Type safe builders, HTML templates and other goodies for GWT Elemental2
Java FreeMarker Other
Latest commit 4804b6c Jul 20, 2018


TC Build License Chat on Gitter


Elemento simplifies working with GWT Elemental2. In a nutshell Elemento brings the following features to the table:


Get Started

Elemento is available in Maven Central. To use it add the following dependency to your POM:


In your GWT module inherit from org.jboss.gwt.elemento.Core:

    <inherits name="org.jboss.gwt.elemento.Core"/>

Builder API

When working with GWT Elemental it is often awkward and cumbersome to create an hierarchy of elements. Even simple structures like

<section class="main">
    <input class="toggle-all" type="checkbox">
    <label for="toggle-all">Mark all as complete</label>
    <ul class="todo-list">
            <div class="view">
                <input class="toggle" type="checkbox" checked>
                <label>Taste Elemento</label>
                <button class="destroy"></button>
            <input class="edit">

lead to a vast amount of Document.createElement() and chained Node.appendChild() calls. With Elemento creating the above structure is as easy as

import static org.jboss.gwt.elemento.core.Elements.*;
import static org.jboss.gwt.elemento.core.InputType.checkbox;
import static org.jboss.gwt.elemento.core.InputType.text;

HTMLElement section = section().css("main")
                .apply(l -> l.htmlFor = "toggle-all")
                .textContent("Mark all as complete"))
                                        .apply(cb -> cb.checked = true))
                                .add(label().textContent("Taste Elemento"))

The class Elements provides convenience methods to create the most common elements. It uses a fluent API to create and append elements on the fly. Take a look at the API documentation for more details.


When creating large hierarchies of elements you often need to assign an element somewhere in the tree. Use an inline assignment together with asElement() to create and assign the element in one go:

final HTMLElement count;
final HTMLElement footer = footer()
        .add(count = span().css("todo-count").asElement())

Event Handlers

Elemento provides methods to easily register event handlers. There are constants for most of the known event types.

You can either add event handlers when building the element hierarchy:

import static org.jboss.gwt.elemento.core.Elements.*;
import static org.jboss.gwt.elemento.core.EventType.*;
import static org.jboss.gwt.elemento.core.InputType.checkbox;
import static org.jboss.gwt.elemento.core.InputType.text;

HTMLLIElement listItem = li()
                        .apply(cb -> cb.checked = true)
                        .on(change, event -> toggle()))
                        .textContent("Taste Elemento")
                        .on(dblclick, event -> edit()))
                        .on(click, event -> destroy())))
                .on(keydown, this::keyDown)
                .on(blur, event -> blur()))

or register them later using EventType.bind():

EventType.bind(listItem, click, event -> DomGlobal.alert("Clicked"));

The latter approach returns org.gwtproject.event.shared.HandlerRegistration which you can use to remove the handler again.

In order to make it easier to work with keyboard events, Elemento provides an enum with the most common keyboard codes:

import static org.jboss.gwt.elemento.core.Key.Escape;
import static org.jboss.gwt.elemento.core.Key.Enter;

void keyDown(KeyboardEvent event) {
    if (Escape.match(event)) {
    } else if (Enter.match(event)) {

Custom Elements

Elemento makes it easy to create custom elements. As for Elemento custom elements are a composite of HTML elements and/or other custom elements. They're ordinary classes which can hold state or register event handlers. The only requirement is to implement IsElement<E extends HTMLElement> and return a root element:

import static org.jboss.gwt.elemento.core.Elements.*;

class TodoItemElement implements IsElement<HTMLElement> {
    private final HTMLElement root;
    private final HTMLInputElement toggle;
    private final HTMLElement label;
    private final HTMLInputElement summary;

    TodoItemElement(TodoItem item) {
        this.root = li().data("item", item.id)
                        .add(toggle = input(checkbox).css("toggle").asElement())
                        .add(label = label().textContent(item.text).asElement())
                        .add(destroy = button().css("destroy").asElement()))
                .add(summary = input(text).css("edit").asElement())
        this.root.classList.toggle("completed", item.completed);
        this.toggle.checked = item.completed;
    public HTMLElement asElement() {
        return root;
    // event handlers omitted

The builder API has support for IsElement<E extends HTMLElement> which makes it easy to use custom elements when building the element hierarchy:

import static org.jboss.gwt.elemento.core.Elements.ul;

TodoItemRepository repository = ...;
TodoItemElement[] itemElements = repository.items().stream()
HTMLUListElement ul = ul().addAll(itemElements).asElement();

HTML Templates

Elemento provides an easy way to take existing HTML content and use it in your application. Templates can be either HTML snippets or full HTML documents where you select an element and its children. This allows you to preview your templates more easily during design without running the application.

HTML templates rely on annotation processing. The annotation processors are configured in META-INF/services/javax.annotation.processing.Processor and should be detected automatically. They pick the HTML content from your template and generate a subclass of your annotated class. This code generation is inspired by Google's AutoValue and it might take some time to get used to it.

Get Started

To use HTML templates add the following maven dependency to your POM:


In your GWT module inherit from org.jboss.gwt.elemento.Template:

    <inherits name="org.jboss.gwt.elemento.Template"/>

Prepare HTML

Let's say you've got the following HTML document called Todo.html:

<!doctype html>
<html lang="en">
    <link rel="stylesheet" href="<path-to>/node_modules/todomvc-common/base.css">
    <link rel="stylesheet" href="<path-to>/node_modules/todomvc-app-css/index.css">
<section data-element="todos" class="todoapp">
    <header class="header">
        <input data-element="newTodo" class="new-todo" placeholder="What needs to be done?" autofocus>
    <section data-element="main" class="main">
        <input data-element="toggleAll" class="toggle-all" id="toggle-all" type="checkbox">
        <label for="toggle-all">Mark all as complete</label>
        <ul data-element="list" class="todo-list">
            <!-- Todo items are mapped to an extra template class -->
    <footer data-element="footer" class="footer">
        <span data-element="count" class="todo-count"><strong>0</strong> item left</span>
        <ul class="filters">
            <li><a data-element="all" href="#/">All</a></li>
            <li><a data-element="active" href="#/active">Active</a></li>
            <li><a data-element="completed" href="#/completed">Completed</a></li>
        <button data-element="clearCompleted" class="clear-completed">Clear completed</button>

The HTML is enriched with data-element attributes. Elemento needs these attributes to select the root element and to map specific HTML elements to fields in the template class.

Template Class

To create a template class which maps to the <section data-element="todos"/> element, create an abstract class and annotate it with @Templated:

abstract class ApplicationElement implements IsElement<HTMLElement> {

    static ApplicationElement create() {
        return new Templated_ApplicationElement();

The @Templated annotation selects the root element. Without specifying a selector, Elemento looks in the current package for a parallel ".html" template next to the template class and use the first child of the <body> as root element.

The static method static ApplicationElement create() might look strange at first and it will most likely be marked as an error if you haven't run the annotation processor so far. But that's the central idea behind Elemento's code generation: You create an abstract base class that has one reasonable implementation. This implementation is generated by the annotation processor and is named Templated_<YourClassName>. That's the class you need to create in the static factory method.

Finally your template class needs to implement IsElement<E extends HTMLElement>. The generated code implements the method E asElement() and returns the selected root element of your HTML template.

To sum it up here are the steps necessary to create and use an HTML template

  1. Place data-element attributes in your HTML markup
  2. Create an abstract class annotated with @Templated which implements IsElement<HTMLElement>
  3. Add a static factory method (name does not matter) which returns the template class and calls the generated constructor Templated_<YourClassName>
  4. Run the annotation processor to get rid of compile errors

Besides these rules you're free to implement additional interfaces or extend from arbitrary base classes.

Data Elements

To map specific elements from the HTML to your template class use the @DataElement annotation:

abstract class ApplicationElement implements IsElement<HTMLElement> {
    @DataElement HTMLInputElement newTodo;
    @DataElement HTMLElement main;
    @DataElement HTMLInputElement toggleAll;
    @DataElement HTMLElement list;
    @DataElement HTMLElement footer;
    @DataElement HTMLElement count;
    @DataElement("all") HTMLElement filterAll;
    @DataElement("active") HTMLElement filterActive;
    @DataElement("completed") HTMLElement filterCompleted;
    @DataElement HTMLButtonElement clearCompleted;

If no value is provided for the @DataElement annotation, the name of the field / method is taken as default. Elemento uses the following CSS selector to pick elements from the HTML template:


The @DataElement can be applied to fields and methods. Those fields and methods must not be private. Elemento defines three simple rules for the mapping between the HTML template and the related class:

Java Element Direction
1. Fields w/o an initializer
The element, its attributes and all children are mapped from the HTML template to the annotated field. The type of the field should be elemental2.dom.HTMLElement or a subclass of elemental2.dom.HTMLElement.
HTML → Java
2. Fields w/ an initializer
The element in the HTML template is replaced with the initializer. The type of the field should be one of: elemental2.dom.HTMLElement or IsElement.
Java → HTML
3. Methods
The element in the HTML template is replaced with the return value of the method. The method should return one of: elemental2.dom.HTMLElement or IsElement and must not have any parameters.
Java → HTML

You're also free to use custom types when mapping fields and methods. In this case the generated code will use Js.cast(). It's up to you to make sure the cast works. Otherwise this can lead to strange errors which are hard to detect.


Now what if your template class has dependencies which you want to pass as constructor arguments? In that case change the signature of the static factory method to accept your dependencies:

abstract class ApplicationElement implements IsElement<HTMLElement> {

    static ApplicationElement create(TodoItemRepository repository, I18n i18n) {
        return new Templated_ApplicationElement(repository, i18n);

    abstract TodoItemRepository repository();
    abstract I18n i18n();

For each constructor parameter you can define an abstract accessor method which returns the dependency and does not accept parameters. The generated code will implement these methods. Please make sure that parameters are passed to the auto-generated constructor in the same order the corresponding accessor methods are defined in the file. Take extra care that the order remains valid if you refactor your code!


Since templated classes cannot have a constructor it's possible to annotate a method with javax.annotation.PostConstruct. This method will be called from the generated constructor after all initialization steps (element mapping, expressions). So it's safe to use the abstract accessor methods and data elements from the initialization method:

abstract class ApplicationElement implements IsElement<HTMLElement> {

    static ApplicationElement create(TodoItemRepository repository, I18n i18n) {
        return new Templated_ApplicationElement(repository, i18n);

    abstract TodoItemRepository repository();
    abstract I18n i18n();

    @DataElement HTMLElement list;

    void init() {
        // load the todo items from local storage
        for (TodoItem item : repository().items()) {
            // TodoItemElement is another template class
            list.appendChild(TodoItemElement.create(this, repository(), item).asElement());

The method annotated with javax.annotation.PostConstruct must not be private, return void and must not have any parameters.


Elemento supports expressions like ${foo} in HTML templates:

<section data-element="todos" class="todoapp">
    <header class="header">
        <input data-element="newTodo" class="new-todo" placeholder="${i18n().constants().new_todo()}" autofocus>

The expressions between ${ and } need to be valid Java expressions. They're executed in the scope of the generated class. ${<expression>} is replaced with the safe HTML result of String.valueOf(<expression>). The example above makes use of the template's i18n() method (see example above) and inserts the language specific header. But you could also use any other valid expressions like ${com.google.gwt.i18n.client.DateTimeFormat.getShortDateFormat().format(new java.util.Date())}.

Expressions are supported in text nodes and attribute values.



Errai is a very powerful, general-purpose framework for writing rich web applications using next-generation web technologies. You can use many EE related technologies such as CDI, JPA or JAX-RS on the client side. As such Errai is particularly useful when you want to develop large RIAs.

Elemento on the other hand is very focused around Elemental2. The main goal is to provide a type safe API for working with HTML elements.

Although there are overlapping parts like HTML templates you can still use Elemento's builder API as part of an Errai application. This can also be combined with Errai HTML templates. Since Errai UI is API agnostic when it comes to the DOM, you can use any native JS type DOM wrapper in an Errai UI template. However I would not recommend using Elemento's HTML templates in an Errai application.

The samples section contains an Errai version of the TodoMVC app using Errai for dependency injection and the builder API to create the elements.


RxGWT provides reactive extensions for GWT. It enables you to use RxJava on the client. RxJava is a library for composing asynchronous and event-based programs by using observable sequences.

Speaking of Elemento you can use RxGWT to bind events and turn them into an observable. To use RxGWT inherit from com.intendia.rxgwt.RxElemento:

<module rename-to="elemental2">
    <inherits name="com.intendia.rxgwt.RxElemento"/>

Then use <T extends Event> Observable<T> fromEvent(EventTarget src, EventType<T, ?> type) to listen for events and turn them into an observable:

HTMLElement info; 
document.body.appendChild(info = span().asElement());

Observable<double[]> touch$ = RxElemento.fromEvent(document, touchmove)
        .map(ev -> ev.touches.item(0))
        .map(touch -> new double[] { touch.clientX, touch.clientY });
Observable<double[]> mouse$ = RxElemento.fromEvent(document, mousemove)
        .map(ev -> new double[] { ev.clientX, ev.clientY });
Observable.merge(touch$, mouse$)
        .map(p -> "position (" + p[0] + "," + p[1] + ")")
        .forEach(n -> info.textContent = n);

Dagger and GIN

Elemento provides support for dependency injection in HTML templates using Dagger or GIN. If one of these libraries is on the classpath, the annotation processor will place a @javax.inject.Inject annotation on the generated constructor.

See the samples how this works and how you can inject the templates classes into other classes.

Helper Methods

Besides the builder API and the HTML templates, Elemento comes with a bunch of static helper methods that roughly fall into four categories:

  1. Element iterator / stream methods
  2. Element append, insert & remove methods
  3. Misc element helper methods (show, hide, toggle CSS classes)

See the API documentation of Elements for more details.


Elemento is ready to be used with J2CL and GWT 3. It has no dependencies to gwt-user. There are methods that use safe HTML classes. But these classes are taken from org.gwtproject.safehtml:safehtml. In case you still need to use the safe HTML classes from gwt-user, you can switch to versions GWT2-SNAPSHOT resp. x.y.z-gwt2. In general Elemento provides different branches which produce the following versions:

  • The main branches develop and master have no gwt-user dependency. They use safe HTML classes from org.gwtproject.safehtml:safehtml. These branches produce versions HEAD-SNAPSHOT resp. x.y.z
  • The branch gwt2 use safe HTML classes from gwt-user. This branch produces versions GWT2-SNAPSHOT resp. x.y.z-gwt2

Methods to convert from com.google.gwt.user.client.ui.Widget and com.google.gwt.dom.client.Element to elemental2.dom.HTMLElement and vice versa are part of class org.jboss.gwt.elemento.core.Widgets in module elemento-widget. This module is available in all versions and branches and has obvisiouly a dependeny to gwt-user.


Elemento comes with five different implementations of the TodoMVC sample app.

All samples use the same key to persist the todo items in the local storage. So you can switch between the samples and continue working on your tasks seamlessly ;-)


If you want to contribute to Elemento, please follow the steps in contribution.

Get Help

If you need help feel free to contact us at Gitter, browse the API documentation or file an issue.