Understanding EclipseLink,
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The following sections describe the descriptor files that can be used for object-relational and MOXy mapping.
Use the orm.xml file to apply the metadata to the persistence unit. This metadata is a union of all the mapping files and the annotations (if there is no xml-mapping-metadata-complete element). If you use one mapping orm.xml file for your metadata and place this file in a META-INF directory on the classpath, then you do not need to explicitly list it. The persistence provider will automatically search for this file (orm.xml) and use it.
The schema for the JPA 2.0 orm.xml is orm_2_0.xsd. (http://java.sun.com/xml/ns/persistence/orm_2_0.xsd)
If you use a different name for your mapping files or place them in a different location, you must list them in the mapping-file element of the persistence.xml file.
EclipseLink supports an extended JPA orm.xml mapping configuration file called eclipselink-orm.xml. This mapping file can be used in place of JPA's standard mapping file or can be used to override a JPA mapping file. In additional to allowing all of the standard JPA mapping capabilities it also includes advanced mapping types and options.
For more information on the eclipselink-orm.xml file, see "eclipselink-orm.xml Schema Reference" in Java Persistence API (JPA) Extensions Reference for EclipseLink.
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Using this mapping file enables many EclipseLink advanced features, but it may prevent the persistence unit from being portable to other JPA implementations. |
For more information, on overriding values, see:
"XML Overriding Rules" in the JPA Specification.
The schema for eclipselink-orm.xml is eclipselink_orm_2_2.xsd:
http://www.eclipse.org/eclipselink/xsds/eclipselink_orm_2_2.xsd
To override the orm.xml file's mapping, you must define the META-INF/eclipselink-orm.xml file in the project. The contents of eclipselink-orm.xml override orm.xml and any other JPA mapping file specified in the persistence unit. If there are overlapping specifications in multiple ORM files, the files are merged if there are no conflicting entities.
For more information, see "Overriding and Merging" in Java Persistence API (JPA) Extensions Reference for EclipseLink.
By default the content of your .orm XML file is not validated against the JPA .orm XML schema.
During development it is a good idea to validate your .orm XML file against the schema to ensure it is valid. In EclipseLink, validating the .orm XML schema can be enabled using the persistence unit property "eclipselink.orm.validate.schema" in your persistence.xml file.
Some advantages of using XML instead of annotations include:
No coupling between the metadata and the source code
Compliance with the existing, pre-EJB 3.0 development process
Support in IDEs and source control systems
The main disadvantages of mapping with XML include:
It is inherently complex (when compared to annotations)
The need for replication of the code context (that is, defining the structure in both the XML and the source code)
For more information, see Chapter 10 "Metadata Annotations" in the JPA Specification:
You can use Java annotations to specify JAXB features in your projects. In addition to Java annotations, EclipseLink provides an XML mapping configuration file called eclipselink-oxm.xml. This mapping file contains the standard JAXB mappings and configuration options for advanced mapping types. You can use the eclipselink-oxm.xml file in place of or to override JAXB annotations in source code.
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Using this mapping file will enable many advanced features but it can prevent the model from being portable to other JAXB implementations. |
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