XML::LibXML provides an light-wight interface to modify a node of the document tree generated by the XML::LibXML parser. This interface
follows as far as possible the DOM Level 3 specification. Additionally to the
specified functions the XML::LibXML supports some functions that are more handy
to use in the perl environment.
One also has to remember, that XML::LibXML is an interface to libxml2 nodes
which actually reside on the C-Level of XML::LibXML. This means each node is a
reference to a structure different than a perl hash or array. The only way to
access these structures values is through the DOM interface provided by
XML::LibXML. This also means, that one cant simply inherit an XML::LibXML node and add new member variables as they were
The DOM interface of XML::LibXML does not intend to implement a full DOM
interface as it is done by XML::GDOME and used for full featured application.
Moreover, it offers an simple way to build or modify documents that are created
by XML::LibXMLs parser.
Another target of the XML::LibXML interface is to make the interfaces of
libxml2 available to the perl community. This includes also some workarounds to
some features where libxml2 assumes more control over the C-Level that most
perl users dont have.
One of the most important parts of the XML::LibXML DOM interface is, that the
interfaces try do follow the DOM Level 3 specification (<http://www.w3.org/TR/DOM-Level-3-Core/>) rather strictly. This means the interface functions are named as the DOM
specification says and not what widespread Java interfaces claim to be
standard. Although there are several functions that have only a singular
interface that conforms to the DOM spec XML::LibXML provides an additional Java
style alias interface.
Also there are some function interfaces left over from early stages of
XML::LibXML for compatibility reasons. These interfaces are for compatibility
reasons only. They might disappear in one of the future versions of XML::LibXML, so a user
is requested to switch over to the official functions.
XML::LibXMLs DOM implementation is limited by the DOM implementation of
libxml2 which treats namespaces slightly differently than required by the DOM
Level 2 specification.
According to the DOM Level 2 specification, namespaces of elements and
attributes should be persistent, and nodes should be permanently bound to
namespace URIs as they get created; it should be possible to manipulate the
special attributes used for declaring XML namespaces just as other attributes
without affecting the namespaces of other nodes. In DOM Level 2, the
application is responsible for creating the special attributes consistently
and/or for correct serialization of the document.
This is both inconvenient, causes problems in serialization of DOM to XML, and
most importantly, seems almost impossible to implement over libxml2.
In libxml2, namespace URI and prefix of a node is provided by a pointer to a
namespace declaration (appearing as a special xmlns attribute in the XML
document). If the prefix or namespace URI of the declaration changes, the
prefix and namespace URI of all nodes that point to it changes as well.
Moreover, in contrast to DOM, a node (element or attribute) can only be bound
to a namespace URI if there is some namespace declaration in the document to
Therefore current DOM implementation in XML::LibXML tries to treat namespace
declarations in a compromise between reason, common sense, limitations of
libxml2, and the DOM Level 2 specification.
In XML::LibXML, special attributes declaring XML namespaces are often created
automatically, usually when a namespaced node is attached to a document and no
existing declaration of the namespace and prefix is in the scope to be reused.
In this respect, XML::LibXML DOM implementation differs from the DOM Level 2
specification according to which special attributes for declaring the
appropriate XML namespaces should not be added when a node with a namespace
prefix and namespace URI is created.
Namespace declarations are also created when XML::LibXML::Documents createElementNS() or createAttributeNS() function are used. If the a
namespace is not declared on the documentElement, the namespace will be locally
declared for the newly created node. In case of Attributes this may look a bit
confusing, since these nodes cannot have namespace declarations itself. In this
case the namespace is internally applied to the attribute and later declared on
the node the attribute is appended to (if required).
The following example may explain this a bit:
my $doc = XML::LibXML->createDocument;
my $root = $doc->createElementNS( "", "foo" );
$doc->setDocumentElement( $root );
my $attr = $doc->createAttributeNS( "bar", "bar:foo", "test" );
$root->setAttributeNodeNS( $attr );
This piece of code will result in the following document:
<foo xmlns:bar="bar" bar:foo="test"/>
The namespace is declared on the document element during the
Namespaces can be also declared explicitly by the use of XML::LibXML::Elements
setNamespace() function. Since 1.61, they can also be manipulated with
functions setNamespaceDeclPrefix() and setNamespaceDeclURI() (not available in
DOM). Changing an URI or prefix of an existing namespace declaration affects
the namespace URI and prefix of all nodes which point to it (that is the nodes
in its scope).
It is also important to repeat the specification: While working with namespaces
you should use the namespace aware functions instead of the simplified
versions. For example you should never use setAttribute() but setAttributeNS().