CSC/ECE 517 Fall 2014/ch1a 6 rl

Google Web Toolkit (GWT) is an open source tool consisting of a set of Java libraries and API that allows web developers to create browser-based applications entirely in Java environment. Developers are not required to be familiar with JavaScript, HTML, or XMLHttpRequest. GWT emphasizes reusable approaches to common web development tasks, including asynchronous remote procedure calls, history management, bookmarking, UI abstraction, and internationalization. In addition to web application, GWT is also used to develop mobile and tablet applications.

Background

GWT Environment

GWT can be run in development mode and production mode. Under the development mode, application is run as Java bytecode on the Java Virtual Machine (JVM), and can be viewed and debugged directly on a browser via the Google Web Toolkit Developer Plugin. The browsers being supported are Apple Safari, Google Chrome, and Internet Explorer. In production mode, application is run as JavaScript and HTML. This design allows developers to adopt object-oriented design for their applications and use Java language syntax and library without coding in JavaScript and XML. Since GWT is based on Java, it also supports full-featured Java debugging and unit testing. In addition, GWT allows developers to insert JavaScript in the code using GWT’s JavaScript Native Interface (JSNI) and is able to catch common JavaScript and CSS errors during compiling. The GWT complier performs comprehensive static analysis and optimizations on the entire GWT codebase and safely eliminates dead code, unused classes, methods and their parameters in order to minimize application size. The applications in general loads and runs faster than the hand-written JavaScript. The produced pure JavaScript codes are in general easy to read by other programmers. Because of this design, the compiling of JavaScript and HTML scripts can be tailored for different web browsers, which improves cross-browser compatibility.

Client-side application, such as AJAX, short for JavaScript+XML, can be developed on Eclipse IDE via Google plugin or using WebAppCreator, a command-line tool included in the GWT. The developing steps in general include 1. create a GWT application directory, 2. Analyze and design the application by examining the functional requirements and identifying the elements of the UI design. 3. Select GWT widgets and UI elements and layout the UI elements on GWT panels. 4. Manage the requirements for event handling (e.g., user events). 5. Add client-side codes to create data structure, tables, and functionalities. 6. Debug in GWT development mode. 7. Compile GWT code and run in production mode.

History

GWT was first released by Google at the JavaOne conference, 2006. Four years later, Google added GWT Designer, now part of Google Plugin for Eclipse. Since 2012 GWT has become an open sourced project. Until May of 2014, there have been 16 versions released. GWT is currently being used by Google to create many products, including AdWords, AdSense, Flights, Hotel Finder, Offers, Wallet, and Blogger. Other real world projects include GoGrid, Scenechronize, and Whirled.

Examples

GWT application is a client-side application, meaning the app will be run on viewers’ web browser as JavaScript. This client-side processing requires developers to follow libraries and Java language constructs.

• EntryPoint Class creation

EntryPoint is a public interface that allows a class to act as a module entry point in the application.

package com.example.foo.client;

import com.google.gwt.core.client.EntryPoint;
import com.google.gwt.core.client.GWT;

/**
 * Entry point classes define onModuleLoad().
 */
public class Foo implements EntryPoint {

  /**
   * This is the entry point method. Initialize you GWT module here.
   */
  public void onModuleLoad() {

    // Writes Hello World to the module log window.
    GWT.log("Hello World!", null);
  }
}

• Creating a simple pop-up dialog and a clickable button on the browser

In the EntryPoint, several components can be created. For example, a Button widget can be created with the text “Click me” and a handler can be created to respond to user’s action. A dialog window is shown after the button is clicked.

public class Hello implements EntryPoint {

  public void onModuleLoad() {
    Button b = new Button("Click me", new ClickHandler() {
      public void onClick(ClickEvent event) {
        Window.alert("Hello, AJAX");
      }
    });

    RootPanel.get().add(b);
  }
}

• Writing native JavaScript in GWT

JSNI methods allows native JavaScript code in a formatted comment block to be inserted in the code. JSNI comment blocks begins with the exact token /*-{ and ends with }-*/ Here is the alter method signature :

public static native void alert(String msg) /*-{
  $wnd.alert(msg);
}-*/;

Here $wnd references the JavaScript window object.

• Calling Java method from embedded JavaScript

The JavaScript is able to communicate with Java methods using the following approach.

package mypackage;

public MyUtilityClass
{
    public static int computeLoanInterest(int amt, float interestRate,
                                          int term) { ... }
    public static native void exportStaticMethod() /*-{
       $wnd.computeLoanInterest =
          $entry(@mypackage.MyUtilityClass::computeLoanInterest(IFI));
    }-*/;
}

• Identifying browser type

For cross-browser compatibility, browser type can be identified using the following embedded JavaScript method.

public static native String getUserAgent() /*-{
     return navigator.userAgent.toLowerCase();
  }-*/

• Cascading Style Sheets (CSS)

GWT application may use CSS. Each class of GWT widget class has an associated style name for each CSS rule. For example, to assign a larger font size to button text, use

  .gwt-Button { font-size: 150%; } 

• To set an id for a GWT widget and set the id attribute for DOM element

Button b = new Button();
  DOM.setElementAttribute(b.getElement(), "id", "my-button-id")

GWT SDK

Browser based applications are tough to build as one has to deal with browser quirks and the speed of java script, also team co-ordination needs to maintained for support and enhancements.GWT is a development toolchain that is designed to build high performance web application that are easy to build and optimize. ANy GWT application be built in 4 stages:

Write

Core Java API and widgets which help in creating new new browser-based applications and compile the source in the highly optimized javascript that runs across various browsers.High level of abstraction is provided over the AJAX concepts of DOM manipulation and XHR communication. Anything that can be done with AJAX DOM and Javascript can be easily done in GWT. The toolkit can also manipulate hand-written javascript as well. The UI can be constructed in a UI template called the template Binder The client-side source is available in the client folder which is compiled and converted into javascript which runs on the client-side browsers. The code which needs to be run on the server side can be added in a server folder. The code is this folder is converted into byte code and can run on any servlet container. Communication between the client side and the server side is done using the RPC mechanism so need to worry about the low level http calls. If the server does not support javascript, json can be used to write the server application.

Debug

GWT provides debugging of the java code just like desktop application using IDE (Eclipse, IntelliJ, JProfiler). Client-side javascript code can be debugged just the way it can be debugged in any browser. Debugging java byte code is easier , one can set breakpoints, monitor variable values and so on just like a desktop application. The javascript can be debugged using the simple edit-save and refresh of javascript code and the changes can be seen in the browser without compiling. GWT's direct integration with JUnit lets you unit test both in a debugger and in a browser and one can even unit test asynchronous RPCs.

Optimize

GWT compiler provides excellent optimisation across code base by inlining method, removing dead code, optimising string strings and so on. By the user’s guidance, the code can be compiled into multiple java script fragments which improves performance by splitting up large applications for faster startup time. Browser layout and css may also be the reason for bottleneck and so the GWT provides with Speed Tracer is a new extension which helps to diagnose problems in the browser performance. After the code is compiled to javascript, the performance of the javascript on the browser can be monitored. Speed tracer shows the bottlenecks by a sluggishness graph and also a drill down into the code.

Run

The GWT SDL compiles the java code into clientside standalone javascript which can be found in the cache.html folder. the compiled script is heavily optimized and not in the readable format. But the “pretty compile” option provided by the GWT gives the output in the human readable format. Browser specific files are also created . For e.g. if there are certain files which are only for Mozilla Firefox browser, they are not downloaded by Internet Explorer.

Resources: http://www.gwtproject.org/

Features

GWT features include:
Dynamic and reusable UI components: GWT allows creating reusable Widgets by composing other Widgets, then easily lay them out automatically in Panels. If you want to reuse your widget with other components, package it in a JAR file for usage in other applications

Browser history management: GWT lets you make your site more usable by easily adding state to the browser's back button history.

Asynchronous remote procedure calls: GWT can handle all of the client-server communications for you, whether you use JSON, XML, or GWT's optimized Remote Procedure Call (RPC) system. You don't need to know the lower level details and frustrations of XHR calls.

UI abstraction, and internationalization: If your application is successful, you'll want to support the world. Making good architecture decisions up front helps. With GWT you can easily create efficient internationalized applications and libraries, including bi-directionality. Use the backend language of your choice: You don't have to run Java on your server to use GWT to build your client. Because GWT works with many standard communication protocols, you can easily communicate back and forth.

Optimize the JavaScript script downloads based on user profile: GWT creates a separate compiled version of your application that is optimized for a particular user's environment. This means that a Firefox browser displaying an application in French doesn't need to download extra code for other browsers or languages.

Use other JavaScript libraries and native JavaScript code: You can mix handwritten JavaScript in your Java source code to interface with existing JavaScript APIs. You can write individual JavaScript methods directly in your Java source files and even encapsulate JavaScript objects inside a Java class.

Be productive with your choice of development tools Because GWT uses Java, you'll be catching errors like typos and type mismatches as you write the code, not at runtime. Harness the productivity gains of an IDE's automated Java refactoring and code prompting/completion. Now you can use all of your favorite Java development tools (Eclipse, IntelliJ, JProfiler) for your AJAX development.

Test your code with JUnit: GWT's direct integration with JUnit lets you unit test both in a debugger and in a browser...and you can even unit test asynchronous RPCs.

It's free and open source: GWT is free, and all of the code is available under the Apache 2.0 license.

Resources: http://www.gwtproject.org/learnmore-sdk.html

Widgets components

GWT offers a set of UI widgets and panels.

Widgets:
Button, PushButton, RadioButton, CheckBox, DatePicker, ToogleButton, TextBox, PasswordTextBox, TextArea, Hyperlink, ListBox, CellList, MenuBar, Tree, CellTree, SuggestBox, RichTextArea, FlexTable, Grid, CellBrowser, TabBar, DialogBox

Panels:
PopupPanel, StackPanel, StackLayoutPanel, HorizontalPanel, VerticalPanel, FlowPanel, VerticalSplitPanel, HorizontalSplitPanel, SplitLayoutPanel, DockPanel, DockLayoutPanel, TabPanel, TabLayoutPanel, DisclosurePanel

You can have a code example of each widget in the Sample application.

Components

Java to JavaScript Compiler
The heart of GWT is a compiler that converts Java source into JavaScript, transforming your working Java application into an equivalent JavaScript application. Generally speaking, if your GWT application compiles and runs in hosted mode as you expect and GWT compiles your application into JavaScript output without complaint, then your application will work the same way in a web browser as it did in hosted mode.

The GWT compiler supports the vast majority of the Java language. The GWT runtime library emulates a relevant subset of the Java runtime library. If a JRE class or method is not supported, the compiler will emit an error.

You can run the compiler with the name of the module you want to compile in one of the following manners:

Run the main class com.google.gwt.dev.GWTCompiler using java from the command-line. If you used the 'applicationCreator' script to create your project, you can use the generated convenience shell script <project-name>-compile. Use the Compile/Browse button from inside of hosted mode. Once compilation completes sucessfully, directories will be created containing the JavaScript implementation of your project. The compiler will create one directory for each module it compiles.

GWT Web UI Class library
The GWT includes a class library designed to help you build your user interface, make RPC calls to your server, internationalize your application, perform unit testing, and lots more. For complete details about the GWT class library, click below to view the javadoc-style API reference.
http://www.gwtproject.org/javadoc/latest/index.html

JRE Emulation library
GWT includes a library that emulates a subset of the Java runtime library. The list below shows the set of JRE packages, types and methods that GWT can translate automatically. Note that in some cases, only a subset of methods is supported for a given type.

java.lang
java.lang.annotation
java.math
java.io
java.sql
java.util
java.util.logging

Resources: https://code.google.com/p/google-web-toolkit-doc-1-4/wiki/JavaToJavaScriptCompiler

http://www.gwtproject.org/doc/latest/RefJreEmulation.html
Hyperlinks to important terms