Developing, Applying and Optimizing XSLT with Java Servlets

The missing link at this point is the servlet framework, which has the following design.

Figure 3: Servlet and XSLT architecture.

This may seem overwhelming at first glance, but it's about everything that is needed for a basic XSLT driven web site. The entire design is very modular, so different programmers can work on different pieces in parallel.

First of all, a single servlet is used rather than many. This means that you only have one point of entry into the application, making security, logging, and deployment easier. Since a single servlet is responsible for the entire application, classes called RequestHandler are used for validation logic. All the servlet does is analyze the request to figure out which RequestHandler subclass to invoke. This keeps the servlet small, even if your application has hundreds of pages.

There are two basic approaches to locating request handlers. You can either add an additional parameter to the HTTP request or add extra path information. For example, this URL could invoke the SubmitCustomer request handler:

http://hostname/webapp/xsltservlet?requestHandler=SubmitCustomer

With extra path information, the URL looks a little cleaner:

http://hostname/webapp/xsltservlet/SubmitCustomer

In either approach, your servlet will extract the text "SubmitCustomer" and convert that into a class name for the request handler. This can be accomplished via reflection or via a lookup table. In either case, once the object is located, it is then responsible for the remainder of this request.

The RequestHandler object may then execute methods on EJB components that return data objects back to the web tier. These data objects are converted to XML, which is then passed on to Apache's Xalan or some other XSLT processor. The output from the XSLT processor is finally delivered to the client browser.

Optimization techniques

The primary disadvantage of the XSLT approach may be runtime performance. Every request to the servlet requires an XSLT transformation. For most applications, performance is fast enough; a few optimization techniques can improve the situation where maximum throughput is required. First of all, the XSLT stylesheets are just static XML files. These can be loaded into memory once and reused over and over again. This substantially improves performance because the XML only has to be parsed once.

The second major improvement is to use the Document Object Model (DOM) for your data objects, instead of converting those to XML text files first. For example, the following code takes the JDOM output from the CreditInfo class and converts it to DOM, without first writing it to a text XML file.

Example 6: Converting JDOM to DOM

DOMOutputter domOutputter = new DOMOutputter();

org.w3c.dom.Document domDoc = domOutputter.output(jdomDoc);

The domDoc object can then be passed to Xalan directly, which should be much faster than parsing in the raw XML text from a file or input stream. The following example is a simple test servlet that creates some XML using JDOM, then applying an XSLT stylesheet. The stylesheet is parsed once and then cached in memory, and the JDOM is converted into a DOM tree that is passed to Xalan directly. Comments in the code explain each step.

Example 7: A Servlet Example

import java.io.*;

import javax.servlet.*;

import javax.servlet.http.*;

import org.apache.xalan.xslt.*;

import org.jdom.*;

import org.jdom.input.*;

import org.jdom.output.*;



public class JDomServlet extends HttpServlet {

    // reuse the same processor over and over

    private XSLTProcessor processor = XSLTProcessorFactory.getProcessor(

            new org.apache.xalan.xpath.xdom.XercesLiaison());



    // initialize the Servlet.  This code is executed once.

    public void init(ServletConfig config) throws ServletException {

        super.init(config);



        ServletContext context = config.getServletContext();



        try {

            // read the XSLT stylesheet and cache it for future reuse

            InputStream xsltStream = context.getResourceAsStream(

                    "/WEB-INF/viewCreditInfo.xslt");

            StylesheetRoot parsedStylesheet = processor.processStylesheet(

                    new XSLTInputSource(xsltStream));

            processor.setStylesheet(parsedStylesheet);

        } catch (Exception ex) {

            throw new UnavailableException(ex.toString());

        }

    }



    // handle a single request from the client

    public void doGet(HttpServletRequest request,

            HttpServletResponse response) throws IOException,

            ServletException {

        try {

            response.setContentType("text/html");



            // in a real app, the CreditInfo object would be retrieved

            // from an EJB component

            CreditInfo ci = new CreditInfo("John Q. Public",

                    "Visa", "111-222-333", "05/2000");



            // convert the JDOM into DOM

            Document jdomDoc = ci.getDocument();

            org.w3c.dom.Document domDoc = 

                    new DOMOutputter().output(jdomDoc);

            

            // transform the XML into HTML

            processor.process(new XSLTInputSource(domDoc),

                              null,  // use pre-compiled stylesheet

                              new XSLTResultTarget(response.getWriter()));

        } catch (Exception ex) {

            ex.printStackTrace();

        }

    }

}

Summary

The XSLT approach to web development has clear advantages over traditional approaches. It does not lock you in to any vendor-specific APIs or frameworks. Perhaps the biggest roadblock to widespread acceptance of XSLT is programmer resistance. Many people have the impression that XSLT is simply too difficult to use, although much of this opinion is probably based on lack of experience rather than the complexity of XSLT. The main reason for the perceived complexity of XSLT is its syntax, which is a direct result of XML. Trying to express a programming language as XML is challenging and can lead to somewhat quirky syntax.

Beyond training and acceptance issues, an XML/Java/XSLT approach has the distinct advantage that its constituent parts are clearly segregated. From a project management perspective, this allows every team member to work in parallel, rather than overloading the servlet guru while the remainder of the team struggles to contribute.

Eric M. Burke is an O'Reilly author and a principal software engineer with Object Computing, Inc. in St. Louis, MO.

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