David Mertz, Ph.D. <[email protected]>
Gnosis Software, Inc. <http://gnosis.cx/publish/>
Several very distinct conceputation models exist for manipulation of XML documents. This series will look at each model. The Document Object Model is a World Wide Web Consortium Recommendation for an object oriented approach to XML programming. This article gives a primer on how DOM represents XML trees conceptually. Source code examples of using DOM are provided, along with pointers to further resources.
As XML has developed into a widely used data format, a number of programming models have arisen for manipulating XML documents. Some of these models--or "paradigms"--have been enshrined as standards, while others remain only informally specified (but equally widely used nonetheless). In a general way, the several models available for manipulating XML documents closely mirror the underlying approaches and techniques that programmers in different traditions bring to the task of working with XML. It is worth noticing that "models" are at a higher level of abstraction than a particular programming language; most of the models discussed in this series are associated with APIs that have been implemented in multiple programming languages.
In part, the richness of available XML programming models simply allows programmers and projects to work in the ways that are most comfortable and familiar to them. In many ways, there is overlap--at least in achievable outcomes--between all the XML programming models. However, different models also carry with them specific pros and cons in the context of XML manipulation; and these might urge the use of particular models for particular projects. This series of five articles aims to provide readers with an overview of the costs, benefits, and motivations for all of the major approaches to programmatic manipulation of XML documents (manipulation here, should be understood also to mean "using XML to drive or communicate other application processes").
The current article, Part 1, discusses the Document Object Model (DOM), which is a W3C Recommendation. DOM is an Object Oriented Programming (OOP) approach to the manipulation of XML documents. Part 2 of this series will address the Simple API for XML (SAX), which is an event-driven and procedural style of XML programming. Part 3 will look at XSLT, which brings a declarative programming style to transformations of XML documents. In Part 4, we will see the application of full-fledged Functional Programming (FP) techniques to XML manipulation--these in some ways unify the earlier models (but are less commonly used). The final installment, Part 5, will look briefly at a number of tools and techniques that did not quite fit into the previous discussion, but that readers would do well to be aware of.
DOM represents an XML document as a
Node, which contains,
hierarchically, zero or more "child"
Node's. What is OOP
about a particular DOM tree is its containment tree, not any inheritence
trees (although there is also an inheritence structure to the special
Node types). In fact, every such descendent
has pretty much the same structure as the
Node is a fairly ordinary in-memory
object from the point-of-view of an object-oriented programming language.
Node is just an object which has a few API-specified
attributes/data-members, and a few methods/member-functions (the terminology
varies between languages).
There are two wrinkles to the system to address immediately. First,
there are actually two sorts of "children" that a
might have. Some types of
Node's have children, while others
do not. One type of child is an 'ELEMENT_NODE's subelements, the other
is its XML attributes. These two types of children are both specialized
'Node's themselves. Subelements are found in the DOM
childNodes; XML attributes are found
in the data-member
This brings us to the second wrinkle. As well as
objects, DOM has two types of collection objects. Ordered collections
Node objects live in a
Unordered collections of named
Node objects line in a
NamedNodeMap . The value of the
data-member, for example, is a
Node methods also return a
NodeList). The value
attributes data-member, in contrast, is a
Node collections have a nice property: they are "live" connections
to the underlying DOM
Document. A list or map is not
just a snapshot of an XML document at a moment in time, but is the
actual collection of
Node objects that fulfill some property
(like living under a particular parent). The W3C DOM FAQ emphasizes
NodeList, although it resembles an array or vector (it has a length attribute, and you can access the members of the list via an integer index), is not an array. Think of it instead as another way of looking at the DOM's document tree. If that tree changes if something inserts or appends or removes Nodes the NodeList will be automatically adjusted at the same time. The result is that a NodeList is always an accurate representation of the getChildNodes or getElementsByTagName results as if you had just issued that call, so there is no need to refresh the NodeList to pick up changes to the underlying document.
The overall picture here is that a DOM object represents an entire XML document, but it is possible to provide "handles" or "pointers" or "proxies" into particular parts or samples of the structure for convenience.
Nodes are lower level and more numerous than one might expect
at first. Basically, everything is a
Node, but they
come in different types. In terms of OOP inheritance, all the specific
types descend from the abstract class
Node. Each concrete
node is one of the following types: ELEMENT_NODE, ATTRIBUTE_NODE, TEXT_NODE,
CDATA_SECTION_NODE, ENTITY_NODE, PROCESSING_INSTRUCTION_NODE, COMMENT_NODE,
DOCUMENT_NODE, DOCUMENT_TYPE_NODE, NOTATION_NODE. These types are
fairly self-explanatory, and the uppercase names indicated are usually
defined as constants in a particular DOM implementation (import syntax
and name qualification will depend on the programming language).
Node of any DOM representation of an XML document
is a DOCUMENT_NODE. This
Document node will contain zero
or more PROCESSING_INSTRUCTION_NODE's and COMMENT_NODE's, and
exactly one ELEMENT_NODE. The one ELEMENT_NODE is the root element
of the XML document. The root element, in turn, may contain nodes of
the various other types--and perhaps children of the root will contain
nodes of the various types.
TEXT_NODE's in particular are a little suprising in their behavior.
On the one hand, one might expect the text of an element to simply
be a string-type data-member of an ELEMENT_NODE. In some ways that would
be more convenient. But instead, the textual contents inside an XML
element is contained in its own TEXT_NODE. And being a type of
Node , a TEXT_NODE is not itself a string, but is instead
a DOM object whose attribute
nodeValue has a string
in it. Its
nodeName, for what it's worth, contains the
But the thing that is most surprising about TEXT_NODE's is that
they are not guaranteed to contain all the text inside a particular
element, even if there is nothing other than text in the element.
A TEXT_NODE is simply promised to contain some text--the entire
textual content of an ELEMENT_NODE might be split between any number
of TEXT_NODE's. Fortunately, the DOM method
can be used to transform a DOM tree, and minimize the number of TEXT_NODE's
contained (and more importantly, make each such node of maximal relevant
If you are like me, it is hard to process the abstract descriptions
of DOM without some concrete code to look at. To show readers what
is going on, I will use some commands pasted from a Python interactive
shell. DOM implementations exist in Java, C++, ECMAScript, Perl, Ruby,
and many other programming languages. A nice thing about DOM is that
the code usually looks almost the same across languages. But Python
has two advantages for presentation: (1) it has an interactive shell
to try variations on commands (what the user types is preceded by
>>> ); (2) its code is particularly concise, and resembles
pseudo-code. In general, in this series I will make efforts to avoid
using constructs which are idiomatic to one particular programming language
(so Python programmers may find that there is a more "Pythonic" way of
implementing my examples). In the code samples, those elements that
are generic DOM (and common to all languages) are marked in red.
For this simple demonstration, I created the following trivial XML document:
Let's work with the XML document a bit:
>>> from xml.dom import minidom
Our start shows off several DOM features. After the requisite
imports, we generally create a DOM object by parsing an XML document using
parseString() method. The latter
allows XML documents to come from a source other than a file. Neither
of these methods is actually part of the W3C standard, but both are
present in almost every implementation. We also might have created
a new DOM object from scratch, but starting with an existing XML document
is more common.
When we check, we find that a
Document node has no
attributes; it does, however, have some
. One of the
childNodes is the same as the
documentElement (as is demonstrated). In our example, another
Document is a processing instruction. In
this paricular DOM implementation--and in a number of others--comment
nodes are ignored by the parser (and do not appear in the
). While this limitation is not usually a problem since the programmatic
content should not be in comments, it is something to be aware of.
Let us start working with the root node of the document, which is what we are usually concerned with (and the descendents thereof). Any descendent element will behave almost exactly the same way the root node does.
>>> Spam = dom.documentElement
The attributes are interesting in their behavior. If you happen
to know that an ELEMENT_NODE has a specific attribute, it is easy to
get its value with a method call. But if you are not sure what XML
attributes are present, you have to use a slightly more roundabout
technique of first looking at the
length , then iterating through the attributes
based on the number of them. Guessing an XML attribute that might be
present is not really reliable--this Python DOM implementation returns
an empty string if the attribute does not exist. But this implementation
is flawed, since it cannot distinguish the below cases (vis-a-vis the
<Spam color="" />
To round the brief examples, let us take a look at TEXT_NODE objects, which is usually our main interest in the end. First, let's see the non-normalized and normalized forms:
>>> for node in Spam.childNodes: print node
To follow, let us see a typical usage of the contents in a TEXT_NODE:
>>> firstEggs = Spam.getElementsByTagName('Eggs')
The strong point of DOM is that it provides an OOP framework for
manipulating XML documents that will be familiar to programmers in
many object oriented languages. The method and attribute names suggest
a particular affinity with Java, but the model is common to OOP thinking.
Moreover, although the short article has not touched on it in any detail,
DOM provides a strong set of
Node methods for filtering
and modifying nodes and collections.
The weak point of DOM is that it is very poorly suited for handling
large XML documents. For the trivial case we have presented there
is no issue, but DOM is extremely memory hungry. A DOM representation
of an XML document is likely to be several times the size of the underlying
Node needs an object, and carries a variety
of instance data). While DOM readers that operate incrementally upon
file-based XML documents are in experimental stages, by far the rule
is to read in an entire XML document at one time in order to create a
DOM object. OOP programmers are often lured by the familiarity of DOM
techniques, but consequently make decisions that use resources inefficiently.
In some ways, the SAX approach that the next installment will address
answers the limits of DOM (but introduces its own set of limitations.
The official word on everything DOM is at the World Wide Web Consortium's (W3C) website. Many links to further resources can be found there also:
One particularly useful source of hints at the W3C's DOM site is the DOM FAQ:
A good starting point for the conceptual framework of DOM is Jonathan Robie's "What is the Document Object Model?":