Input and Output
XSLT processors, like other XML tools, can read their input data from
many different sources. In the most basic scenario, you will load a
static stylesheet and XML document using the java.io.File
System Identifiers, Files, and URLs
The simple examples presented earlier in this chapter introduced the
concept of a system identifier. As mentioned before, system identifiers
are nothing more than URIs and are used frequently by XML tools. For
example, javax.xml.transform.Source,
public interface Source {
String getSystemId( );
void setSystemId(String systemId);
}
The second method, setSystemId( ), is crucial. By
providing a URI to the Source, the XSLT processor can
resolve URIs encountered in XSLT stylesheets. This allows XSLT code
like this to work:
<xsl:import href="commonFooter.xslt"/>
When it comes to XSLT programming, you will use methods in
java.io.File and java.net.URL to
convert platform-specific file names into system IDs. These can
then be used as parameters to any methods that expect a system ID
as a parameter. For example, you would write the following code to
convert a platform-specific filename into a system ID:
public static void main(String[] args) {
// assume that the first command-line arg contains a file name
// - on Windows, something like "C:\home\index.xml"
// - on Unix, something like "/usr/home/index.xml"
String fileName = args[0];
File fileObject = new File(fileName);
URL fileURL = fileObject.toURL( );
String systemID = fileURL.toExternalForm( );
This code was written on several lines for clarity; it can be
consolidated as follows:
String systemID = new File(fileName).toURL().toExternalForm( );
Converting from a system identifier back to a filename or a
File object can be accomplished with this code:
URL url = new URL(systemID);
String fileName = url.getFile( );
File fileObject = new File(fileName);
And once again, this code can be condensed into a single line as
follows:
File fileObject = new File((new URL(systemID)).getFile( ));
JAXP I/O Design
The Source and Result interfaces in
javax.xml.transform provide the basis for all
transformation input and output in JAXP 1.1. Regardless of whether
a stylesheet is obtained via a URI, filename, or InputStream, its data is fed into JAXP via an implementation of the Source interface. The output is then sent to an implementation of the Result interface. The implementations provided by JAXP are shown in Figure 5-3.
Figure 5-3.
Source and Result interfaces
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As you can see, JAXP is not particular about where it gets its data or
sends its results. Remember that two instances of Source
are always specified: one for the XML data and another for the XSLT
stylesheet.
JAXP Stream I/O
As shown in Figure 5-3,
StreamSource is one of the implementations of the
Source interface. In addition to the system identifiers
that Source provides, StreamSource allows
input to be obtained from a File, an
InputStream, or a Reader. The
SimpleJaxp class in Example 5-3 showed how to use StreamSource to read from a File object. There are also four constructors that allow you to construct a StreamSource from either an InputStream or Reader. The complete list of constructors is shown here:
public StreamSource( )
public StreamSource(File f)
public StreamSource(String systemId)
public StreamSource(InputStream byteStream)
public StreamSource(InputStream byteStream, String systemId)
public StreamSource(Reader characterStream)
public StreamSource(Reader characterStream, String systemId)
For the constructors that take InputStream and
Reader as arguments, the first argument provides
either the XML data or the XSLT stylesheet. The second argument,
if present, is used to resolve relative URI references in the
document. As mentioned before, your XSLT stylesheet may include
the following code:
<xsl:import href="commonFooter.xslt"/>
By providing a system identifier as a parameter to the
StreamSource, you are telling the XSLT processor
where to look for commonFooter.xslt. Without this
parameter, you may encounter an error when the processor cannot
resolve this URI. The simple fix is to call the setSystemId( )
method as follows:
// construct a Source that reads from an InputStream
Source mySrc = new StreamSource(anInputStream);
// specify a system ID (a String) so the Source can resolve
// relative URLs that are encountered in XSLT stylesheets
mySrc.setSystemId(aSystemId);
The documentation for StreamSource also advises that
InputStream is preferred to Reader because
this allows the processor to properly handle the character encoding
as specified in the XML declaration.
StreamResult is similar in functionality to
StreamSource, although it is not necessary to
resolve relative URIs. The available constructors are as follows:
public StreamResult( )
public StreamResult(File f)
public StreamResult(String systemId)
public StreamResult(OutputStream byteStream)
public StreamResult(Writer characterStream)
Let's look at some of the other options for StreamSource
and StreamResult. Example 5-4
is a modification of the SimpleJaxp program that was
presented earlier. It downloads the XML specification from the W3C
web site and stores it in a temporary file on your local disk. To
download the file, construct a StreamSource with a
system identifier as a parameter. The stylesheet is a simple one
that merely performs an identity transformation, copying the
unmodified XML data to the result tree. The result is then sent to
a StreamResult using its File constructor.
Example 5-4:
Streams.java
package chap5;
import java.io.*;
import javax.xml.transform.*;
import javax.xml.transform.stream.*;
/**
* A simple demo of JAXP 1.1 StreamSource and StreamResult. This
* program downloads the XML specification from the W3C and prints
* it to a temporary file.
*/
public class Streams {
// an identity copy stylesheet
private static final String IDENTITY_XSLT =
"<xsl:stylesheet xmlns:xsl=
'http://www.w3.org/1999/XSL/Transform'"
+ " version='1.0'>"
+ "<xsl:template match='/'>
<xsl:copy-of select='.'/>"
+ "</xsl:template></xsl:stylesheet>";
// the XML spec in XML format
// (using an HTTP URL rather than a file URL)
private static String xmlSystemId =
"http://www.w3.org/TR/2000/REC-xml-20001006.xml";
public static void main(String[] args) throws IOException,
TransformerException {
// show how to read from a system identifier and a Reader
Source xmlSource = new StreamSource(xmlSystemId);
Source xsltSource = new StreamSource(
new StringReader(IDENTITY_XSLT));
// send the result to a file
File resultFile = File.createTempFile("Streams", ".xml");
Result result = new StreamResult(resultFile);
System.out.println("Results will go to: "
+ resultFile.getAbsolutePath( ));
// get the factory
TransformerFactory transFact =
TransformerFactory.newInstance( );
// get a transformer for this particular stylesheet
Transformer trans =
transFact.newTransformer(xsltSource);
// do the transformation
trans.transform(xmlSource, result);
}
}
Note: Color coded lines have been broken for display purposes.
The "identity copy" stylesheet simply matches
"/", which is the document itself. It
then uses <xsl:copy-of select='.'/> to select
the document and copy it to the result tree. In this case, we
coded our own stylesheet. You can also omit the XSLT stylesheet
altogether as follows:
// construct a Transformer without any XSLT stylesheet
Transformer trans = transFact.newTransformer( );
In this case, the processor will provide its own stylesheet and do
the same thing that our example does. This is useful when you need to
use JAXP to convert a DOM tree to XML text for debugging purposes
because the default Transformer will simply copy the
XML data without any transformation.
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