EclipseLink/Examples/JPA/Caching

User's Guide

How to use EclipseLink Caching

By default EclipseLink uses a shared object cache, that caches a subset of all objects read and persisted for the persistence unit. The EclipseLink shared cache differs from the local EntityManager cache. The shared cache exists for the duration of the persistence unit (EntityManagerFactory, or server) and is shared by all EntityManagers and users of the persistence unit. The local EntityManager cache is not shared, and only exists for the duration of the EntityManager or transaction.

The benefit of the shared cache, is that once an object has been read, if it is read again using the find operation, the database does not need to be accessed. Also if the object is read through any Query, it will not need to be rebuilt, and its relationships will not need to be re-fetched.

The limitation of the shared cache, is that if the database is changed directly through JDBC, or by another application or server, the objects in the shared cache will be stale.

EclipseLink offers several mechanism to deal with stale data including:

• Refreshing
• Invalidation
• Optimistic locking
• Cache coordination

The shared cache can also be disabled. This can be done using the persistence unit property:

<property name="eclipselink.cache.shared.default" value="false"/>

Or, in JPA 2.0 the shared-cache-mode element in the persistence.xml can be used,

<shared-cache-mode>NONE</shared-cache-mode>

Or can be selectively enabled/disabled using the @Cache annotation. Or in JPA 2.0 using the @Cacheable annotation.

EclipseLink also offers several different caching strategies, the configure how many objects are cached, and how much memory is used.

Note, the CacheType NONE on the @Cache annotation should not be used to disable the cache, instead shared should be set to false.

See Introduction to Cache for details on these types.

How to refresh the cache

If the application knows the cache is out of date, it can clear, refresh or invalidate it programmatically.

If JPA 2.0 is used, this can be done using the JPA Cache interface:

em.getEntityManagerFactory().getCache().evictAll();

In JPA 1.0 this can be done using the EclipseLink IdentityMapAccessor:

((JpaEntityManager)em.getDelegate()).getServerSession().getIdentityMapAccessor().invalidateAll();

Both of these invalidate all of the objects in the cache. The cache can also be cleared, clearing the cache can cause object identity issues if any of the cached object is in use, so invalidating is safer. If you know that none of the cached objects are is use then you can just clear the cache.

((JpaEntityManager)em.getDelegate()).getServerSession().getIdentityMapAccessor().initializeAllIdentityMaps();

EclipseLink provides more cache API, see JpaCache and IdentityMapAccessor.

None of these actively refresh the cached objects, the objects will not be refreshed until next accessed.

To actively refresh an object the EntityManager refresh() API can be used. However this only refreshes a single object.

In JPA 2.0 a query hint can be used to trigger any query to refresh the cache.

Query query = em.createQuery("Select e from Employee e");
query.setHint("javax.persistence.cache.storeMode", "REFRESH");

In JPA 1.0 this can be done using the EclipseLink refresh query hint:

Query query = em.createQuery("Select e from Employee e");
query.setHint("eclipselink.refresh", "true");

Using the @Cache Annotation

You may define the @Cache annotation on the following:

• @Entity
• @MappedSuperclass;
• the root of the inheritance hierarchy (if applicable).
  

If you define the @Cache annotation on an inheritance subclass, the annotation will be ignored. If you define the @Cache annotation on @Embeddable EclipseLink will throw an exception.

Example of @Cache Annotation

...
@Entity
@Cache(
  type=CacheType.SOFT, // Cache everything until the JVM decides memory is low.
  size=64000  // Use 64,000 as the initial cache size.
  expiry=360000,  // 6 minutes
  coordinationType=CacheCoordinationType.INVALIDATE_CHANGED_OBJECTS  // if cache coordination is used, only send invalidation messages.
)
public class Employee {
  ...
}

For more information on @Cache options and configuring caching, see Configuring Caching.

Caching in Clustered Environments

Caching in a clustered environment can have issues as changes made on one server will not be reflected on objects cached in other servers. For read-only objects, this is not an issue, but for objects that are frequently updated this can be an issue.

EclipseLink offers several solutions to this issue.

• The cache can be disabled for the classes that frequently change.
• Cache coordination can be used to broadcast changes between the servers in the cluster to update of invalidate changed objects.
• Cache invalidation based on time-to-live or time-of-day.
• Optimistic locking will prevent updates to stale objects, and trigger the objects to be invalidated in the cache.

See, How to enable cache coordination