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## How to marshal Hibernate proxies

Intended Audience
Prerequisite
Concept

## Intended Audience

Anyone who want’s to marshal Hibernate POJOs. This document shows how prevent undesirable XML output caused by Hibernate's lazy-loading technique.

## Prerequisite

A general understanding about the Hibernate persistence framework, its use of dynamic proxies for lazy-loading and Castor XML.

## Concept

Assume that we have a Hibernate POJO named `Person`, which stores basic information about persons and set of `Address` objects. Address itself is another Hibernate POJO. This set of address objects is mapped in the Hibernate mapping file to load lazily, as shown below in the Hibernate mapping for the `Person` object:

    <hibernate-mapping>
        <class name="xxx.yyy.Person" table="person">

            <id name="id" type="integer">
                <column name="id" length="10" />
                <generator class="native" />
            </id>

            <!-- Properties section -->
            <property name="firstName" type="string">
                <column name="first_name" length="80" not-null="true" />
            </property>

            <property name="lastName" type="string">
                <column name="last_name" length="128" not-null="true" />
            </property>

            <set name="Address" table="address" inverse="true"
                cascade="all-delete-orphan" lazy="true">
                <key column="person_id" />
                <one-to-many class="xxx.yyy.Address" />
            </set>
        </class>
    </hibernate-mapping>

When we want to marshal a `Person` object to XML, we use a standard Castor XML mapping file to map the `Person` class to the desired XML layout.

    <mapping>
        <class name="xxx.yyy.Person">
            <map-to xml="person"/>
            <field name="firstName" type="string" >
                <bind-xml name="firstName" node="element" />
            </field>
            <field name="lastName" type="string">
                <bind-xml name="lastName" node="element" />
            </field>
            <field name="address" type="xxx.yyy.Address" collection="set">
                <bind-xml name="address" node="element" location="addressList" />
            </field>
        </class>
    </mapping>

As shown below, marshalling the `Person` instance produces undesirable XML output. This is because hibernate (or other ORM frameworks) use dynamic proxy classes for lazy-loading.

        <person>
            <firstName>John</firstName>
            <lastName>Smith</lastName>
            <addressList>
                <address xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                         xsi:type="java:xxx.yyy.Address_$$_javassist_11">
                    <hibernate-lazy-initializer unwrap="false" uninitialized="false"
                         xsi:type="java:org.hibernate.proxy.pojo.javassist.JavassistLazyInitializer">
                        <implementation xsi:type="address">
                            <street>street</street>
                            <country>England</country>
                        </implementation>
                        <entity-name>xxx.yyy.Address</entity-name>
                    </hibernate-lazy-initializer>
                    <street>street</street>
                    <country>England</country>
                </address>
            </addressList>
        </person>

As Hibernate returns dynamic proxies for lazy-loaded objects, Castor won't be able to find a mapping for these (proxy) classes, and in this case fall back to using introspection. To avoid this behaviour, Castor XML during marshalling must be (en)able(d) to detect such proxy classes and handle them in a special way.

As proxy objects usually implement proxy interfaces, Castor XML can be instructed to check for such interfaces at marshal time, and marshal classes that implement these interfaces in a different way. Castor XML, in its `castor.properties` configuration file, allows you to enlist (proxy) interfaces that should be used for checking. The property `org.exolab.castor.xml.proxyInterfaces` can be used to specify whether or not to search for interfaces. If the property is set, the objects to be marshalled will be analysed if they implement one of the given interface names. If that's the case, the superclass will be marshalled instead of the proxy class itself.

    org.exolab.castor.xml.proxyInterfaces = net.sf.cglib.proxy.Factory, org.hibernate.proxy.HibernateProxy

Setting this property as hown above now enables Castor XML to produce the desired output.

    <person>
        <firstName>John</firstName>
        <lastName>Smith</lastName>
        <addressList>
            <address>
                <street>street</street>
                <country>Engand</country>
            </address>
        </addressList>
    </person>

<notice> Copyright © 1999-2005 ExoLab Group, Intalio Inc., and Contributors. All rights reserved. </notice>
<notice> Java, EJB, JDBC, JNDI, JTA, Sun, Sun Microsystems are trademarks or registered trademarks of Sun Microsystems, Inc. in the United States and in other countries. XML, XML Schema, XSLT and related standards are trademarks or registered trademarks of MIT, INRIA, Keio or others, and a product of the World Wide Web Consortium. All other product names mentioned herein are trademarks of their respective owners. </notice>