Java and the XML DOM

An excerpt from New Riders Inside XML by Steven Holzner.

This chapter is all about using XML with Java to create standalone programs. In fact, I'll even create a few browsers in this chapter. Here, the programs we write will be based on the XML DOM, and I'll use the XML for Java (XML4J) packages from IBM alphaWorks (http://www.alphaworks.ibm.com/tech/xml4j).

This is the famous XML parser that adheres to the W3C standards and has implemented the W3C DOM level 1 (and part of level 2). It's the most widely used standalone XML Java parser available. 

As of this writing, the current version is 3.0.1, and it's based on the Apache Xerces XML Parser Version 1.0.3.

XML Parser for Java is a validating XML parser written in 100% pure Java. The package (com.ibm.xml.parser) contains classes and methods for parsing, generating, manipulating, and validating XML documents. XML Parser for Java is believed to be the most robust XML processor currently available and conforms most closely to the XML 1.0 Recommendation.

In fact, this points out one of the problems with working with modern XML Java parsers-they're always in a state of flux. It turns out that the com.ibm.xml.parser package mentioned here is now deprecated, which in Java terms means that it's obsolete (although still supported) and scheduled to be removed in a future release. Instead, we'll use the org.apache.xerces.parsers package, which is the successor to com.ibm.xml.parser.

This is an occupational hazard when working with third-party parsers, which historically have been extremely volatile. For example, when XML was still very young, I wrote a book based largely on the Microsoft XML Java parser, which was the only commercial-grade Java XML parser available at that time. And just before the book appeared on shelves, Microsoft changed its parser utterly so that virtually none of the code in the book worked. (The Microsoft XML Java parser is not even available as a standalone package anymore.) That's not an uncommon experience.

On the other hand, the alphaWorks parser has been changed so that it's now based on the W3C DOM (the package we'll be using to support nodes and elements in code will be alphaWork's org.w3c.dom package), which means that things have finally become standardized. However, the package names and the actual parsers we'll use, such as org.apache.xerces.parsers.DOMParser in this chapter, are still subject to change. By the time you read this, the alphaWorks packages may well have changed, something that's beyond our control here. In that case, you should refer to the XML for Java documentation to see what changes you need to make to your code-now that the W3C DOM is available, those changes should be minimized compared to what happened in the past.

This chapter and the next one provide you with a good introduction to the XML for Java parser. However, there's enough material here to take up a whole book-in fact, such books have been published, as recently as last year. (Those books are now obsolete because of changes in the parser-surprise!) The XML for Java packages are extensive and come with hundreds of pages of documentation, so if you want to pursue XML for Java programming beyond the techniques that you see in these chapters, dig into that documentation.

We saw XML for Java in this book as early as Chapter 1, "Essential XML," where I used an example that comes with XML for Java named DOMWriter that lets you validate XML documents based on DTDs. In Chapter 1, we saw this document, greeting.xml:

<?xml version="1.0" encoding="UTF-8"?>

<DOCUMENT>

    <GREETING>

        Hello From XML

    </GREETING>

    <MESSAGE>

        Welcome to the wild and woolly world of XML.

    </MESSAGE>

</DOCUMENT>

I tested this document using DOMWriter like this, where you can see that it reports validation errors:

%java dom.DOMWriter greeting.xml

greeting.xml:

[Error] greeting.xml:2:11: Element type "DOCUMENT" must be declared

[Error] greeting.xml:3:15: Element type "GREETING" must be declared

[Error] greeting.xml:6:14: Element type "MESSAGE" must be declared.

<?xml version="1.0" encoding="UTF-8"?>

<DOCUMENT>

    <GREETING>

        Hello From XML

    </GREETING>

    <MESSAGE>

        Welcome to the wild and woolly world of XML.

    </MESSAGE>

</DOCUMENT>

In this chapter, we'll build our own Java programs using XML for Java directly, including parsing and filtering XML documents, as well as creating standalone browsers and even a specialized graphical browser that uses XML documents not to display text, but to display circles.

That's one advantage of being able to create your own programs using parsers like the ones in XML for Java: You can create your own specialized browsers.