XML data binding refers to a means of representing information in an XML document as a business 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.
Approaches to data binding can be distinguished as follows:
- XML schema based: Based on an existing XML schema, classes that correspond to the schema are generated.
- Class based: Based on a set of classes to be serialized, a corresponding XML schema is generated.
- Mapping-based: A mapping description, usually itself an XML document, describes how an existing XML schema maps to a set of classes, and vice versa.
Since XML is inherently sequential and objects are (usually) not, XML data binding mappings often have difficulty preserving all the information in an XML document. Specifically, information like comments, XML entity references, and sibling order may fail to be preserved in the object representation created by the binding application. This is not always the case; sufficiently complex data binders are capable of preserving 100% of the information in an XML document.
Similarly, since objects in computer memory are not inherently sequential, and may include links to other objects (including self-referential links), XML data binding mappings often have difficulty preserving all the information about an object when it is marshalled to XML.
An alternative approach to automatic data binding relies instead on hand-crafted XPath expressions that extract the data from XML. This approach has a number of benefits. First, the data binding code only needs proximate knowledge (e.g., topology, tag names, etc.) of the XML tree structure, which developers can determine by looking at the XML data; XML schemas are no longer mandatory. Furthermore, XPath allows the application to bind the relevant data items and filter out everything else, avoiding the unnecessary processing that would be required to completely unmarshall the entire XML document. The drawback of this approach is the lack of automation in implementing the object model and XPath expressions. Instead the application developers have to create these artifacts manually.
Data binding in general
One of XML data binding's strengths is the ability to un/serialize objects across programs, languages, and platforms. You can dump a time series of structured objects from a datalogger written in C (programming language) on an embedded processor, bring it across the network to process in Perl and finally visualize in Octave. The structure and the data remain consistent and coherent throughout the journey, and no custom formats or parsing is required. This is not unique to XML. YAML, for example, is emerging as a powerful data binding alternative to XML. JSON (which can be regarded as a subset of YAML) is often suitable for lightweight or restricted applications.
XML data binding frameworks
|Name||Programming Language||License||First release||Last stable release||Code generation from XSD||Custom mapping||Note|
|Apache Commons Betwixt||Java||Apache||January 28, 2003||0.8||Unknown||Unknown||Dormant. Serializes objects to XML without requiring an XML schema definition|
|Apache XMLBeans||Java||Apache License 2.0||5.1.1, August 29, 2022||Yes||Unknown|
|Castor||Java||Apache 2.0||1.4.1, May 15, 2016||Unknown||Unknown||Earlier versions also supported Java-to-SQL persistence but this has since been forked into a separate project|
|CodeSynthesis XSD||C++||GNU GPL and proprietary||4.0.0, July 22, 2014||Unknown||Unknown||with SAX or tree-like mapping into C++ classes|
|gSOAP||C and C++||GNU GPL and proprietary||December 8, 2000||2.8.94, October 17, 2019||Yes||Yes||Supports XML schema, WSDL, and SOAP; XML schemas are not required to serialize C/C++ data to XML; custom mapping of XML schema types to C/C++ types via a type mapping file and from C/C++ types to compatible XML schema types by source code annotation|
|Java Architecture for XML Binding (JAXB)||Java||?||Yes||Yes|
|JiBX||Java||BSD License||1.2.6, January 1, 2015||Yes||Yes||Maps classes to XML schemas via bytecode manipulation|
|Simple||Java||Apache 2.0||2.7.1, February 9, 2017||No||Yes|
|System.Xml.Serialization||C#||?||Yes||No||Part of the .NET framework, contains XML data binding classes; includes |
|xmlbeansxx||C++||Apache 2.0||0.9.1, April 1, 2008||Unknown||Unknown||C++ port of Apache XMLBeans|
|XStream||Java||BSD-style license||January 1, 2004||1.4.10, May 23, 2017||Unknown||Unknown||Also capable of serializing to JSON|
|Zeus||Java||?||3.5 beta, August 16, 2002||Unknown||Unknown|