9.4 The XML Parser

The XML parser is responsible for reading an XML document and making sure it complies with the necessary rules. There are two kinds of parsers: non-validating and validating. The non-validating parser is the simpler of the two, and simply checks to see if a document is well-formed. The more complex validating parser will not only check for well-formedness, but also for validity (i.e., that the document actually follows all the rules laid out in an associated DTD).

If the document passes these tests, then the parser breaks it into a structure called a document tree. As the name implies, a document tree is simply a hierarchical data structure created from the nested elements in the document. The left-hand side of Figure 9.1 illustrates a typical document tree.

Once the document is parsed and loaded, you can use a wide variety of languages (such as Java, JavaScript, etc.) to write programs that use the Document Object Model (DOM) API to traverse and manipulate the information in the tree. For example, you could write a JavaScript program to build a hierarchical view of an XML invoice by programmatically expanding and collapsing the tree's branches. This sort of client-side manipulation is very fast because it acts on information stored in memory, rather than having to requery the server each time a user requests a new view of the same information.

In the next two sections, we'll see how you can generate XML documents from information stored in the Oracle database. In the first, we'll develop a package to build an XML invoice. In the second, we'll look at a set of experimental packages that you can use to build a variety of XML applications.