IndexResource.java

    @Path("/index.html")
    @GET
    @Produces(MediaType.TEXT_HTML)
    public String goIndex() throws IOException, TemplateException, URISyntaxException {
        return view("index.ftl", Map.of(
                "hello", "Hello",
                "world", "World"
        ));
    }

    String view(String name, Map<String, String> model) throws TemplateException, IOException, URISyntaxException {
        var config = new Configuration(Configuration.VERSION_2_3_23);
        config.setDirectoryForTemplateLoading(new File(getClass().getResource("/WEB/").toURI()));

        var out = new StringWriter();
        var template = config.getTemplate(name);
        template.process(model, out);
        out.flush();

        return out.toString();
    }