Program guide > Access data with client applications > Cache objects and their relationships (EntityManager API) > Interacting with EntityManager
Entity listeners and callback methods
Applications can be notified when the state of an entity transitions from state to state. Two callback mechanisms exist for state change events: life cycle callback methods that are defined on an entity class and are invoked whenever the entity state changes, and entity listeners, which are more general because the entity listener can be registered on several entities.
Life cycle of an entity instance
An entity instance has the following states:
- New: A newly created entity instance that does not exist in the eXtreme Scale cache.
- Managed: The entity instance exists in the eXtreme Scale cache and is retrieved or persisted using the entity manager. An entity must be associated with an active transaction to be in the managed state.
- Detached: The entity instance exists in the eXtreme Scale cache, but is no longer associated with an active transaction.
- Removed: The entity instance is removed, or is scheduled to be removed, from the eXtreme Scale cache when the transaction is flushed or committed.
- Invalidated: The entity instance is invalidated, or is scheduled to be invalidated, from the eXtreme Scale cache when the transaction is flushed or committed.
When entities change from state to state, you can invoke life-cycle, call-back methods.
The following sections further describe the meanings of New, Managed, Detached, Removed and Invalidated states as the states apply to an entity.
Entity life cycle callback methods
Entity life cycle callback methods can be defined on the entity class and are invoked when the entity state changes. These methods are useful for validating entity fields and updating transient state that is not usually persisted with the entity. Entity life cycle callback methods can also be defined on classes that are not using entities. Such classes are entity listener classes, which can be associated with multiple entity types. life cycle callback methods can be defined using both metadata annotations and a entity metadata XML descriptor file:
- Annotations: life cycle callback methods can be denoted using the PrePersist, PostPersist, PreRemove, PostRemove, PreUpdate, PostUpdate, and PostLoad annotations in an entity class.
- Entity XML descriptor : life cycle callback methods can be described using XML when annotations are not available.
Entity listeners
An entity listener class is a class that does not use entities that defines one or more entity life cycle callback methods. Entity listeners are useful for general purpose auditing or logging applications. Entity listeners can be defined using both metadata annotations and a entity metadata XML descriptor file:
- Annotation: The EntityListeners annotation can be used to denote one or more entity listener classes on an entity class. If multiple entity listeners are defined, the order in which they are invoked is determined by the order in which they are specified in the EntityListeners annotation.
- Entity XML descriptor: The XML descriptor can be used as an alternative to specify the invocation order of entity listeners or to override the order that is specified in metadata annotations.
Callback method requirements
Any subset or combination of annotations can be specified on an entity class or a listener class. A single class cannot have more than one life cycle callback method for the same life cycle event. However, the same method can be used for multiple callback events. The entity listener class must have a public no-arg constructor. Entity listeners are stateless. The life cycle of an entity listener is unspecified. eXtreme Scale does not support entity inheritance, so callback methods can only be defined in the entity class, but not in the superclass.
Callback method signature
Entity life cycle callback methods can be defined on an entity listener class, directly on an entity class, or both. Entity life cycle callback methods can be defined using both metadata annotations and the entity XML descriptor. The annotations used for callback methods on the entity class and on the entity listener class are the same. The signatures of the callback methods are different when defined on an entity class versus an entity listener class. Callback methods defined on an entity class or mapped superclass have the following signature:
void <METHOD>()Callback methods that are defined on an entity listener class have the following signature:
void <METHOD>(Object)The Object argument is the entity instance for which the callback method is invoked. The Object argument can be declared as a java.lang.Object object or the actual entity type.
Callback methods can have public, private, protected, or package level access, but must not be static or final.
The following annotations are defined to designate life cycle event callback methods of the corresponding types:
- com.ibm.websphere.projector.annotations.PrePersist
- com.ibm.websphere.projector.annotations.PostPersist
- com.ibm.websphere.projector.annotations.PreRemove
- com.ibm.websphere.projector.annotations.PostRemove
- com.ibm.websphere.projector.annotations.PreUpdate
- com.ibm.websphere.projector.annotations.PostUpdate
- com.ibm.websphere.projector.annotations.PostLoad
See the API Documentation for more details. Each annotation has an equivalent XML attribute defined in the entity metadata XML descriptor file.
Life cycle callback method semantics
Each of the different life cycle callback methods has a different purpose and is called in different phases of the entity life cycle:
- PrePersist
- Invoked for an entity before the entity has been persisted to the store, which includes entities that have been persisted due to a cascading operation. This method is invoked on the thread of the EntityManager.persist operation.
- PostPersist
- Invoked for an entity after the entity has been persisted to the store, which includes entities that have been persisted due to a cascading operation. This method is invoked on the thread of the EntityManager.persist operation. It is called after the EntityManager.flush or EntityManager.commit is called.
- PreRemove
- Invoked for an entity before the entity has been removed, which includes entities that have been removed due to a cascading operation. This method is invoked on the thread of the EntityManager.remove operation.
- PostRemove
- Invoked for an entity after the entity has been removed, which includes entities that have been removed due to a cascading operation. This method is invoked on the thread of the EntityManager.remove operation. It is called after the EntityManager.flush or EntityManager.commit is called.
- PreUpdate
- Invoked for an entity before the entity has been updated to the store. This method is invoked on the thread of the transaction flush or commit operation.
- PostUpdate
- Invoked for an entity after the entity has been updated to the store. This method is invoked on the thread of the transaction flush or commit operation.
- PostLoad
- Invoked for an entity after the entity has been loaded from the store which includes any entities that are loaded through an association. This method is invoked on the thread of the loading operation, such as EntityManager.find or a query.
Duplicate life cycle callback methods
If multiple callback methods are defined for an entity life cycle event, the ordering of the invocation of these methods is...
- life cycle callback methods defined in the entity listeners: The life cycle callback methods that are defined on the entity listener classes for an entity class are invoked in the same order as the specification of the entity listener classes in the EntityListeners annotation or the XML descriptor.
- Listener super class: Callback methods defined in the super class of the entity listener are invoked before the children.
- Entity life cycle methods: WebSphere eXtreme Scale does not support entity inheritance, so the entity life cycle methods can only be defined in the entity class.
Exceptions
Life cycle callback methods might result in run time exceptions. If a life cycle callback method results in a run time exception within a transaction, the transaction is rolled back. No further life cycle callback methods are invoked after a runtime exception results.
Parent topic:
Interacting with EntityManager