How to Embed and Bind JSON Data to HTML Template in Java Web Service

Question

I am working on a Java PDF Generation micro service using spring boot. The pdf generation is meant to be a 2 stage process.

1. Templating - html template with some sort of expression language, which reads a json structure directly

2. HTML to PDF - this generates a pdf from the html produced in step 1

Note: I have some java and JavaScript (nunjucks/nodejs) solution for steps 1 and 2, but I really need a more maintainable approach as follows.

• My restful endpoint will take 2 parameters an html template and a json
• The json structure maps to the html template one to one and both files have been predefined to strict contract.
• The service endpoint should not do any object mapping of json data to html Dom elements e.g. table, rows, etc
• The endpoint only embeds the json data to the html using logic in java code
• The html gets executed/rendered and reads the json directly using some sort of expression language, since it contains the json structure
• The endpoint then response with an html with the dynamic data, which can be sent to the pdf generator web service endpoint

Below is a sample code:

@POST
 @Path("createPdf")   
 public Response createPdf(String htmlTemplateParam, String json) {

 //Read template from endpoint param or start off with reading local html template from resource folder
String templateHtml = IOUtils.toString(getClass().getResourceAsStream(HTMLTemplateFiles.INCOMING_TEMPLATE));

RequestJson requestJson = [Prepare json before passing to HTML Template]];

if(requestJson.isContentValid()) {

    LOG.info("incoming data successfully validated");

    // TODO     
    // Pass the requestJson (Endpoint Param JSON ) to templateHtml
    // Trigger the reading of the Json data and populating different HTML DOM elements using some sort of expression predefined in HTML

    // Get hold of the rendered HTML         
    String resolvedHtml = [HTML with data from json param into endpoint];

    // The next bit is done
    String pdf = htmlToPdfaHandler.generatePdfFromHtml(resolvedHtml);

    javax.ws.rs.core.Response response = Response.ok().entity(Base64.decodeBase64(pdf)).build();
            }
}

The templating stage one is where I need help.

Please, what is the best technical solution for this?

I am comfortable with Java and JavaScript framework and happy to learn any framework you suggest.

But, my main design goal is to ensure that as we have new templates and a template and data changes, a non-techie can change html/json and generate pdf.

Also, no java code changes should be required for template and data changes.

There are a few things in my head like jsonpath, thyme leaf, JavaScript, etc But, I love best practice and like to learn from someone with the real-life experience of similar use case.

After further research and first answer I am also thinking of the freemarker solution below.

But, how would I create a free marker template-data auto-magically from reading input json.i.e. without creating POJO/DTO?

Based on first answer :

     Configuration cfg = new Configuration(new Version("2.3.23"));
            cfg.setDefaultEncoding("UTF-8");

            // Loading you HTML template (via file or input stream):
            Template template = cfg.getTemplate("template.html");

  // Will this suffice for all JSON Structure, including nested deep ones

           Type mapType = new TypeToken<Map<String, Object>>(){}.getType(); 

    Map<String, String[]> templateData = new Gson().fromJson(json, mapType);

            try (StringWriter out = new StringWriter()) {
         // In output stream the result will be template with values from map:
                template.process(templateData, out);
                System.out.println(out.getBuffer().toString());

                out.flush();
            }

Thanks in advance.

NOTE: Code snippets, pseudo code, references are welcomed.

Answer 1

One of the options might be FreeMarker usage:

    Configuration cfg = new Configuration(new Version("2.3.23"));
    cfg.setDefaultEncoding("UTF-8");

    // Loading you HTML template (via file or input stream):
    Template template = cfg.getTemplate("template.html");

    // You need convert json to map of parameters (key-value):
    Map<String, Object> templateData = new HashMap<>();
    templateData.put("msg", "Today is a beautiful day");

    try (StringWriter out = new StringWriter()) {
        // In output stream the result will be template with values from map:
        template.process(templateData, out);
        System.out.println(out.getBuffer().toString());

        out.flush();
    }