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An Overview of the Google Web Toolkit (GWT). November 15, 2006. Introduction. Who am I? My development experience: C++, Java, .NET Why GWT?. Why Google Webtoolkit?. Web-based application framework AJAX enabled Web 2.0 <insert yet another buzzword here>

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Presentation Transcript
  • Who am I?
  • My development experience: C++, Java, .NET
  • Why GWT?
why google webtoolkit
Why Google Webtoolkit?
  • Web-based application framework
  • AJAX enabled
  • Web 2.0
  • Implement an HTML User Interface in Java
  • Compiles to:
      • HTML and Javascript
      • AJAX-enables app using a binary remoting protocol (proprietary Google technology)
what is ajax
What is AJAX?
  • Asyncrhonous Xml And Javascript
  • Centered around the XMLHttpRequest object
  • AJAX request types:
      • Fire and forget (doPost in Servlet parlance)
      • Asynchronous (request/callback)
  • Enables Event Handling paradigm/semantics
  • Implemented using Javascript
everybody loves javascript just not me
Everybody loves Javascript - Just not me
  • Originally created by Netscape
  • Now based on ECMA Spec
  • Not a strongly typed language
  • Not fully dynamically typed either (think Ruby)
  • Browser implementations are inconsistent
  • Fragile and difficult to debug (I write buggy code)
  • Unit testing Javascript? jsUnit
ajax advantages
AJAX Advantages
  • Sexy
  • Looks great on a resume!
  • Lends itself to great pick up lines
real ajax advantages
Real AJAX Advantages
  • Finer grained requests/responses
  • Avoid full-page refreshes to update data
  • Just-in-time data retrieval
  • Reuse existing server-side logic/components
      • Data validations in native language
  • Richer UI functionality
ajax disadvantages
AJAX Disadvantages
  • Implemented differently by browser vendors
  • Javascript not an OO language
  • Inheritance, Polymorphism is possible, but difficult
  • Encapsulation using Javascript objects works
  • Knowledge of DOM to manipulate elements (really it’s a Javascript issue, not really AJAX)
frameworks frameworks frameworks
Frameworks, Frameworks, Frameworks
  • Scriptaculous, DWR, home-grown in-house etc.
  • No clear leader, definitely no standard
  • Java developers are inherently averse to Javascript – that’s the reality (or is it just me?)
  • Provide tested code and crosss-browser support
so what does google do
So what does Google do?
  • Recognize these issues - they develop webapps too!
  • Ask the question:
      • How should a Java developer develop sexy web-based AJAX-enabled applications?
so what is google webtoolkit
So what is Google Webtoolkit?
  • A Rich Client Architecture for developing rich internet apps
  • How Google describes it:

“Google Web Toolkit (GWT) is a Java software development framework that makes writing AJAX applications like Google Maps and Gmail easy for developers who don't speak browser quirks as a second language.”

  • Conceptually similar to Swing but HTML specific with web remoting capabilities
  • Includes UI Widgets/Components, RPC mechanisms and native Javascript support
  • UI’s based on Panels and Layout Managers
  • UI Widgets for trees, lists, text, labels etc.
  • Event Handling (Action Listeners, Keyboard Handling, Mouse and Focus events)
  • Swing’s implementation of UI is based on UI Delegate (pattern?)
  • Widgets include the usual suspects - text, passwords, tables and HTML abstractions
  • Layouts based on Panels (vertical, horizontal, deck, dock, scroll, tab etc.)
  • Event Handlers/Listeners (Change, Click, Focus, Keyboard, Mouse, Scroll, Tab)
  • Also utilizes the UI Delegate pattern
what where
What? Where?
  • Can be downloaded from
  • Supported platforms include: Windows, Linux (GTK+) and Mac OSX
  • OSS-friendly license:
      • UI Widgets: Apache 2.0
      • GWT Compiler: Proprietary non-distributable license
  • Initial support for Eclipse
archive contents
Archive Contents
  • Command-line utilities: projectCreator, applicationCreator, i18nCreator, junitCreator
  • Platform Development Jar: gwt-dev-xxx.jar – where xxx is win32, linux, mac
  • Deployment Jar: gwt-user.jar
  • Sample Applications
  • API Documentation
layout of significant packages
Layout of Significant Packages
  • core.client:
      • GWT (uncaught exception handler)
      • JavascriptException
      • EntryPoint Interface
  • user.client – Browser history, DOM manipulation, event handling etc
  • user.client.rpc – Client side implementation classes of RPC (IsSerializable, AsyncCallback)
  • user.client.ui – UI Widgets, Panels and other classes to support GUI
getting started developing with gwt
Getting Started Developing with GWT
  • First things first, we need an Eclipse project:
      • Manually create a directory in your workspace
      • Create project files with projectCreator
      • Create application with applicationCreator
      • Import project into Workspace
      • Grip it and rip it! …err, run the app
project creator
Project Creator
  • projectCreator, creates specific project artifacts
      • Src/bin directories
      • .project file
      • .classpath file
  • In the case of Eclipse, workspace folder must exist already:

projectCreator -ant Foo -eclipse Foo

  • Ant build file creates targets for compile, package and clean.
application creator
Application Creator
  • Command line utility to generate application artifacts:
      • Default client packages
      • Main class, entry point implementation
      • Module XML file (more later)
      • .launch file for Hosted Mode (debug mode)

applicationCreator –eclipse Foo


project structure
Project Structure
  • com/example/cal - The project root package contains module XML files
  • com/example/cal/client - Client-side source files and subpackages
  • com/example/cal/server - Server-side code and subpackages
  • com/example/cal/public - Static resources that can be served publicly (think HTML, images etc.)
modes of operandi
Modes of Operandi
  • GWT supports to modes:
      • Hosted - uses a built-in Tomcat instance for run-time debug environment
      • Web - compiled application deployed to a production (or non-production as the case may be) environment
the application
The Application
  • Synonymous with C/C++, Java and C# main methods
  • Implementation of the Module entry point

public interface EntryPoint {

public abstract void onModuleLoad();


what is a module
What is a Module?
  • An XML configuration
  • Specifies an entry point - an application class that renders to HTML
  • Specifies servlet mapping for Hosted Mode
  • May inherit from other Modules
application example
Application Example

publicclass Application implements EntryPoint {

publicvoid onModuleLoad() {

final Button button = new Button("Click me");

final Label label = new Label();

button.addClickListener(new ClickListener() {

publicvoid onClick(Widget sender) {

if (label.getText().equals(""))

label.setText("Hello World!");









user client ui package
user.client.ui Package
  • Contains basic UI abstractions: TextBox, PasswordTextBox, Grid, Label, Listbox, MenuBar, MenuItem, Tree, HTMLTable
  • All UI elements descend from Widget
  • Panel abstractions include: Panel, VerticalPanel, HorizontalPanel, DeckPanel, DockPanel, RootPanel
  • Panels are composites, support whole/part hierarchies
more detailed sample
More Detailed Sample

VerticalPanel display = new VerticalPanel(); HorizontalPanel panel = new HorizontalPanel(); panel.setSpacing(2);

// Add a label

panel.add(new Label("Time:"));

// Create a text box

textbox = new TextBox();

textbox.setSize("75px", "25px");

textbox.addFocusListener(createFocusListener()); textbox.setFocus(true);





event handling
Event Handling
  • GWT supports a wide selection of event handling interfaces and semantics, samples include:
      • KeyboardListener
      • MouseListener
      • ClickListener
      • SourceClickListeners
      • And many many more - seriously, tons more.
  • UI elements have methods for adding and removing the event handlers
clicklistener sample
ClickListener Sample
  • An interface used for click events.
  • Buttons etc.

ClickListener listener = new ClickListener() {

publicvoid onClick(Widget sender)


String value = textbox.getText();

if (value != null && !"".equals(value))


// do something with value…





service remoting
Service Remoting
  • Proprietary binary remoting protocol.
  • AJAX under the covers
  • Server-side service target is a Servlet
  • Hosted Mode deployed in Module XML
  • Repetitive steps to create a Service - opportunity to refactor/abstract and write some “cool” OO code
  • Steps are well defined
so how do we do this
So how do we do this?
  • Create a client-side interface to represent the service
  • Server-side abstraction is a Servlet - extends GWT’s RemoteServiceServlet
  • Create an asynchronous interface – GWT uses this to generate the proxy
  • Make the call
Sample User Story:
  • User enters time which is validated and formatted (data entry validation)
create client side interface
Create Client-side Interface
  • Extends GWT RemoteService
  • Regular Java interface - nothing special

publicinterface TimeFormatService extends RemoteService


publicabstract String formatTime(String unformatted);


server side servlet
Server-side Servlet
  • Lives in app.server package structure
  • This is different than the client package - “real” Java code, does not get compiled by the GWT compiler
  • Standard Servlet
  • Extends RemoteServiceServlet (GWT base class for remote services)
  • Implements client-side interface
  • Hosted mode testing by mapping in Module XML
servlet code
Servlet Code

publicclass TimeFormatServiceImpl extends RemoteServiceServlet implements TimeFormatService


public String formatTime(String unformatted)


String result = "";

if (unformatted != null)


Time time = TimeFactory.createTime(unformatted);

result = time.toString();


return result;



aysnchronous interface
Aysnchronous Interface
  • GWT generates remote proxy from this
  • Interface that mimics the client-side interface
  • Subtle differences
  • This is the actual wiring between client and server - the underpinnings of GWT’s remoting

publicinterface TimeFormatServiceAsync


publicabstractvoid formatTime(String unformatted, AsyncCallback callback);


making the call
Making the Call
  • Create an instance of the Service Proxy - GWT.create()
  • This is the weird part: Cast the client interface to a ServiceDefTarget
  • Set the Module Entry point on the above target (URL)
  • Create Asynchronous callback - handles onSuccess and onFailure
  • Call client-side remote proxy passing the arguments and the callback
what this looks like
What this looks like

TimeFormatServiceAsync timeService = (TimeFormatServiceAsync) GWT.create(TimeFormatService.class);

ServiceDefTarget endpoint = (ServiceDefTarget) timeService;

String moduleRelativeURL = GWT.getModuleBaseURL() + "time";


AsyncCallback callback = new AsyncCallback() {

publicvoid onFailure(Throwable object)

{ // Failure path


publicvoid onSuccess(Object object)

{ // Success path



timeService.formatTime(value, callback);

well did it work
Well? Did it work?
  • Hosted mode allows rapid development feedback loop - with a caveat.
  • Supports Hosted Mode debugging through Eclipse
  • Full debugging capabilities of the IDE
wrapping up
Wrapping Up
  • Full UI abstractions for Web applications
  • Allows for remoting of Objects that implement IsSerializable
  • Surprisingly mature API
  • Well supported
  • Finally! Java code that generates to a Web GUI
  • Much much more than has been shown here