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javax.xml.transform
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
{@link javax.xml.transform.Source} and
{@link javax.xml.transform.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
{@link javax.xml.transform.sax}, {@link javax.xml.transform.dom},
and {@link javax.xml.transform.stream}.
Creating Objects
The API allows a concrete
{@link javax.xml.transform.TransformerFactory} object to be created from
the static function
{@link javax.xml.transform.TransformerFactory#newInstance}.
Specification of Inputs and Outputs
This API defines two interface objects called
{@link javax.xml.transform.Source} and
{@link javax.xml.transform.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:
{@link javax.xml.transform.stream.StreamSource} and
{@link javax.xml.transform.stream.StreamResult},
{@link javax.xml.transform.sax.SAXSource} and
{@link javax.xml.transform.sax.SAXResult}, and
{@link javax.xml.transform.dom.DOMSource} and
{@link javax.xml.transform.dom.DOMResult}. Each of these objects defines
a FEATURE string (which is i the form of a URL), which can be passed into
{@link javax.xml.transform.TransformerFactory#getFeature} 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)) {
...
}
Qualified Name Representation
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
{@link javax.xml.transform.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.
Result Tree Serialization
Serialization of the result tree to a stream can be controlled with
the {@link javax.xml.transform.Transformer#setOutputProperties} and the
{@link javax.xml.transform.Transformer#setOutputProperty} 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
{@link javax.xml.transform.OutputKeys} class. Other strings can be
specified as well.
If the transformer does not recognize an output key, a
{@link java.lang.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 {@link javax.xml.transform.TransformerFactory}
provides a
{@link javax.xml.transform.TransformerFactory#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.
Exceptions and Error Reporting
The transformation API throw three types of specialized exceptions. A
{@link javax.xml.transform.TransformerFactoryConfigurationError} is parallel to
the {@link javax.xml.parsers.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 {@link javax.xml.transform.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
{@link javax.xml.transform.TransformerFactory#newTransformer} is
called.
{@link javax.xml.transform.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
{@link javax.xml.transform.TransformerException#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
{@link javax.xml.transform.SourceLocator} object which indicates where
in the source tree or transformation instructions the error occurred.
{@link javax.xml.transform.TransformerException#getMessageAndLocation()}
may be called to get an error message with location info, and
{@link javax.xml.transform.TransformerException#getLocationAsString()}
may be called to get just the location string.
Transformation warnings and errors are sent to an
{@link javax.xml.transform.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
{@link javax.xml.transform.TransformerFactory#setErrorListener} for
reporting errors that have to do with syntax errors in the transformation
instructions, or via
{@link javax.xml.transform.Transformer#setErrorListener} 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.
Resolution of URIs within a transformation
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
{@link javax.xml.transform.URIResolver} interface, with its one method,
{@link javax.xml.transform.URIResolver#resolve}, and use this class to
set the URI resolution for the transformation instructions or transformation
with {@link javax.xml.transform.TransformerFactory#setURIResolver} or
{@link javax.xml.transform.Transformer#setURIResolver}. 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 {@link javax.xml.transform.Source} object must be usable by
the transformer, as specified in its implemented features.