Monday, January 25, 2010

XML Data Binding

XML data binding refers to the process of representing the information in an XML document as an object in computer memory. This allows applications to access the data in the XML from the object rather than using the DOM or SAX to retrieve the data from a direct representation of the XML itself.

Java Architecture for XML Binding (JAXB)
JABX allows Java developers to map Java classes to XML representations. JAXB provides two main features: the ability to marshal Java objects into XML and the inverse, i.e. to unmarshal XML back into Java objects. In other words, JAXB allows storing and retrieving data in memory in any XML format, without the need to implement a specific set of XML loading and saving routines for the program's class structure.

Usage of JAXB
The tool "xjc" can be used to convert XML Schema and other schema file types (as of Java 1.6, RELAX NG, XML DTD, and WSDL are supported experimentally) to class representations. Classes are marked up using annotations from javax.xml.bind.annotation.* namespace, for example, @XmlRootElement and @XmlElement. XML list sequences are represented by attributes of type java.util.List. Marshallers and Unmarshallers are created through an instance of JAXBContext.
In addition, JAXB includes a "schemagen" tool which can essentially perform the inverse of "xjc", creating an XML Schema from a set of annotated classes.

It is a tool that allows access to the full power of XML in a Java friendly way. The idea is to take advantage of the richness and features of XML and XML Schema and have these features mapped as naturally as possible to the equivalent Java language and typing constructs. XMLBeans uses XML Schema to compile Java interfaces and classes that can then be used to access and modify XML instance data. Using XMLBeans is similar to using any other Java interface/class: with methods like getFoo or setFoo, just as when working with Java. While a major use of XMLBeans is to access XML instance data with strongly typed Java classes there are also APIs that allow access to the full XML infoset (XMLBeans keeps XML Infoset fidelity) as well as to allow reflection into the XML schema itself through an XML Schema Object model.

Usage of XMLBean
To accomplish the above objectives, XMLBeans provides three major APIs:
  • XmlObject
  • XmlCursor
  • SchemaType
    XmlObject: The java classes that are generated from an XML Schema are all derived from XmlObject. These provide strongly typed getters and setters for each of the elements within the defined XML. Complex types are in turn XmlObjects. For example getCustomer might return a CustomerType (which is an XmlObject). Simple types turn into simple getters and setters with the correct java type. For example getName might return a String.
    XmlCursor: From any XmlObject the developer can get an XmlCursor. This provides efficient, low level access to the XML Infoset. A cursor represents a position in the XML instance. The cursor can be moved around the XML instance at any level of granularity needed from individual characters to Tokens.
    SchemaType: XMLBeans provides a full XML Schema object model that can be used to reflect on the underlying schema meta information. For example, the developer might generate a sample XML instance for an XML schema or perhaps find the enumerations for an element so that they may be displayed.
  • No comments: