3. Metadata Extensions

3. Metadata Extensions
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OpenJPA extends standard metadata to allow you to access advanced OpenJPA functionality. This section covers persistence metadata extensions; we discuss mapping metadata extensions in Section 13, “ Mapping Extensions ”. All metadata extensions are optional; OpenJPA will rely on its defaults when no explicit data is provided.

3.1. Class Extensions

3.1.1. Fetch Groups
3.1.2. Data Cache
3.1.3. Detached State

OpenJPA recognizes the following class extensions:

3.1.1. Fetch Groups

The org.apache.openjpa.persistence.FetchGroups and org.apache.openjpa.persistence.FetchGroup annotations allow you to define fetch groups in your JPA entities. Section 7, “ Fetch Groups ” discusses OpenJPA's support for fetch groups in general; see Section 7.1, “ Custom Fetch Groups ” for how to use these annotations in particular.

3.1.2. Data Cache

Section 1, “ Data Cache ” examines caching in OpenJPA. Metadata extensions allow individual classes to override system caching defaults.

OpenJPA defines the org.apache.openjpa.persistence.DataCache annotation for caching information. This annotation has the following properties:

boolean enabled: Whether to cache data for instances of the class. Defaults to true for base classes, or the superclass value for subclasses. If you set this property to false, all other properties are ignored.
int timeout: The number of milliseconds data for the class remains valid. Use -1 for no timeout. Defaults to the openjpa.DataCacheTimeout property value.

3.1.3. Detached State

The OpenJPA enhancer may add a synthetic field to detachable classes to hold detached state (see Section 1.3, “ Defining the Detached Object Graph ” for details). You can instead declare your own detached state field or supress the creation of a detached state field altogether. In the latter case, your class must not use datastore identity, and should declare a version field to detect optimistic concurrency errors during detached modifications.

OpenJPA defines the org.apache.openjpa.persistence.DetachedState annotation for controlling detached state. When used to annotate a class, DetachedState recognizes the following properties:

boolean enabled: Set to false to suppress the use of detached state.
String fieldName: Use this property to declare your own detached state field. The field must be of type Object. Typically this property is only used if the field is inherited from a non-persisted superclass. If the field is declared in your entity class, you will typically annotate the field directly, as described below.

If you declare your own detached state field, you can annotate that field with DetachedState directly, rather than placing the annotation at the class level and using the fieldName property. When placed on a field, DetachedState acts as a marker annotation; it does not recognize any properties. Your annotated field must be of type Object.

3.2. Field Extensions

3.2.1. Dependent
3.2.2. Load Fetch Group
3.2.3. LRS
3.2.4. Inverse-Logical
3.2.5. Read-Only
3.2.6. Type
3.2.7. Externalizer
3.2.8. Factory
3.2.9. External Values

OpenJPA recognizes the following field extensions:

3.2.1. Dependent

In a dependent relation, the referenced object is deleted whenever the owning object is deleted, or whenever the relation is severed by nulling or resetting the owning field. For example, if the Magazine.coverArticle field is marked dependent, then setting Magazine.coverArticle to a new Article instance will automatically delete the old Article stored in the field. Similarly, deleting a Magazine object will automatically delete its current cover Article. (This latter processing is analogous to using JPA's CascadeType.REMOVE functionality as described in Section 2.8.1, “ Cascade Type ”.) You can prevent an orphaned dependent object from being automatically deleted by assigning it to another relation in the same transaction.

OpenJPA offers a family of marker annotations to denote dependent relations in JPA entities:

org.apache.openjpa.persistence.Dependent: Marks a direct relation as dependent.
org.apache.openjpa.persistence.ElementDependent : Marks the entity elements of a collection, array, or map field as dependent.
org.apache.openjpa.persistence.KeyDependent: Marks the key entities in a map field as dependent.

3.2.2. Load Fetch Group

The org.apache.openjpa.persistence.LoadFetchGroup annotation specifies a field's load fetch group. Section 7, “ Fetch Groups ” discusses OpenJPA's support for fetch groups in general; see Section 7.1, “ Custom Fetch Groups ” for how to use this annotation in particular.

3.2.3. LRS

This boolean extension, denoted by the OpenJPA org.apache.openjpa.persistence.LRS annotation, indicates that a field should use OpenJPA's special large result set collection or map proxies. A complete description of large result set proxies is available in Section 6.4.2, “ Large Result Set Proxies ”.

3.2.4. Inverse-Logical

This extension names the inverse field in a logical bidirectional relation. To create a logical bidrectional relation in OpenJPA, use the org.apache.openjpa.persistence.InverseLogical annotation. We discuss logical bidirectional relations and this extension in detail in Section 5, “ Managed Inverses ”.

3.2.5. Read-Only

The read-only extension makes a field unwritable. The extension only applies to existing persistent objects; new object fields are always writeable.

To mark a field read-only in JPA metadata, set the org.apache.openjpa.persistence.ReadOnly annotation to an org.apache.openjpa.persistence.UpdateAction enum value. The UpdateAction enum includes:

UpdateAction.IGNORE: Updates to the field are completely ignored. The field is not considered dirty. The new value will not even get stored in the OpenJPA data cache.
UpdateAction.RESTRICT: Any attempt to change the field will result in an immediate exception.

3.2.6. Type

OpenJPA has three levels of support for relations:

Relations that hold a reference to an object of a concrete persistent class are supported by storing the primary key values of the related instance in the database.
Relations that hold a reference to an object of an unknown persistent class are supported by storing the stringified identity value of the related instance. This level of support does not allow queries across the relation.
Relations that hold an unknown object or interface. The only way to support these relations is to serialize their value to the database. This does not allow you to query the field, and is not very efficient.

Clearly, when you declare a field's type to be another persistence-capable class, OpenJPA uses level 1 support. By default, OpenJPA assumes that any interface-typed fields you declare will be implemented only by other persistent classes, and assigns interfaces level 2 support. The exception to this rule is the java.io.Serializable interface. If you declare a field to be of type Serializable, OpenJPA lumps it together with java.lang.Object fields and other non-interface, unrecognized field types, which are all assigned level 3 support.

With OpenJPA's type family of metadata extensions, you can control the level of support given to your unknown/interface-typed fields. Setting the value of this extension to Entity indicates that the field value will always be some persistent object, and gives level 2 support. Setting the value of this extension to the class of a concrete persistent type is even better; it gives you level 1 support (just as if you had declared your field to be of that type in the first place). Setting this extension to Object uses level 3 support. This is useful when you have an interface relation that may not hold other persistent objects (recall that OpenJPA assumes interface fields will always hold persistent instances by default).

This extension is also used with OpenJPA's externalization feature, described in Section 6.5, “ Externalization ”.

OpenJPA defines the following type annotations for field values, collection, array, and map elements, and map keys, respectively:

3.2.7. Externalizer

The OpenJPA org.apache.openjpa.persistence.Externalizer annotation names a method to transform a field value into a value of another type. See Section 6.5, “ Externalization ” for details.

3.2.8. Factory

The OpenJPA org.apache.openjpa.persistence.Factory annotation names a method to re-create a field value from its externalized form. See Section 6.5, “ Externalization ” for details.

3.2.9. External Values

The OpenJPA org.apache.openjpa.persistence.ExternalValues annotation declares values for transformation of simple fields to different constant values in the datastore. See Section 6.5.1, “ External Values ” for details.

3.3. Example

The following example shows you how to specify extensions in metadata.

Example 6.3. OpenJPA Metadata Extensions

import org.apache.openjpa.persistence.*;

@Entity
@DataCache(enabled=false)
public class Magazine
{
    @ManyToMany
    @LRS
    private Collection<Subscriber> subscribers;

    @ExternalValues({"true=1", "false=2"})
    @Type(int.class)
    private boolean weekly;

    ...
}