A simple solution:

<%@page contentType="text/xml" 
        import="java.io.*,java.util.*,javax.xml.parsers.*,javax.xml.transform.*,javax.xml.transform.stream.*" 
        pageEncoding="UTF-8"%><%!
    
   long timestamp;
   List cache;
   File transformFile;
   
   public void jspInit() {
      ServletConfig config = getServletConfig();
      transformFile = new File(config.getServletContext().getRealPath("service.xsl"));
      timestamp = transformFile.lastModified();
      cache = new ArrayList();
   }

   public Transformer getTransformer() {
      synchronized (cache) {
         if (transformFile.lastModified()>timestamp) {
            timestamp = transformFile.lastModified();
            cache.clear();
         }
         if (cache.size()>0) {
            Transformer t = (Transformer)cache.remove(cache.size()-1);
            return t;
         } else {
            return loadTransformer();
         }
      }
   }
   
   public void releaseTransformer(Transformer t,long savedTimestamp) {
      if (savedTimestamp==timestamp) {
         cache.add(t);
      }
   }
   
   public Transformer loadTransformer() {
      ServletConfig config = getServletConfig();
      try {
         TransformerFactory tfactory = TransformerFactory.newInstance();
         return tfactory.newTransformer(new StreamSource(transformFile));
      } catch (Exception ex) {
         config.getServletContext().log("Cannot load stylesheet.",ex);
      }
      return null;
   }
   
%><%

Transformer xslt = getTransformer();
long savedTimestamp = timestamp;

xslt.transform(new StreamSource(request.getInputStream()),new StreamResult(out));

releaseTransformer(xslt,savedTimestamp);
%>