Persisting 3rd party objects with JPA

Question

In my current project I am using a 3rd party library which has no JPA annotations.

How can I persist objects from that library using JPA and external mappings?

Answer 1

Check this and this. In short:

1. Create META-INF/orm.xml
2. Follow (read) the .xsd

You don't have to manually map each column - only some specifics (i.e. collections and the id) are required. All fields are assumed to be columns (if the class is mapped). If there are no collections, something like this suffices:

<?xml version="1.0" encoding="UTF-8" ?>
<entity-mappings xmlns="http://java.sun.com/xml/ns/persistence/orm"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/persistence/orm 
    http://java.sun.com/xml/ns/persistence/orm_1_0.xsd"
    version="1.0">

    <description>External entities from library X</description>
    <package>com.external.library</package>
    <entity class="SomeClassName">
      <id>..</id>
    </entity>
    <entity class="AnotherClassName">
      <id>..</id>
    </entity>
</entity-mapping>

Note that when specifying <package> you don't need fully-qualified names.

In case you want a file named differently than orm.xml, in your persistence.xml specify it via:

<mapping-file>customMappingFile.xml</mapping-file>

Answer 2

Refer to the docs of your JPA implementation; any serious JPA implementation should provide examples of use of XML as well as annotations. See http://www.datanucleus.org/products/accessplatform_2_0/jpa/metadata_xml.html for DataNucleus docs for XML structure, and then refer to the particular relation types for examples of different features.