code: private void apply(String style,
                         String source,
                         HttpServletRequest req,
                         HttpServletResponse res) throws Exception {
  
          // Validate parameters
          if (style == null) {
              throw new TransformerException("No style parameter supplied");
          }
          if (source == null) {
              throw new TransformerException("No source parameter supplied");
          }
  
          InputStream sourceStream = null;
          try {
              // Load the stylesheet (adding to cache if necessary)
              Templates pss = tryCache(style);
              Transformer transformer = pss.newTransformer();
  
              Enumeration<?> p = req.getParameterNames();
              while (p.hasMoreElements()) {
                  String name = (String) p.nextElement();
                  if (!(name.equals("style") || name.equals("source"))) {
                      String value = req.getParameter(name);
                      transformer.setParameter(name, new StringValue(value));
                  }
              }
  
              // Start loading the document to be transformed
              sourceStream = getInputStream(source);
  
              // Set the appropriate output mime type
              String mime =
                      pss.getOutputProperties()
                              .getProperty(OutputKeys.MEDIA_TYPE);
              if (mime == null) {
                  res.setContentType("text/html");
              } else {
                  res.setContentType(mime);
              }
  
              // Transform
              StreamSource ss = new StreamSource(sourceStream);
              ss.setSystemId(source);
              transformer.transform(ss, new StreamResult(res.getOutputStream()));
  
          } finally {
              if (sourceStream != null) {
                  try {
                      sourceStream.close();
                  } catch (Exception e) {
                  }
              }
          }
  
      }
docs: Apply stylesheet to source document