode is set, the persistence provider will cache any entities that have the @Cacheable(true) annotation and any subclasses of that entity that have not been overridden. The persistence provider will not cache entities that have @Cacheable(false) or have no @Cacheable annotation. That is, the ENABLE_SELECTIVE mode will cache only entities that have been explicitly marked for the cache using the @Cacheable annotation.

When the DISABLE_SELECTIVE cache mode is set, the persistence provider will cache any entities that do not have the @Cacheable(false) annotation. Entities that do not have @Cacheable annotations, and entities with the @Cacheable(true) annotation, will be cached. That is, the DISABLE_SELECTIVE mode will cache all entities that have not been explicitly prevented from being cached.

If the cache mode is set to UNDEFINED, or is left unset, the behavior of entities annotated with @Cacheable is undefined. If the cache mode is set to ALL or NONE, the value of the @Cacheable annotation is ignored by the persistence provider.