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.
OpenJPA recognizes the following class extensions.
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.
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.
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.
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.
OpenJPA recognizes the following field extensions.
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.
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.
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.
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:
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.