Combine selected newsletters into one PDF

protected void doPost(SlingHttpServletRequest request,SlingHttpServletResponse response)
   {

       String []newsletters = request.getParameter("selectedNewsLetters").split(",");
       Map<String, Object> mapOfDocuments = new HashMap<String, Object>();
       for(int i= 0;i<newsletters.length;i++)
       {
           Resource resource = request.getResourceResolver().getResource(newsletters[i]);

           log.debug("The resource name is "+resource.getName());
           Document newsletter = new Document(resource.getPath());
           mapOfDocuments.put(resource.getName(), newsletter);
       }
       log.debug("The newsletters selected: "+newsletters);
       Document ddxDocument = createDDXFromMapOfDocuments(mapOfDocuments);
       AssemblerOptionSpec aoSpec = new AssemblerOptionSpec();
       aoSpec.setFailOnError(true);
       AssemblerResult ar = null;
       try {
               ar = assemblerService.invoke(ddxDocument, mapOfDocuments, aoSpec);
               Document assembledPDF = ar.getDocuments().get("GeneratedPDF.pdf");
               // This is my custom code to get fd-service user
               ResourceResolver formsServiceResolver = getResolver.getFormsServiceResolver();

               Resource nodeResource = formsServiceResolver.getResource("/content/newsletters");

               UUID uuid = UUID.randomUUID();
               String uuidString = uuid.toString();
               javax.jcr.Node assembledNewsletters = nodeResource.adaptTo(Node.class);
               javax.jcr.Node assembledNewsletter =  assembledNewsletters.addNode(uuidString + ".pdf", "nt:file");
               javax.jcr.Node resNode = assembledNewsletter.addNode("jcr:content", "nt:resource");
               ValueFactory valueFactory = formsServiceResolver.adaptTo(Session.class).getValueFactory();
               Binary contentValue = valueFactory.createBinary(assembledPDF.getInputStream());
               resNode.setProperty("jcr:data", contentValue);
               formsServiceResolver.commit();
               PrintWriter out = response.getWriter();
               response.setContentType("application/json");
               response.setCharacterEncoding("UTF-8");
               JsonObject asset = new JsonObject();

               asset.addProperty("assetPath", assembledNewsletter.getPath());
               out.print(new Gson().toJson(asset));
               out.flush();

           }
           catch (IOException | OperationException | RepositoryException e)
           {

               log.error("Error is "+e.getMessage());
           }
   }

public Document createDDXFromMapOfDocuments(Map<String, Object> mapOfDocuments)
     {

        final DocumentBuilderFactory docFactory = DocumentBuilderFactory.newInstance();
        org.w3c.dom.Document ddx = null;
        try
               {
                   DocumentBuilder docBuilder = docFactory.newDocumentBuilder();
                   ddx = docBuilder.newDocument();
                   Element rootElement = ddx.createElementNS("http://ns.adobe.com/DDX/1.0/", "DDX");
                ddx.appendChild(rootElement);
                Element pdfResult = ddx.createElement("PDF");
                pdfResult.setAttribute("result","GeneratedPDF.pdf");
                rootElement.appendChild(pdfResult);
                for (String key : mapOfDocuments.keySet())
                    {
                        log.debug(key + " " + mapOfDocuments.get(key));
                        Element pdfSourceElement = ddx.createElement("PDF");
                        pdfSourceElement.setAttribute("source", key);
                        pdfSourceElement.setAttribute("bookmarkTitle",key);
                        pdfResult.appendChild(pdfSourceElement);
                    }
                return orgw3cDocumentToAEMFDDocument(ddx);
            }
            catch (ParserConfigurationException e)
                {
                    log.debug("Error:"+e.getMessage());
                }
            return null;
     }

public Document orgw3cDocumentToAEMFDDocument( org.w3c.dom.Document xmlDocument)
     {
         final ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
         DOMSource source = new DOMSource(xmlDocument);
         log.debug("$$$$In orgW3CDocumentToAEMFDDocument method");
         StreamResult outputTarget = new StreamResult(outputStream);
         try
             {
               TransformerFactory.newInstance().newTransformer().transform(source, outputTarget);
               InputStream is1 = new ByteArrayInputStream(outputStream.toByteArray());
               Document xmlAEMFDDocument = new Document(is1);
               if (log.isDebugEnabled())
                   {
                    xmlAEMFDDocument.copyToFile(new File("dataxmldocument.xml"));
                }
             return xmlAEMFDDocument;
            }
            catch (Exception e)
                 {
                    log.error("Error in generating ddx " + e.getMessage());
                    return null;
                 }
        }