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JAXP DOM I/O

In many cases, the fastest form of transformation available is to feed an instance of org.w3c.dom.Document directly into JAXP. Although the transformation is fast, it does take time to generate the DOM; DOM is also memory intensive, and may not be the best choice for large documents. In most cases, the DOM data will be generated dynamically as the result of a database query or some other operation (see Chapter 1). Once the DOM is generated, simply wrap the Document object in a DOMSource as follows:

org.w3c.dom.Document domDoc = createDomDocument(  );
Source xmlSource = 
	new javax.xml.transform.dom.DOMSource(domDoc);

The remainder of the transformation looks identical to the file-based transformation shown in Example 5-4. JAXP needs only the alternate input Source object shown here to read from DOM.

JAXP SAX I/O

XSLT is designed to transform well-formed XML data into another format, typically HTML. But wouldn't it be nice if we could also use XSLT stylesheets to transform nonXML data into HTML? For example, most spreadsheets have the ability to export their data into Comma Separated Values (CSV) format, as shown here:

Burke,Eric,M
Burke,Jennifer,L
Burke,Aidan,G

One approach is parsing the file into memory, using DOM to create an XML representation of the data, and then feeding that information into JAXP for transformation. This approach works but requires an intermediate programming step to convert the CSV file into a DOM tree. A better option is to write a custom SAX parser, feeding its output directly into JAXP. This avoids the overhead of constructing the DOM tree, offering better memory utilization and performance.

The approach

It turns out that writing a SAX parser is quite easy. [4] All a SAX parser does is read an XML file top to bottom and fire event notifications as various elements are encountered. In our custom parser, we will read the CSV file top to bottom, firing SAX events as we read the file. A program listening to those SAX events will not realize that the data file is CSV rather than XML; it sees only the events. Figure 5-4 illustrates the conceptual model.

Figure 5-4. Custom SAX parser

In this model, the XSLT processor interprets the SAX events as XML data and uses a normal stylesheet to perform the transformation. The interesting aspect of this model is that we can easily write custom SAX parsers for other file formats, making XSLT a useful transformation language for just about any legacy application data.

In SAX, org.xml.sax.XMLReader is a standard interface that parsers must implement. It works in conjunction with org.xml.sax.ContentHandler, which is the interface that listens to SAX events. For this model to work, your XSLT processor must implement the ContentHandler interface so it can listen to the SAX events that the XMLReader generates. In the case of JAXP, javax.xml.transform.sax.TransformerHandler is used for this purpose.

Obtaining an instance of TransformerHandler requires a few extra programming steps. First, create a TransformerFactory as usual:

TransformerFactory transFact = 
	TransformerFactory.newInstance(  );

As before, the TransformerFactory is the JAXP abstraction to some underlying XSLT processor. This underlying processor may not support SAX features, so you have to query it to determine if you can proceed:

if (transFact.getFeature(SAXTransformerFactory.FEATURE)) {

If this returns false, you are out of luck. Otherwise, you can safely downcast to a SAXTransformerFactory and construct the TransformerHandler instance:

SAXTransformerFactory saxTransFact =
            (SAXTransformerFactory) transFact;
  // create a ContentHandler, don't specify a stylesheet.  Without
  // a stylesheet, raw XML is sent to the output.
  TransformerHandler transHand = 
  	saxTransFact.newTransformerHandler(  );

In the code shown here, a stylesheet was not specified. JAXP defaults to the identity transformation stylesheet, which means that the SAX events will be "transformed" into raw XML output. To specify a stylesheet that performs an actual transformation, pass a Source to the method as follows:

Source xsltSource = new StreamSource(myXsltSystemId);
TransformerHandler transHand = 
	saxTransFact.newTransformerHandler(
        xsltSource);

Detailed CSV to SAX design

Before delving into the complete example program, let's step back and look at a more detailed design diagram. The conceptual model is straightforward, but quite a few classes and interfaces come into play. Figure 5-5 shows the pieces necessary for SAX-based transformations.

Figure 5-5. SAX and XSLT transformations

This diagram certainly appears to be more complex than previous approaches, but is similar in many ways. In previous approaches, we used the TransformerFactory to create instances of Transformer; in the SAX approach, we start with a subclass of TransformerFactory. Before any work can be done, you must verify that your particular implementation supports SAX-based transformations. The reference implementation of JAXP does support this, although other implementations are not required to do so. In the following code fragment, the getFeature method of TransformerFactory will return true if you can safely downcast to a SAXTransformerFactory instance:

TransformerFactory transFact = 
	TransformerFactory.newInstance(  );
if (transFact.getFeature(SAXTransformerFactory.FEATURE)) {
    // downcast is allowed
    SAXTransformerFactory saxTransFact = 
		(SAXTransformerFactory) transFact;

If getFeature returns false, your only option is to look for an implementation that does support SAX-based transformations. Otherwise, you can proceed to create an instance of TransformerHandler:

TransformerHandler transHand = 
	saxTransFact.newTransformerHandler(myXsltSource);

This object now represents your XSLT stylesheet. As Figure 5-5 shows, TransformerHandler extends org.xml.sax.ContentHandler, so it knows how to listen to events from a SAX parser. The series of SAX events will provide the "fake XML" data, so the only remaining piece of the puzzle is to set the Result and tell the SAX parser to begin parsing. The TransformerHandler also provides a reference to a Transformer, which allows you to set output properties such as the character encoding, whether to indent the output or any other attributes of <xsl:output>.

Footnote:

4. Our examples use SAX 2.

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