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.
An XML data binder accomplishes this by automatically creating a mapping between elements of the XML schema of the document we wish to bind and members of a class to be represented in memory.
When this process is applied to convert an XML document to an object, it is called unmarshalling. The reverse process, to serialize an object as XML, is called marshalling.
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 manually 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 major 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 on an embedded processor, bring it across the network to process in perl and finally visualize in Mathematica. 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.
Data binding frameworks and tools
C++
- gSOAP - Open source XML data binding compilers and autocoders for C and C++, supports SOAP, WSDL and XSD.
- Liquid XML Studio - Generates cross platform C++ classes from XSD, XDR & DTD Schemas. Memory management is performed using reference counting via smart pointers, the code is thread safe, and compiles on Windows, Linux, Solaris.
- CodeSynthesis XSD - Open source XML data binding compiler for C++ which provides an event-driven, SAX-like C++ mapping in addition to the standard, tree-like in-memory object model. For closed source use, the company also offers a commercial license.
- CodeSynthesis XSD/e - Open source, validating XML parser/serializer and C++ data binding generator for mobile and embedded systems.
- xmlbeansxx - A C++ library for easing the processing of XML data. It is very similar to and in fact was inspired by XMLBeans.
- Codalogic LMX - GUI and command-line driven royalty-free C++ XML data binding tool. Code generator runs on Windows and Linux. Generated code runs on wide range of C++ compliant platforms. Free edition available.
- XMLSpy - A Windows only XML/XSD editor that supports Java and C++ code generation in the Enterprise edition.
Delphi
- XmlDataBinding Standard CodeGear XML binding tool, included in Delphi product (from version 7 onwards)
Java
- Castor, open source (original BSD), object/XML/relational mapping tool
- Schemaless Java-XML data binding with XPath and VTD-XML
- Java Architecture for XML Binding (JAXB), and JaxMe variation
- Eclipse Modeling Framework (EMF) open source (EPL), dynamic and static binding
- XMLBeans data binding over Document Object Model view of data
- Hibernate, open source (GNU LGPL), relational/object/XML mapping tool
- JiBX, open source (modified BSD), a high performance Java/XML binding framework
- CookXml, open source (MIT License), dynamically configurable XML binding tool.
- XStream, a simple Java library to serialize objects to XML and back again.
- Proselyte, a simple java library for converting a JPA Annotated pojo to xml and json.
- Smooks, Smooks is a Java Framework/Engine for binding into a Java Object Model from any data source (CSV, EDI, XML, Java, JSON etc)
JavaScript
- OpenLaszlo, open source, JavaScript based, Rich Internet application programming language
.Net
- Integrated into the .NET Framework, in the System.Xml.Serialization namespace.
- xsd.exe - A tool Supplied with Visual Studio which generates C# or VB.Net code from an XSD.
Python
- Amara open source Python library for XML data-binding
- EaseXML An open source XML Data Binding library Python. Translate XML to Python objects and vice-versa.
Visual Basic
- Liquid XML Studio - Generates Visual Basic 6.0 classes from an XSD, XDR or DTD. This code can then be used from any COM compliant language including VBScript & ASP.
External links
- XML Data Binding Resources, by Ronald Bourret
- XML Schema Patterns for Databinding Working Group