url - Universität zu Lübeck

22 CHAPTER 2. FUNDAMENTALS
2.3 XML Query Languages
The DOM API provides a universal way of accessing an XML document by a programming
language. However, the explicit navigation to nodes by a programming
language is not very comfortable. For instance, the following Java code selects all
elements of items that have the value ’MP3 Player’ in its description.
1 Document doc = (new DOMBuilder ( ) ) . build (XMLSource) ;
2 Element root = doc . getRootElement ( ) ;
3 Element description = root . getChild ( ” description ” )
4 i f ( description != null ) {
5 Element text = description . getChild ( ” text ” )
6 i f ( text != null ) {
7 Element emph = text . getChild ( ”emph” )
8 i f (emph != null && emph. getText ( ) . equals ( ”MP3 Player ” ) ) { // found
9 Element name = root . getChild ( ”name” ) ;
10 return name;
11 }
12 }
13 }
14 return null ; // not found
Figure 2.9: A Java program for accessing the DOM-tree
As one can see quickly the code is not intuitive and descriptive. Rather than defining
which nodes are selected, the programmer has to express how to select them.
For more complex queries with more than one value comparison the code will
quickly become unmanageable. For this reason a more descriptive and compact
query language for XML is needed.
2.3.1 XPath
One common approach is XPath [126] a language for expressing navigational
steps and conditions for selecting nodes (elements and attributes) in an XML document.
XPath 1.0 was introduced in 1999 by the W3C with a definition for the
syntax and the semantics. Specific XPath implementations are offered by third
parties like [4, 7, 101, 108] or are part of a database management system, e.g.
[8, 55].
The following expression motivates XPath: It selects the same nodes as the previous
Java code but is much more compact:
1 \item\name[ . . \ description \ text \emph= ’MP3 Player ’ ]
XPath is a navigation language for selecting parts of an XML document modeled
as a tree of nodes. XPath discriminates between seven types of nodes: The documents
root-node, normal elements, text nodes, attributes and, less important,
comment-nodes, processing instructions, and XML Namespaces .