1.1. Overview of the DOM
Core Interfaces
This section defines a set of objects and interfaces for
accessing and manipulating document objects. The functionality
specified in this section (the Core functionality) is
sufficient to allow software developers and web script authors to
access and manipulate parsed HTML and XML content inside conforming
products. The DOM Core API also allows creation and population of a
Document object
using only DOM API calls; loading a Document and saving it
persistently is left to the product that implements the DOM
API.
1.1.1. The DOM Structure
Model
The DOM presents documents as a hierarchy of Node objects that
also implement other, more specialized interfaces. Some types of
nodes may have child
nodes of various types, and others are leaf nodes that cannot have
anything below them in the document structure. For XML and HTML,
the node types, and which node types they may have as children, are
as follows:
Document -- Element (maximum of
one), ProcessingInstruction,
Comment, DocumentType
(maximum of one)DocumentFragment --
Element, ProcessingInstruction,
Comment, Text, CDATASection, EntityReferenceDocumentType
-- no childrenEntityReference -- Element, ProcessingInstruction,
Comment, Text, CDATASection, EntityReferenceElement -- Element, Text, Comment, ProcessingInstruction,
CDATASection, EntityReferenceAttr -- Text, EntityReferenceProcessingInstruction
-- no childrenComment --
no childrenText -- no
childrenCDATASection
-- no childrenEntity -- Element, ProcessingInstruction,
Comment, Text, CDATASection, EntityReferenceNotation -- no
children
The DOM also specifies a NodeList interface
to handle ordered lists of Nodes, such as the
children of a Node, or the elements returned by
the getElementsByTagName method of the Element interface,
and also a NamedNodeMap
interface to handle unordered sets of nodes referenced by their
name attribute, such as the attributes of an Element. NodeList and NamedNodeMap
objects in the DOM are live; that is, changes to the
underlying document structure are reflected in all relevant NodeList and NamedNodeMap
objects. For example, if a DOM user gets a NodeList object
containing the children of an Element, then
subsequently adds more children to that element (or removes
children, or modifies them), those changes are automatically
reflected in the NodeList, without
further action on the user's part. Likewise, changes to a Node in the tree
are reflected in all references to that Node in NodeList and NamedNodeMap
objects.
Finally, the interfaces Text, Comment, and CDATASection all
inherit from the CharacterData
interface.
1.1.2. Memory Management
Most of the APIs defined by this specification are
interfaces rather than classes. That means that an
implementation need only expose methods with the defined names and
specified operation, not implement classes that correspond directly
to the interfaces. This allows the DOM APIs to be implemented as a
thin veneer on top of legacy applications with their own data
structures, or on top of newer applications with different class
hierarchies. This also means that ordinary constructors (in the
Java or C++ sense) cannot be used to create DOM objects, since the
underlying objects to be constructed may have little relationship
to the DOM interfaces. The conventional solution to this in
object-oriented design is to define factory methods that
create instances of objects that implement the various interfaces.
Objects implementing some interface "X" are created by a
"createX()" method on the Document interface;
this is because all DOM objects live in the context of a specific
Document.
The DOM Level 2 API does not define a standard way to
create DOMImplementation
objects; DOM implementations must provide some proprietary way of
bootstrapping these DOM interfaces, and then all other objects can
be built from there.
The Core DOM APIs are designed to be compatible with a wide
range of languages, including both general-user scripting languages
and the more challenging languages used mostly by professional
programmers. Thus, the DOM APIs need to operate across a variety of
memory management philosophies, from language bindings that do not
expose memory management to the user at all, through those (notably
Java) that provide explicit constructors but provide an automatic
garbage collection mechanism to automatically reclaim unused
memory, to those (especially C/C++) that generally require the
programmer to explicitly allocate object memory, track where it is
used, and explicitly free it for re-use. To ensure a consistent API
across these platforms, the DOM does not address memory management
issues at all, but instead leaves these for the implementation.
Neither of the explicit language bindings defined by the DOM API
(for ECMAScript
and Java) require any memory management methods, but DOM bindings
for other languages (especially C or C++) may require such support.
These extensions will be the responsibility of those adapting the
DOM API to a specific language, not the DOM Working Group.
1.1.3. Naming Conventions
While it would be nice to have attribute and method names that
are short, informative, internally consistent, and familiar to
users of similar APIs, the names also should not clash with the
names in legacy APIs supported by DOM implementations. Furthermore,
both OMG IDL and ECMAScript have significant
limitations in their ability to disambiguate names from different
namespaces that make it difficult to avoid naming conflicts with
short, familiar names. So, DOM names tend to be long and
descriptive in order to be unique across all environments.
The Working Group has also attempted to be internally consistent
in its use of various terms, even though these may not be common
distinctions in other APIs. For example, the DOM API uses the
method name "remove" when the method changes the structural model,
and the method name "delete" when the method gets rid of something
inside the structure model. The thing that is deleted is not
returned. The thing that is removed may be returned, when it makes
sense to return it.
1.1.4. Inheritance vs.
Flattened Views of the API
The DOM Core APIs
present two somewhat different sets of interfaces to an XML/HTML
document: one presenting an "object oriented" approach with a
hierarchy of inheritance, and a
"simplified" view that allows all manipulation to be done via the
Node interface
without requiring casts (in Java and other C-like languages) or
query interface calls in COM environments. These
operations are fairly expensive in Java and COM, and the DOM may be
used in performance-critical environments, so we allow significant
functionality using just the Node interface.
Because many other users will find the inheritance
hierarchy easier to understand than the "everything is a Node" approach to
the DOM, we also support the full higher-level interfaces for those
who prefer a more object-oriented API.
In practice, this means that there is a certain amount of
redundancy in the API.
The Working Group considers the "inheritance"
approach the primary view of the API, and the full set of
functionality on Node to be "extra"
functionality that users may employ, but that does not eliminate
the need for methods on other interfaces that an object-oriented
analysis would dictate. (Of course, when the O-O analysis yields an
attribute or method that is identical to one on the Node
interface, we don't specify a completely redundant one.) Thus, even
though there is a generic nodeName attribute on the Node
interface, there is still a tagName attribute on the
Element
interface; these two attributes must contain the same value, but
the it is worthwhile to support both, given the different
constituencies the DOM API must satisfy.
To ensure interoperability, the DOM specifies the following:
-
- Type Definition DOMString
-
A DOMString is a
sequence of 16-bit
units.
IDL Definition-
valuetype DOMString sequence<unsigned short>;
- Applications must encode
DOMString using UTF-16
(defined in [Unicode] and Amendment 1 of [ISO/IEC
10646]).
The UTF-16 encoding was chosen because of its widespread industry
practice. Note that for both HTML and XML, the document character
set (and therefore the notation of numeric character references) is
based on UCS [ISO-10646]. A single numeric character reference in a
source document may therefore in some cases correspond to two
16-bit units in a DOMString (a high
surrogate and a low surrogate).
Note: Even though the DOM defines the name of the string
type to be DOMString, bindings may
use different names. For example for Java, DOMString is bound to
the String type because it also uses UTF-16 as its
encoding.
Note: As of August 2000, the OMG IDL specification ([OMGIDL]) included
a wstring type. However, that definition did not meet
the interoperability criteria of the DOM API since it relied on
negotiation to decide the width and encoding of a character.
To ensure interoperability, the DOM specifies the following:
-
- Type Definition DOMTimeStamp
-
A DOMTimeStamp
represents a number of milliseconds.
IDL Definition-
typedef unsigned long long DOMTimeStamp;
-
Note: Even though the DOM uses the type DOMTimeStamp,
bindings may use different types. For example for Java, DOMTimeStamp is
bound to the long type. In ECMAScript,
TimeStamp is bound to the Date type
because the range of the integer type is too
small.
1.1.7. String comparisons in
the DOM
The DOM has many interfaces that imply string matching. HTML
processors generally assume an uppercase (less often, lowercase)
normalization of names for such things as elements, while XML is
explicitly case sensitive. For the purposes of the DOM, string
matching is performed purely by binary comparison of
the 16-bit
units of the DOMString. In addition,
the DOM assumes that any case normalizations take place in the
processor, before the DOM structures are built.
Note: Besides case folding, there are additional
normalizations that can be applied to text. The W3C I18N Working
Group is in the process of defining exactly which normalizations
are necessary, and where they should be applied. The W3C I18N
Working Group expects to require early normalization, which means
that data read into the DOM is assumed to already be normalized.
The DOM and applications built on top of it in this case only have
to assure that text remains normalized when being changed. For
further details, please see [Charmod].
1.1.8. XML
Namespaces
The DOM Level 2 supports XML namespaces [Namespaces] by augmenting
several interfaces of the DOM Level 1 Core to allow creating and
manipulating elements and
attributes associated to a namespace.
As far as the DOM is concerned, special attributes used for
declaring XML
namespaces are still exposed and can be manipulated just
like any other attribute. However, nodes are permanently bound to
namespace URIs
as they get created. Consequently, moving a node within a document,
using the DOM, in no case results in a change of its namespace
prefix or namespace URI. Similarly, creating a node with a
namespace prefix and namespace URI, or changing the namespace
prefix of a node, does not result in any addition, removal, or
modification of any special attributes for declaring the
appropriate XML namespaces. Namespace validation is not enforced;
the DOM application is responsible. In particular, since the
mapping between prefixes and namespace URIs is not enforced, in
general, the resulting document cannot be serialized naively. For
example, applications may have to declare every namespace in use
when serializing a document.
DOM Level 2 doesn't perform any URI normalization or
canonicalization. The URIs given to the DOM are assumed to be valid
(e.g., characters such as whitespaces are properly escaped), and no
lexical checking is performed. Absolute URI references are treated
as strings and compared
literally. How relative namespace URI references are
treated is undefined. To ensure interoperability only absolute
namespace URI references (i.e., URI references beginning with a
scheme name and a colon) should be used. Note that because the DOM
does no lexical checking, the empty string will be treated as a
real namespace URI in DOM Level 2 methods. Applications must use
the value null as the namespaceURI parameter for
methods if they wish to have no namespace.
Note: In the DOM, all namespace declaration attributes
are by definition bound to the namespace URI: "http://www.w3.org/2000/xmlns/".
These are the attributes whose namespace
prefix or qualified name
is "xmlns". Although, at the time of writing, this is not part of
the XML Namespaces specification [Namespaces], it is planned to
be incorporated in a future revision.
In a document with no namespaces, the child list of an EntityReference node
is always the same as that of the corresponding Entity. This is not
true in a document where an entity contains unbound namespace
prefixes. In such a case, the descendants of the
corresponding EntityReference nodes
may be bound to different namespace URIs,
depending on where the entity references are. Also, because, in the
DOM, nodes always remain bound to the same namespace URI, moving
such EntityReference nodes
can lead to documents that cannot be serialized. This is also true
when the DOM Level 1 method createEntityReference of
the Document
interface is used to create entity references that correspond to
such entities, since the descendants of the
returned EntityReference are
unbound. The DOM Level 2 does not support any mechanism to resolve
namespace prefixes. For all of these reasons, use of such entities
and entity references should be avoided or used with extreme care.
A future Level of the DOM may include some additional support for
handling these.
The new methods, such as createElementNS and
createAttributeNS of the Document interface,
are meant to be used by namespace aware applications. Simple
applications that do not use namespaces can use the DOM Level 1
methods, such as createElement and
createAttribute. Elements and attributes created in
this way do not have any namespace prefix, namespace URI, or local
name.
Note: DOM Level 1 methods are namespace ignorant.
Therefore, while it is safe to use these methods when not dealing
with namespaces, using them and the new ones at the same time
should be avoided. DOM Level 1 methods solely identify attribute
nodes by their nodeName. On the contrary, the DOM
Level 2 methods related to namespaces, identify attribute nodes by
their namespaceURI and localName. Because
of this fundamental difference, mixing both sets of methods can
lead to unpredictable results. In particular, using
setAttributeNS, an element may have two
attributes (or more) that have the same nodeName, but
different namespaceURIs. Calling
getAttribute with that nodeName could
then return any of those attributes. The result depends on the
implementation. Similarly, using setAttributeNode, one
can set two attributes (or more) that have different
nodeNames but the same prefix and
namespaceURI. In this case
getAttributeNodeNS will return either attribute, in an
implementation dependent manner. The only guarantee in such cases
is that all methods that access a named item by its
nodeName will access the same item, and all methods
which access a node by its URI and local name will access the same
node. For instance, setAttribute and
setAttributeNS affect the node that
getAttribute and getAttributeNS,
respectively, return.
