from locating jaxp.properties JBuilder, for instance, installs its own version of Java 2 that it uses by default.
5.2.3 The Transformer Class
As shown in Example 5-3, a Transformer object can be obtained from the
TransformerFactory as follows:
javax.xml.transform.TransformerFactory transFact =
javax.xml.transform.TransformerFactory.newInstance( ); javax.xml.transform.Transformer trans =
transFact.newTransformer(xsltSource);
The Transformer instance is wrapped around an XSLT stylesheet and allows you to perform as many transformations as you wish. The main caveat is thread safety, because many threads cannot use a single Transformer instance concurrently. For each transformation, invoke the
transform method:
abstract void transform(Source xmlSource, Result outputTarget) throws TransformerException
This method is abstract because the TransformerFactory returns a subclass of
Transformer that does the actual work. The Source interface defines where the XML data comes from and the Result interface specifies where the transformation result is sent. The
TransformerException will be thrown if anything goes wrong during the transformation process and may contain the location of the error and a reference to the original exception. The ability to properly report the location of the error is entirely dependent upon the quality of the underlying XSLT transformer implementation's error reporting. We will talk about specific classes that implement the Source and Result interfaces later in this chapter.
Aside from actually performing the transformation, the Transformer implementation allows you to set output properties and stylesheet parameters. In XSLT, a stylesheet parameter is declared and used as follows:
<?xml version="1.0" encoding="UTF -8"?> <xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"> <xsl:output method="html"/>
<xsl:param name="image_dir" select="'images'"/>
<xsl:template match="/"> <html>
<body>
<h1>Stylesheet Parameter Example</h1>
<img src="{$image_dir}/sample.gif"/>
</body> </html>
</xsl:template> </xsl:stylesheet>
The <xsl:param> element declares the parameter name and an optional select attribute. This attribute specifies the default value if the stylesheet parameter is not provided. In this case, the string 'images' is the default value and is enclosed in apostrophes so it is treated as a string instead of an XPath expression. Later, the image_dir variable is referred to with the attribute value template syntax: {$image_dir}.
Passing a variable for the location of your images is a common technique because your
development environment might use a different directory name than your production web server. Another common use for a stylesheet parameter is to pass in data that a servlet generates dynamically, such as a unique ID for session tracking.
From JAXP, pass this parameter via the Transformer instance. The code is simple enough:
javax.xml.transform.Transformer trans =
transFact.newTransformer(xsltSource);
trans.setParameter("image_dir", "graphics");
You can set as many parameters as you like, and these parameters will be saved and reused for every transformation you make with this Transformer instance. If you wish to remove a
parameter, you must call clearParameters( ), which clears all parameters for this Transformer instance. Parameters work similarly to a java.util.Map; if you set the same parameter twice, the second value overwrites the first value.
Another use for the Transformer class is to get and set output properties through one of the following methods:
void setOutputProperties(java.util.Properties props) void setOutputProperty(String name, String value) java.util.Properties getOutputProperties( ) String getOutputProperty(String name)
As you can see, properties are specified as name/value pairs of Strings and can be set and retrieved individually or as a group. Unlike stylesheet parameters, you can un-set an individual property by simply passing in null for the value. The permitted property names are defined in the javax.xml.transform.OutputKeys class and are explained in Table 5-1.
Table 5-1. Constants defined in javax.xml.transform.OutputKeys
Constant Meaning
CDATA_SECTION_ELEMENTS
Specifies a whitespace-separated list of element names whose content should be output as CDATA sections. See the XSLT specification from the W3C for examples.
DOCTYPE_PUBLIC
Only used if DOCTYPE_SYSTEM is also used, this instructs the processor to output a PUBLIC document type declaration. For example: <!DOCTYPE rootElem PUBLIC "public id" "system id">.
DOCTYPE_SYSTEM Instructs the processor to output a document-type declaration. For
example: <!DOCTYPE rootElem SYSTEM "system id">.
ENCODING Specifies the character encoding of the result tree, such as UTF-8
or UTF-16.
INDENT Specifies whether or not whitespace may be added to the result
and no. Although indentation makes the output more readable, it does make the file size larger, thus harming performance.
MEDIA_TYPE The MIME type of the result tree.
METHOD
The output method, either xml, html, or text. Although other values are possible, such as xhtml, these are implementation- defined and may be rejected by your processor.
OMIT_XML_DECLARATION Acceptable values are yes and no, specifying whether or not to
include the XML declaration on the first line of the result tree.
STANDALONE
Acceptable values are yes and no, specifying whether or not the XML declaration indicates that the document is standalone. For example: <?xml version="1.0" encoding="UTF-8" standalone="yes"?>.
VERSION
Specifies the version of the output method, typically 1.0 for XML output. This shows up in the XML declaration as follows: <?xml version="1.0" encoding="UTF-8"?>.
It is no coincidence that these output properties are the same as the properties you can set on the <xsl:output> element in your stylesheets. For example:
<xsl:output method="xml" indent="yes" encoding="UTF -8"/>
Using JAXP, you can either specify additional output properties or override those set in the stylesheet. To change the encoding, write this code:
// this will take precedence over any encoding specified in the stylesheet
trans.setOutputProperty(OutputKeys.ENCODING, "UTF -16");
Keep in mind that this will, in addition to adding encoding="UTF-16" to the XML declaration, actually cause the processor to use that encoding in the result tree. For a value of UTF-16, this means that 16-bit Unicode characters will be generated, so you may have trouble viewing the result tree in many ASCII-only text editors.