JPA中的Xml类型映射（EclipseLink / Oracle Toplink）

Question

According to James and Tomas Mapping Oracle XMLType on JPA (EclipseLink) , XMLType fields are handled(mapping and persist) using @Customizer and DescriptorCustomizer.

I have few doubts on this approach.

1. Why the xmlField attribute don't need @Column anotation?
2. Why we go for DescriptorCustomizer for maapping the xmlField? This is only for mapping the fields? Or it can also persist that field?
3. DirectToXMLTypeMapping can be used for Object type? (Please refer: http://docs.oracle.com/cd/E15051_01/web.1111/b32441/relmapun.htm#CHDFIFEF )
4. As of Tomas post how that xmlField will be persist? Need to write Callback methods?

Please clarify me.Sorry if my question is pointless. Please provide some links and documentation to understand the XMLType fields mapping in JPA(EclipseLink/Oracle Toplink).

Answer 1

XMLType is a specialized Oracle field type, and I'm guessing does not work well with a simple basic JPA mapping. EclipseLink has a custom DirectToXMLTypeMapping type that can handle conversions of the Oracle XMLType to/from a java String. In the example you've posted, the customizer is first removing the existing mapping that gets created through annotations for the "xmlField" attribute. This essentially wipes out any @Column information that might have been added. It then adds a new DirectToXMLTypeMapping containing all information required for the field, so there is no need to use annotations on the attribute.

Alternatively, you could define the @Column annotation and any other settings, and instead of just removing the mapping, you could get the DatabaseMapping object from the descriptor and pull the column name and other values from it to populate the new DirectToXMLTypeMapping.

A mapping is something that describes how the attribute in your entity relates to a database field. So it should handle retrieval as well as inserts/updates, converting it to/from the java type into/from the database type. In the link you posted, the entity's xmlType attribute mapping converts the Java String into an Oracle XMLType on persist, and reverses it when reading back from the DB.