This package defines the generic APIs for processing transformation
instructions, and performing a transformation from source to result. These
interfaces have no dependencies on SAX or the DOM standard, and try to make as
few assumptions as possible about the details of the source and result of a
transformation. It achieves this by defining
Source and
Result interfaces.
To define concrete classes for the user, the API defines specializations
of the interfaces found at the root level. These interfaces are found in
javax.xml.transform.sax, javax.xml.transform.dom,
and javax.xml.transform.stream.
The API allows a concrete
TransformerFactory object to be created from
the static function
newInstance().
This API defines two interface objects called
Source and
Result. In order to pass Source and Result
objects to the interfaces, concrete classes must be used.
Three concrete representations are defined for each of these
objects:
StreamSource and
StreamResult,
SAXSource and
SAXResult, and
DOMSource and
DOMResult. Each of these objects defines
a FEATURE string (which is i the form of a URL), which can be passed into
getFeature(String) to see if the
given type of Source or Result object is supported. For instance, to test if a
DOMSource and a StreamResult is supported, you can apply the following
test.
TransformerFactory tfactory = TransformerFactory.newInstance();
if (tfactory.getFeature(DOMSource.FEATURE) && tfactory.getFeature(StreamResult.FEATURE)) {
...
}
Namespaces present something of a problem area when dealing with XML objects. Qualified Names appear in XML markup as prefixed names. But the prefixes themselves do not hold identity. Rather, it is the URIs that they contextually map to that hold the identity. Therefore, when passing a Qualified Name like "xyz:foo" among Java programs, one must provide a means to map "xyz" to a namespace.
One solution has been to create a "QName" object that holds the namespace URI, as well as the prefix and local name, but this is not always an optimal solution, as when, for example, you want to use unique strings as keys in a dictionary object. Not having a string representation also makes it difficult to specify a namespaced identity outside the context of an XML document.
In order to pass namespaced values to transformations,
for
instance when setting a property or a parameter on a
Transformer object,
this specification defines that a
String "qname" object parameter be passed as two-part string, the namespace URI
enclosed in curly braces ({}), followed by the local name. If the qname has a
null URI, then the String object only contains the local name. An application
can safely check for a non-null URI by testing to see if the first character of
the name is a '{' character.
For example, if a URI and local name were obtained from an element defined with <xyz:foo xmlns:xyz="http://xyz.foo.com/yada/baz.html"/>, then the Qualified Name would be "{http://xyz.foo.com/yada/baz.html}foo". Note that the prefix is lost.
Serialization of the result tree to a stream can be controlled with
the setOutputProperties(Properties) and the
setOutputProperty(String, String) methods.
These properties only apply to stream results, they have no effect when
the result is a DOM tree or SAX event stream.
Strings that match the XSLT
specification for xsl:output attributes can be referenced from the
OutputKeys class. Other strings can be
specified as well.
If the transformer does not recognize an output key, a
IllegalArgumentException is thrown, unless the
key name is namespace qualified. Output key names
that are namespace qualified are always allowed, although they may be
ignored by some implementations.
If all that is desired is the simple identity transformation of a
source to a result, then TransformerFactory
provides a
newTransformer() method
with no arguments. This method creates a Transformer that effectively copies
the source to the result. This method may be used to create a DOM from SAX
events or to create an XML or HTML stream from a DOM or SAX events.
The transformation API throw three types of specialized exceptions. A
TransformerFactoryConfigurationError is parallel to
the FactoryConfigurationError, and is thrown
when a configuration problem with the TransformerFactory exists. This error
will typically be thrown when the transformation factory class specified with
the "javax.xml.transform.TransformerFactory" system property cannot be found or
instantiated.
A TransformerConfigurationException
may be thrown if for any reason a Transformer can not be created. A
TransformerConfigurationException may be thrown if there is a syntax error in
the transformation instructions, for example when
newTransformer() is
called.
TransformerException is a general
exception that occurs during the course of a transformation. A transformer
exception may wrap another exception, and if any of the
printStackTrace()
methods are called on it, it will produce a list of stack dumps, starting from
the most recent. The transformer exception also provides a
SourceLocator object which indicates where
in the source tree or transformation instructions the error occurred.
getMessageAndLocation()
may be called to get an error message with location info, and
getLocationAsString()
may be called to get just the location string.
Transformation warnings and errors are sent to an
ErrorListener, at which point the
application may decide to report the error or warning, and may decide to throw
an Exception for a non-fatal error. The ErrorListener may be set via
setErrorListener(ErrorListener) for
reporting errors that have to do with syntax errors in the transformation
instructions, or via
setErrorListener(ErrorListener) to report
errors that occur during the transformation. The ErrorListener on both objects
will always be valid and non-null, whether set by the application or a default
implementation provided by the processor.
The default implementation provided by the processor will report all warnings and errors to System.err
and does not throw any Exceptions.
Applications are strongly encouraged to register and use
ErrorListeners that insure proper behavior for warnings and
errors.
The API provides a way for URIs referenced from within the stylesheet
instructions or within the transformation to be resolved by the calling
application. This can be done by creating a class that implements the
URIResolver interface, with its one method,
resolve(String, String), and use this class to
set the URI resolution for the transformation instructions or transformation
with setURIResolver(URIResolver) or
setURIResolver(URIResolver). The
URIResolver.resolve method takes two String arguments, the URI found in the
stylesheet instructions or built as part of the transformation process, and the
base URI
against which the first argument will be made absolute if the
absolute URI is required.
The returned Source object must be usable by
the transformer, as specified in its implemented features.