Using a Second-Level Cache with Jakarta Persistence Applications

This chapter explains how to modify the second-level cache mode settings to improve the performance of applications that use the Jakarta Persistence.

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.

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 Jakarta Persistence.

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:

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

Controlling whether Entities May Be Cached

The jakarta.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.

Specifying the Cache Mode Settings to Improve Performance

To adjust the cache mode settings for a persistence unit, specify one of the cache modes as the value of the shared-cache-mode element in the persistence.xml deployment descriptor (shown in bold):

<persistence-unit name="examplePU" transaction-type="JTA">
    <provider>org.eclipse.persistence.jpa.PersistenceProvider</provider>
    <jta-data-source>java:comp/DefaultDataSource</jta-data-source>
    <shared-cache-mode>DISABLE_SELECTIVE</shared-cache-mode>
</persistence-unit>
Because support for a second-level cache is not required by the Jakarta Persistence specification, setting the second-level cache mode in persistence.xml will have no effect when you use a persistence provider that does not implement a second-level cache.

Alternatively, you can specify the shared cache mode by setting the jakarta.persistence.sharedCache.mode property to one of the shared cache mode settings:

EntityManagerFactory emf =
    Persistence.createEntityManagerFactory(
        "myExamplePU", new Properties().add(
            "jakarta.persistence.sharedCache.mode", "ENABLE_SELECTIVE"));

Setting the Cache Retrieval and Store Modes

If you have enabled the second-level cache for a persistence unit by setting the shared cache mode, you can further modify the behavior of the second-level cache by setting the jakarta.persistence.cache.retrieveMode and jakarta.persistence.cache.storeMode properties. You can set these properties at the persistence context level by passing the property name and value to the EntityManager.setProperty method, or you can set them on a per-EntityManager operation (EntityManager.find or EntityManager.refresh) or on a per-query level.

Cache Retrieval Mode

The cache retrieval mode, set by the jakarta.persistence.retrieveMode property, controls how data is read from the cache for calls to the EntityManager.find method and from queries.

You can set the retrieveMode property to one of the constants defined by the jakarta.persistence.CacheRetrieveMode enumerated type, either USE (the default) or BYPASS.

When the property is set to USE, data is retrieved from the second-level cache, if available. If the data is not in the cache, the persistence provider will read it from the database.

When the property is set to BYPASS, the second-level cache is bypassed and a call to the database is made to retrieve the data.

Cache Store Mode

The cache store mode, set by the jakarta.persistence.storeMode property, controls how data is stored in the cache.

The storeMode property can be set to one of the constants defined by the jakarta.persistence.CacheStoreMode enumerated type: either USE (the default), BYPASS, or REFRESH.

When the property is set to USE, the cache data is created or updated when data is read from or committed to the database. If data is already in the cache, setting the store mode to USE will not force a refresh when data is read from the database.

When the property is set to BYPASS, data read from or committed to the database is not inserted or updated in the cache. That is, the cache is unchanged.

When the property is set to REFRESH, the cache data is created or updated when data is read from or committed to the database, and a refresh is forced on data in the cache upon database reads.

Setting the Cache Retrieval or Store Mode

To set the cache retrieval or store mode for the persistence context, call the EntityManager.setProperty method with the property name and value pair:

EntityManager em = ...;
em.setProperty("jakarta.persistence.cache.storeMode", "BYPASS");

To set the cache retrieval or store mode when calling the EntityManager.find or EntityManager.refresh methods, first create a Map<String, Object> instance and add a name/value pair as follows:

EntityManager em = ...;
Map<String, Object> props = new HashMap<String, Object>();
props.put("jakarta.persistence.cache.retrieveMode", "BYPASS");
String personPK = ...;
Person person = em.find(Person.class, personPK, props);
The cache retrieval mode is ignored when calling the EntityManager.refresh method, as calls to refresh always result in data being read from the database, not the cache.

To set the retrieval or store mode when using queries, call the Query.setHint or TypedQuery.setHint methods, depending on the type of query:

EntityManager em = ...;
CriteriaQuery<Person> cq = ...;
TypedQuery<Person> q = em.createQuery(cq);
q.setHint("jakarta.persistence.cache.storeMode", "REFRESH");
...

Setting the store or retrieve mode in a query or when calling the EntityManager.find or EntityManager.refresh method overrides the setting of the entity manager.

Controlling the Second-Level Cache Programmatically

The jakarta.persistence.Cache interface defines methods for interacting with the second-level cache programmatically.

Overview of the jakarta.persistence.Cache Interface

The Cache interface defines methods to do the following:

  • Check whether a particular entity has cached data

  • Remove a particular entity from the cache

  • Remove all instances (and instances of subclasses) of an entity class from the cache

  • Clear the cache of all entity data

If the second-level cache has been disabled, calls to the Cache interface’s methods have no effect, except for contains, which will always return false.

Checking whether an Entity’s Data Is Cached

To find out whether a given entity is currently in the second-level cache:

  1. Call the Cache.contains method . The contains method returns true if the entity’s data is cached, and false if the data is not in the cache:

    EntityManager em = ...;
Cache cache = em.getEntityManagerFactory().getCache();
String personPK = ...;
if (cache.contains(Person.class, personPK)) {
  // the data is cached
} else {
  // the data is NOT cached
}

Removing an Entity from the Cache

To remove a particular entity or all entities of a given type from the second-level cache:

  1. Call one of the Cache.evict methods.

    1. To remove a particular entity from the cache, call the evict method and pass in the entity class and the primary key of the entity:

      EntityManager em = ...;
Cache cache = em.getEntityManagerFactory().getCache();
String personPK = ...;
cache.evict(Person.class, personPK);

    2. To remove all instances of a particular entity class, including subclasses, call the evict method and specify the entity class:

      EntityManager em = ...;
Cache cache = em.getEntityManagerFactory().getCache();
cache.evict(Person.class);

All instances of the Person entity class will be removed from the cache. If the Person entity has a subclass, Student, calls to the above method will remove all instances of Student from the cache as well.

Removing All Data from the Cache

To completely clear the second-level cache, call the Cache.evictAll method:

EntityManager em = ...;
Cache cache = em.getEntityManagerFactory().getCache();
cache.evictAll();