J2EE Support for XML

XML is portable data, and the Java platform is portable code. Add Java APIs for XML that make it easy to use XML and, together, you have the ideal combination:

• Portability of data

• Portability of code

• Ease of use

The J2EE platform bundles all these advantages together.

Enterprises are rapidly discovering the benefits of using J2EE for developing Web Services that use XML for the dissemination and integration of data; particularly because XML eases the sharing of legacy data both internally among departments and with other enterprises.

J2EE includes the Java API for XML Processing (JAXP) that makes it easy to process XML data with applications written in Java. JAXP embraces the parser standards:

• Simple API for XML Parsing (SAX) for parsing XML as a stream.

• Document Object Model (DOM) to build an in-memory tree representation of an XML document.

• XML Stylesheet Language Transformations (XSLT) to control the presentation of the data and convert it to other XML documents or to other formats, such as HTML. XLST is covered on Day 17, “Transforming XML Documents.”

JAXP also provides namespace support, allowing you to work with multiple XML documents that might otherwise cause naming conflicts.

Parsing XML

So far, you have used Internet Explorer or other third-party tools to parse your XML documents. Now you will look at three APIs that provide a way to access and manipulate the information stored in an XML document so you can build your own XML applications.

The Simple API for XML (SAX) defines parsing methods and Document Object Model (DOM) defines a mechanism for accessing and manipulating well-formed XML. A third API is the Java API for XML Processing (JAXP) that you will use to build a simple SAX and DOM parser. The two parsers you will develop effectively echo the input XML structure. Usually, you will want to parse XML to perform some useful function, but simply echoing the XML is a good way to learn the APIs.

JAXP has the benefit that it provides a common interface for creating and using SAX and DOM in Java.

SAX and DOM define different approaches to parsing and handling an XML document. SAX is an event-based API, whereas DOM is tree-based.

With event-based parsers, the parsing events (such as the start and end tags) are reported directly to the application through callback methods. The application implements these callback methods to handle the different components in the document, much like handling events in a graphical user interface (GUI).

Using the DOM API, you will transform the XML document into a tree structure in memory. The application then navigates the tree to parse the document.

Each method has its advantages and disadvantages. Using DOM

• Simplifies the mapping of the structure of the XML.

• Is a good choice when the document is not too large. If the document is large, it can place a strain on system resources.

• Most, or all, of the document needs to be parsed.

• The document is to be altered or written out in a structure that is very different from the original.

Using SAX is a good choice

• If you are searching through an XML document for a small number of tags

• The document is large

• Processing speed is important

• If the document does not need to be written out in a structure that is different from the original

SAX is a public domain API developed cooperatively by the members of the XML-DEV (XML DEVelopment) Internet discussion group (http://www.xml.org/).

The DOM is a set of interfaces defined by the W3C DOM Working Group. The latest DOM recommendation can be obtained from the W3C Web site (http://www.w3.org).