9. Mapping Extensions

9. + Mapping Extensions +
Prev	Chapter 7. + Mapping +	Next

9. + Mapping Extensions +

+ +

+Mapping extensions allow you to access OpenJPA-specific functionality from your +mappings. Note that all extensions below are specific to mappings. If you store +your mappings separately from your persistence metadata, these extensions must +be specified along with the mapping information, not the persistence metadata +information. +

9.1. + Class Extensions +

9.1.1. + Subclass Fetch Mode +
9.1.2. + Strategy +
9.1.3. + Discriminator Strategy +
9.1.4. + Version Strategy +

+ +

+OpenJPA recognizes the following class extensions. +

9.1.1. + Subclass Fetch Mode +

+ + +

+This extension specifies how to eagerly fetch subclass state. It overrides the +global +openjpa.jdbc.SubclassFetchMode property. Set the OpenJPA + +org.apache.openjpa.persistence.jdbc.SubclassFetchMode + annotation to a value from the + +org.apache.openjpa.persistence.jdbc.FetchMode + enum: JOIN, PARALLEL, or +NONE. See Section 8, “ + Eager Fetching + ” + for a discussion of eager fetching. +

9.1.2. + Strategy +

+ + +

+The +org.apache.openjpa.persistence.jdbc.Strategy +class annotation allows you to specify a custom mapping strategy for your class. +See Section 10, “ + Custom Mappings + ” for information on custom +mappings. +

9.1.3. + Discriminator Strategy +

+ + +

+The + +org.apache.openjpa.persistence.jdbc.DiscriminatorStrategy + class annotation allows you to specify a custom discriminator strategy. +See Section 10, “ + Custom Mappings + ” for information on custom +mappings. +

9.1.4. + Version Strategy +

+ + +

+The + +org.apache.openjpa.persistence.jdbc.VersionStrategy + class annotation allows you to specify a custom version strategy. See +Section 10, “ + Custom Mappings + ” for information on custom +mappings. +

9.2. + Field Extensions +

9.2.1. + Eager Fetch Mode +
9.2.2. + Nonpolymorphic +
9.2.3. + Class Criteria +
9.2.4. + Strategy +

+ +

+OpenJPA recognizes the following field extensions. +

9.2.1. + Eager Fetch Mode +

+ + +

+This extension specifies how to eagerly fetch related objects. It overrides the +global +openjpa.jdbc.EagerFetchMode property. Set the OpenJPA + +org.apache.openjpa.persistence.jdbc.EagerFetchMode + annotation to a value from the + +org.apache.openjpa.persistence.jdbc.FetchMode + enum: JOIN, PARALLEL, or +NONE. See Section 8, “ + Eager Fetching + ” + for a discussion of eager fetching. +

9.2.2. + Nonpolymorphic +

+ + +

+All fields in Java are polymorphic. If you declare a field of type T +, you can assign any subclass of T to the field as +well. This is very convenient, but can make relation traversal very inefficient +under some inheritance strategies. It can even make querying across the field +impossible. Often, you know that certain fields do not need to be entirely +polymorphic. By telling OpenJPA about such fields, you can improve the +efficiency of your relations. +

Note

+OpenJPA also includes the type metadata extension for +narrowing the declared type of a field. See Section 4.2.6, “ + Type + ”. +

+OpenJPA defines the following extensions for nonpolymorphic values: +

+The value of these extensions is a constant from the + +org.apache.openjpa.persistence.jdbc.NonpolymorphicType + enumeration. The default value, EXACT, indicates +that the relation will always be of the exact declared type. A value of +JOINABLE, on the other hand, means that the relation might +be to any joinable subclass of the declared type. This value only excludes +table-per-class subclasses. +

9.2.3. + Class Criteria +

+ + + +

+This family of boolean extensions determines whether OpenJPA will use the +expected class of related objects as criteria in the SQL it issues to load a +relation field. Typically, this is not needed. The foreign key values uniquely +identify the record for the related object. Under some rare mappings, however, +you may need to consider both foreign key values and the expected class of the +related object - for example, if you have an inverse relation that shares the +foreign key with another inverse relation to an object of a different subclass. +In these cases, set the proper class criteria extension to true + to force OpenJPA to append class criteria to its select SQL. +

+OpenJPA defines the following class criteria annotations for field relations and +array or collection element relations, respectively: +

9.2.4. + Strategy +

+ + +

+OpenJPA's + +org.apache.openjpa.persistence.jdbc.Strategy +extension allows you to specify a custom mapping +strategy or value handler for a field. See +Section 10, “ + Custom Mappings + ” for information on custom +mappings. +

5. + Mapping Factory +
Prev	Chapter 7. + Mapping +	Next

5. + Mapping Factory +

+ + + +

+An important decision in the object-relational mapping process is how and where +to store the data necessary to map your persistent classes to the database +schema. +

+Section 1, “ + Metadata Factory + ” introduced OpenJPA's +MetaDataFactory interface. OpenJPA uses this same interface to +abstract the storage and retrieval of mapping information. OpenJPA includes the +built-in mapping factories below, and you can create your own factory if you +have custom needs. You control which mapping factory OpenJPA uses with the ++openjpa.jdbc.MappingFactory configuration property. +

+The bundled mapping factories are: +

+
+-: Leaving the openjpa.jdbc.MappingFactory + property unset allows your metadata factory to take over mappings as +well. If you are using the default jpa metadata factory, +OpenJPA will read mapping information from your annotations and +orm.xml when you leave the mapping factory unspecified. +
+

Example 7.13. + Standard JPA Configuration +

+ +

+In the standard JPA configuration, the mapping factory is left unset. +

+<property name="openjpa.MetaDataFactory" value="jpa"/>
+

Prev	Up	Next
8. + Mapping Limitations +	Home	10. + Custom Mappings +

Prev	Up	Next
4. + Mapping Defaults +	Home	6. + Non-Standard Joins +