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Title: Java and XSLT
ISBN: 0-596-00143-6, Order Number: 143-6
US Price: $39.95
© O'Reilly & Associates, Inc.

JAXP XSLT Design

Now that we have seen some example code and have begun our exploration of the Transformer class, let's step back and look at the overall design of the XSLT plugability layer. JAXP support for XSLT is broken down into the packages listed in Table 5-2.

Table 5-2: JAXP transformation packages

Package

Description

javax.xml.transform

Defines a general-purpose API for XML transformations without any dependencies on SAX or DOM. The Transformer class is obtained from the TransformerFactory class. The Transformer transforms from a Source to a Result.

javax.xml.transform.dom

Defines how transformations can be performed using DOM. Provides implementations of Source and Result: DOMSource and DOMResult.

javax.xml.transform.sax

Supports SAX2 transformations. Defines SAX versions of Source and Result: SAXSource and SAXResult. Also defines a subclass of TransformerFactory that allows SAX2 events to be fed into an XSLT processor.

javax.xml.transform.stream

Defines I/O stream implementations of Source and Result: StreamSource and StreamResult.

The heart of JAXP XSLT support lies in the javax.xml.transform package, which lays out the mechanics and overall process for any transformation that is performed. This package mostly consists of interfaces and abstract classes, except for OutputKeys and a few exception and error classes. Figure 5-2 presents a UML class diagram that shows all of the pieces in this important package.

Figure 5-2. javax.xml.transform class diagram

 

As you can see, this is a small package, indicative of the fact that JAXP is merely a wrapper around the tools that actually perform transformations. The entry point is TransformerFactory, which creates instances of Transformer, as we have already seen, as well as instances of the Templates abstract class. A Templates object represents a compiled stylesheet and will be covered in detail later in this chapter. [3] The advantage of compilation is performance: the same Templates object can be used over and over by many threads without reparsing the XSLT file.

The URIResolver is responsible for resolving URIs found within stylesheets and is generally something you will not need to deal with directly. It is used when a stylesheet imports or includes another document, and the processor needs to figure out where to look for that document. For example:

<xsl:import href="commonFooter.xslt"/>

ErrorListener, as you may guess, is an interface that allows your code to register as a listener for error conditions. This interface defines the following three methods:

void error(TransformerException ex)
void fatalError(TransformerException ex)
void warning(TransformerException ex)

The TransformerException has the ability to wrap around another Exception or Throwable object and may return an instance of the SourceLocator class. If the underlying XSLT implementation does not provide a SourceLocator, null is returned. The SourceLocator interface defines methods to locate where a TransformerException originated. In the case of error() and warning(), the XSLT processor is required to continue processing the document until the end. For fatalError(), on the other hand, the XSLT processor is not required to continue. If you do not register an ErrorListener object, then all errors, fatal errors, and warnings are normally written to System.err. TransformerFactoryConfigurationError and TransformerConfigurationException round out the error-handling APIs for JAXP, indicating problems configuring the underlying XSLT processor implementation. The TransformerFactoryConfigurationError class is generally used when the implementation class cannot be found on the CLASSPATH or cannot be instantiated at all. TransformerConfigurationException simply indicates a "serious configuration error" according to its documentation.


Footnote:

3. The exact definition of a "compiled" stylesheet is vague. XSLT processors are free to optimize cached stylesheets however they see fit.

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