Java and JDOM: the perfect couple
by Keld H. Hansen
Introducing JDOM
JDOM is a very user-friendly
Java API for handling XML documents.
JDOM builds a tree structure of an XML document in memory, and
therefore resembles the
DOM API.
This is quite a different approach than the one used by the
SAX API which doesn't
build anything, but fires off a set of events when it reads an
XML file. The events may then be caught by your program for
extracting the data in the XML file. You may read
more about SAX
in my previous article at javaboutique.com.
Now I've mentioned 3 API's (and there are actually more), so
which one should you choose?
Well, JDOM and DOM are the obvious candidates if you need the
whole tree structure available to your program.
DOM is a language (and platform) neutral API defined by the W3C,
which allows you to define and manipulate a document -
not only an XML document - from a program. JDOM is made
specifically for Java and XML, and is therefore much simpler to
use for a Java programmer. So in most situations I'd recommend
JDOM, but you should note, that JDOM is not (yet) a standard,
so the API may still change. JDOM has however been accepted as a
Java Specification Request
(JSR102).
SAX could be the best choice if your program reads an XML-file
and you only need to use parts of it, or if the
XML-file is very large so it can't be held in memory. But you
could also use SAX to build your own favorite Document Model.
This article is about JDOM. It's easy to read, edit and write
XML documents with JDOM, since it has been designed
and implemented with the Java programmer in mind. I have an idea
about how JDOM was conceived. Maybe the authors thought:
"How easily can we implement an XML tree structure in Java, using
standard Java classes and features? Well, an XML tree
consists of elements with certain properties, and every element
can have a number of child elements. So let's start by
creating an Element class with the necessary properties. Since
the Collection interfaces from the java.util package are
well-known let's use a List of Elements to represent the children.
Now, let's see how far we can go with this approach".
In this article I'll show you what the authors ended up with.
I'll briefly show you how to read and write an XML file,
but the focus will be on how you edit the document tree: add,
change, or delete the elements. If you hang on until the end
of the article you'll be rewarded with a complete web application
to manage your DVD library.
By the way: When reading an XML file JDOM actually uses SAX to
build the document tree!
A one-liner
How difficult would you think it is to read an XML-file, build
a JDOM document, and finally write the document (in XML-format)
to System.out? Well I'll show you:
new XMLOutputter().output(new SAXBuilder().build("d:\\ex1.xml"), System.out);
OK, I'd never put such a one-liner into one of my own programs,
I'd use four lines (maybe with some comments):
SAXBuilder builder = new SAXBuilder(); // Build a document ...
Document doc = builder.build("d:\\ex1.xml"); // ... from a file
XMLOutputter output = new XMLOutputter(); // And output the document ...
output.output(doc, System.out); // ... unchanged to System.out
Even on four lines it's still simple. The document is kept in the
"Document" class, which is a JDOM class that contains
a variety of information about the document. To work with the
document we need two other classes: "Element" and "Attribute",
which contain--you guessed it: the elements and their attributes.
"Document" contains a method, getRootElement(), which gives
us the root "Element"--you know: every XML document must contain
one and only one root element.
First we'll look into the properties of the Element class.
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