Introduction
JAXB
defines annotations to
control the mapping of Java objects to XML, but it also defines a
default set of mappings. Using the defaults, TopLink can
marshall
a set of objects into XML and unmarshall an XML documenet into
objects. This is particularly useful when you have an
existing
domain model and you need an XML representation of those
objects.
This
how-to describes how to use TopLink to marshall an object and it's
contained objects to XML using the default JAXB mappings. It
also
shows how to unmarshall an XML document into an exisiting Java domain
model
.Step
1: Setup
Unzip
JAXBPojos.zip
into the <DEMO_HOME> folder of your choice.
After
unzipping, <DEMO_HOME> will contain the folder JAXBPojos.
<DEMO_HOME>/JAXBPojos/build.xml
is an ANT build file that can be used to build and run this how-to.
Open build.xml in a text editor and set the property
"toplink.home" to reference your TopLink install. It should
look something like:
<property name="toplink.home" value="C:\bin\Toplink11"/>
Step
2: Creating a JAXBContext
Open
<DEMO_HOME>/JAXBPojos/src/examples/jaxb/marshall/MarshallPojos.java
in a text editor and review the main() method. The top of the
method looks like:public static void main(String[] args) throws Exception {
Class[] classes = new Class[3];
classes[0] = Address.class;
classes[1] = Customer.class;
classes[2] = PhoneNumber.class
JAXBContext jaxbContext = JAXBContext.newInstance(classes);
Marshaller marshaller = jaxbContext.createMarshaller();
marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT,Boolean.TRUE);The
JAXBContext is created with an array of classes that will be included
in the context. All classes statically referenced
by the
listed cases will also be included in the context and can be marshalled
and unmarshalled. In the example, with the context initialized as in
the method, the Address, Customer, and PhoneNumber classes will be
included in the context. Once created, a marshaller can e
created
for converting objects to XML.
If you
open the Address,
Customer, and PhoneNumber classes you'll notice that there are no JAXB
mappings or imports. The classes are simple POJOs and so the
mappings for the classes are all defaulted.
Step 3:
Selecting the TopLink 11 JAXB runtime
When
you create a JAXBContext you need to identify the JAXB runtime to use.
To use TopLink 11 you need to provide a
jaxb.properties file
in the directory that contains the domain model classes. In
the
example, those classes are in the examples.jaxb.pojo package and so the
<DEMO_HOME>/JAXBPojos/src/examples/jaxb/pojo
directory should contain the jaxb.properties and contain:javax.xml.bind.context.factory = oracle.toplink.ox.jaxb20.JAXBContextFactory
Step
4: Marshalling Java to
XML
Once
created with the classes explictly listed, the context can be used to
marshall objects. In the example, a set of objects are
created
and associated. In the expert below, a customer object is
created
and marshalled to XML on the console.
Customer customer = new Customer();
customer.setFirstName("William");
customer.setLastName("Gibson");
System.out.println("\nCUSTOMER");
marshaller.marshal(customer, System.out);
The resulting XML looks like:
<?xml version="1.0" encoding="UTF-8"?>
<customer>
<firstName>William</firstName>
<lastName>Gibson</lastName>
</customer>
You can use the ANT build.xml file to compile the example and marshall objects. To do so
you need to invoke ANT with the "marshall" target. From a command prompt, with your current
directory <DEMO_HOME>/JAXBPojos (the directory that contains build.xml), you can issue the command:
<ANT_HOME>/bin/ant marshall
Summary
In
this
how-to we've seen how to write a Java program to marshall unannotated
Java POJOs to XML. The important steps are to:
- Create
the JAXBContext with an array of classses you intend to marshall.
- Define
the JAXB runtime in the jaxb.properties file in the package containing
your domain classes.
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