XML binding with JAXB 1/3

I’ve been watching for an XML binding tool for one of my projects, which make use of XML to store datas.

I looked at hibernate, which now make XML bindings, but, from what I’ve seen, the binding is managed by the relational mapping. As I do not have a database in my application, I had to search elsewhere.

So I took a look at JAXB. It’s the tool I needed : you specify your xml schema, run xjc and you have your object mapping done. As example, I’ll use a schema I’m writing at the moment (phonebook.xsd). Now, all you have to write, once the classes are generated is 4 lines of code (plus catching the exceptions:

        Phonebook phonebook = null;
        JAXBContext jc;

        try {

            // Create the context with package containing mapping classes
            jc = JAXBContext.newInstance(BINDING_ID);

            // Load the xml file
            Unmarshaller u = jc.createUnmarshaller();

            u.setSchema(SchemaFactory.newInstance(
                    XMLConstants.W3C_XML_SCHEMA_NS_URI).newSchema(
                    new File("phonebook.xsd")));

          phonebook  =
    	            (Phonebook)u.unmarshal( new FileInputStream( phonebookFilename ) );

            // Do some work ...

            // Save the modified datas (here writes on stdout)
            Marshaller m = jc.createMarshaller();
             m.marshal( b, System.out);

        } catch (SAXException e) {
            logger.log(Level.ERROR, "Altered schema file.", e);
            throw new Exception("Schema file has been altered.", e);
        } catch (FileNotFoundException e) {
            logger.log(Level.ERROR, "Phonebook file not found.", e);
            throw new Exception("Phonebook file not found.");
        } catch (JAXBException e) {
            logger.log(Level.ERROR, "Corrupted phonebook file ", e);
            throw new Exception("Corrupted phonebook file ");
        }

Let’s comment the source :

1. Create a context : the constant BINDING_ID is the package name of the generated xml binding classes.
2. Create the unmarshaller : the object that will read the xml file and bind it to objects
3. Set the schema of the xml file read to assume xml validation
4. Unmarshal : load the file in the objects

But you have three things to know to better use it : first if you want to directly unmarshal to your root class and to write an object representation to an xml file, the master object class has to be tagged @xmlRootElement, if you want to write simplier code.

Take the following schema, that does not generate the @XmlRootElement annotation :

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"
    xmlns:jxb="http://java.sun.com/xml/ns/jaxb" jxb:version="2.0" >

    <xsd:element name="phonebook" type="Phonebook" />;

    <xsd:complexType name="Phonebook">
    ...
    </xsd:complexType>

If you use that schema, you’ll have an error (java.lang.ClassCastException: javax.xml.bind.JAXBElement cannot be cast to package.name.Phonebook) when unmarshalling and an other (unable to marshal type "package.name.Phonebook" as an element because it is missing an @XmlRootElement annotation) when marshalling. You’ll have to write the following java code to unmarshall and marshall without errors with the schema above :

phonebook  =
     (Phonebook) ((JAXBElement)u.unmarshal( new FileInputStream( phonebookFilename ) )).getValue;

     m.marshal( new JAXBElement( new QName("", "phonebook"), Phonebook.class, phonebook ), System.out);

Instead, a better way to declare your schema (and have simplier java code) is :

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"
    xmlns:jxb="http://java.sun.com/xml/ns/jaxb" jxb:version="2.0" >

    <xsd:element name="phonebook" >;

    <xsd:complexType >
    ...
    </xsd:complexType>
    </xsd:element>

The two others tips to knows will be the subjects of the next posts.