DUEL » Examples

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Here is an example view where the result differs if there is zero, one, or many items.

<view name="foo.bar">

<div class="items">
<h3><%= data.title %></h3>

<if test="!data.items || data.items.length === 0">
  <call view="foo.warning" data="data">
      <part name="messageArea">
          <p class="item empty"><%= data.title %> contains no items.</p>

<else if="data.items.length === 1">
  <p class="item">
      <b><%= data.items[0].name %></b> is <i><%= data.items[0].detail %></i>

  <ul><for each="data.items">
      <li class="item">
          <b><%= data.name %></b> is <i><%= data.detail %></i>

Client-Side Views

To invoke this view from JavaScript, call the view as a function:

var data = {
title: "Hello world!",
items: [
  { name: "One", detail: 101 },
  { name: "two", detail: 2.718 }

// either get a string as output...
var markup = foo.bar(data).toString();
document.getElementById("baz").innerHTML = markup;

// ...or get DOM objects as output
var result = foo.bar(data).toDOM();

Which would output:

<div class="items">
<h3>Hello world!</h3>

  <li class="item">
      <b>One</b> is <i>101</i>
  <li class="item">
      <b>Two</b> is <i>2.718</i>

Server-Side Views

Servlet-based Controller

To invoke that same view from within Java, instantiate the view and call its render method. Here is a simple Servlet-as-controller example:

public class FooServlet extends HttpServlet {

public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException {

  DuelView view = new foo.bar();

  // can use helper methods to declaratively construct view data graph
  // or can directly use POJO/JavaBean types
  Object data = DuelData.asMap(
      "title", "Hello world!",
      "items", DuelData.asList(
              "name", "One",
              "detail", 101),
              "name", "Two",
              "detail", 2.718)

  // response headers

  // response body
  view.render(response.getWriter(), data);


DUEL views can be used to implement an MVC architecture in a JAX-RS (JSR-311) implementation (e.g. Jersey, JBoss RESTeasy, or Apache CXF). The JAX-RS resource acts as the controller which produces different content (XML, JSON or a DUEL view) using MIME-type negotiation. ViewResult is a lightweight adapter between DuelView and StreamingOutput.

import javax.ws.rs.*;
import org.duelengine.duel.rs.ViewResult;

public class OrdersController {

@Path("{id : \\d+}")
public Order getOrderJSON(@PathParam("id") long id) {
  return findOrder(id);

@Path("{id : \\d+}")
public Order getOrderXML(@PathParam("id") long id) {
  return findOrder(id);

@Path("{id : \\d+}")
public ViewResult getOrderView(@PathParam("id") long id) {
  Order orderData = findOrder(id);
  return new ViewResult(SingleOrderView.class).data(orderData);

// ...