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## Chapter 20. XML Mapping

XML Mapping is an experimental feature in Hibernate 3.0 and is currently under active development.

## 20.1. Working with XML data

Hibernate allows you to work with persistent XML data in much the same way you work with persistent POJOs. A parsed XML tree can be thought of as another way of representing the relational data at the object level, instead of POJOs.

Hibernate supports dom4j as API for manipulating XML trees. You can write queries that retrieve dom4j trees from the database and have any modification you make to the tree automatically synchronized to the database. You can even take an XML document, parse it using dom4j, and write it to the database with any of Hibernate's basic operations: `persist(), saveOrUpdate(), merge(), delete(), replicate()` (merging is not yet supported).

This feature has many applications including data import/export, externalization of entity data via JMS or SOAP and XSLT-based reporting.

A single mapping can be used to simultaneously map properties of a class and nodes of an XML document to the database, or, if there is no class to map, it can be used to map just the XML.

### 20.1.1. Specifying XML and class mapping together

Here is an example of mapping a POJO and XML simultaneously:

    <class name="Account"
            table="ACCOUNTS"
            node="account">

        <id name="accountId"
                column="ACCOUNT_ID"
                node="@id"/>

        <many-to-one name="customer"
                column="CUSTOMER_ID"
                node="customer/@id"
                embed-xml="false"/>

        <property name="balance"
                column="BALANCE"
                node="balance"/>

        ...

    </class>

### 20.1.2. Specifying only an XML mapping

Here is an example where there is no POJO class:

    <class entity-name="Account"
            table="ACCOUNTS"
            node="account">

        <id name="id"
                column="ACCOUNT_ID"
                node="@id"
                type="string"/>

        <many-to-one name="customerId"
                column="CUSTOMER_ID"
                node="customer/@id"
                embed-xml="false"
                entity-name="Customer"/>

        <property name="balance"
                column="BALANCE"
                node="balance"
                type="big_decimal"/>

        ...

    </class>

This mapping allows you to access the data as a dom4j tree, or as a graph of property name/value pairs or java `Map`s. The property names are purely logical constructs that can be referred to in HQL queries.

## 20.2. XML mapping metadata

A range of Hibernate mapping elements accept the `node` attribute. This lets you specify the name of an XML attribute or element that holds the property or entity data. The format of the `node` attribute must be one of the following:

- `"element-name"`: map to the named XML element

- `"@attribute-name"`: map to the named XML attribute

- `"."`: map to the parent element

- `"element-name/@attribute-name"`: map to the named attribute of the named element

For collections and single valued associations, there is an additional `embed-xml` attribute. If `embed-xml="true"`, the default, the XML tree for the associated entity (or collection of value type) will be embedded directly in the XML tree for the entity that owns the association. Otherwise, if `embed-xml="false"`, then only the referenced identifier value will appear in the XML for single point associations and collections will not appear at all.

Do not leave `embed-xml="true"` for too many associations, since XML does not deal well with circularity.

    <class name="Customer"
            table="CUSTOMER"
            node="customer">

        <id name="id"
                column="CUST_ID"
                node="@id"/>

        <map name="accounts"
                node="."
                embed-xml="true">
            <key column="CUSTOMER_ID"
                    not-null="true"/>
            <map-key column="SHORT_DESC"
                    node="@short-desc"
                    type="string"/>
            <one-to-many entity-name="Account"
                    embed-xml="false"
                    node="account"/>
        </map>

        <component name="name"
                node="name">
            <property name="firstName"
                    node="first-name"/>
            <property name="initial"
                    node="initial"/>
            <property name="lastName"
                    node="last-name"/>
        </component>

        ...

    </class>

In this case, the collection of account ids is embedded, but not the actual account data. The following HQL query:

    from Customer c left join fetch c.accounts where c.lastName like :lastName

would return datasets such as this:

    <customer xml:id="123456789">
        <account short-desc="Savings">987632567</account>
        <account short-desc="Credit Card">985612323</account>
        <name>
            <first-name>Gavin</first-name>
            <initial>A</initial>
            <last-name>King</last-name>
        </name>
        ...
    </customer>

If you set `embed-xml="true"` on the `<one-to-many>` mapping, the data might look more like this:

    <customer xml:id="123456789">
        <account xml:id="987632567" short-desc="Savings">
            <customer xml:id="123456789"/>
            <balance>100.29</balance>
        </account>
        <account xml:id="985612323" short-desc="Credit Card">
            <customer xml:id="123456789"/>
            <balance>-2370.34</balance>
        </account>
        <name>
            <first-name>Gavin</first-name>
            <initial>A</initial>
            <last-name>King</last-name>
        </name>
        ...
    </customer>

## 20.3. Manipulating XML data

You can also re-read and update XML documents in the application. You can do this by obtaining a dom4j session:

    Document doc = ....;

    Session session = factory.openSession();
    Session dom4jSession = session.getSession(EntityMode.DOM4J);
    Transaction tx = session.beginTransaction();

    List results = dom4jSession
        .createQuery("from Customer c left join fetch c.accounts where c.lastName like :lastName")
        .list();
    for ( int i=0; i<results.size(); i++ ) {
        //add the customer data to the XML document
        Element customer = (Element) results.get(i);
        doc.add(customer);
    }

    tx.commit();
    session.close();

    Session session = factory.openSession();
    Session dom4jSession = session.getSession(EntityMode.DOM4J);
    Transaction tx = session.beginTransaction();

    Element cust = (Element) dom4jSession.get("Customer", customerId);
    for ( int i=0; i<results.size(); i++ ) {
        Element customer = (Element) results.get(i);
        //change the customer name in the XML and database
        Element name = customer.element("name");
        name.element("first-name").setText(firstName);
        name.element("initial").setText(initial);
        name.element("last-name").setText(lastName);
    }

    tx.commit();
    session.close();

When implementing XML-based data import/export, it is useful to combine this feature with Hibernate's `replicate()` operation.

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