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XML Mapping is an experimental feature in Hibernate 3.0 and is currently under active development.
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.
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>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 Maps. The property names
are purely logical constructs that can be referred to in HQL queries.
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 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 id="123456789">
<account id="987632567" short-desc="Savings">
<customer id="123456789"/>
<balance>100.29</balance>
</account>
<account id="985612323" short-desc="Credit Card">
<customer id="123456789"/>
<balance>-2370.34</balance>
</account>
<name>
<first-name>Gavin</first-name>
<initial>A</initial>
<last-name>King</last-name>
</name>
...
</customer>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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