GWT 101

What is GWT?
A set of tools to write AJAX applications in JAVA.
GWT is an open source, cross-browser framework that allows developers to write AJAX applications in Java and get the bytecode compiled into JavaScript.

What makes GWT interesting is Java to JavaScript compiler?
GWT is more than just a Java-JavaScript Compiler.
Based on the code that you write in Java, it performs dead code elimination, code optimization etc straightening polymorphic calls, method call in lining and string interning. basically the features of what a "compiler" does.

Well why Java ?
Because Java is a strongly typed language unlike python and that is when you get benefit of the compiler optimizations. But then again Java is the chosen one, because of the availability of IDE's, debugging tools etc and not that its the only best language in the world. Google didn't choose Java because of a particular attachment to the language, but because of the abundance of tools out there available for Java. According to Rasmussen the final code you end up with will be faster and smaller than what you would write yourself.

GWT enables software development capabilities with AJAX. These are some of its advantages:
>>Java IDEs -- you can use any Java IDE or Notepad if you prefer
>>Quick editing, testing, debugging and refactor cycle -- the browser lets you debug code at Java level. You start your hosted browser in debug mode, edit your Java code, refresh your browser and you will see the changes straight away.
>>Unit testing -- GWTTestCase class is provided so that it can be integrated with JUnit
>>Reuse through jars -- Modules or jar files of one application can be referenced from another
>>Make the most of Object Oriented design principles
>>Javadoc -- GWT API documentation
>>Compile time errors -- With Eclipse the errors are fixed as you are typing code.

How to build the UI in GWT?
GWT preserves the principles of Web usability by: providing regular UI elements; enabling keyboard-only use and font size preferences; allowing the user to be in control of the browser; facilitating a fast start-up; and, allowing the user to feel like they are using a standard Web app, but with an enhanced experience.

The User Interface is created using a GWT widget library. The widgets got to be added to the panels. And its very similar to Swings. It follows the observer pattern to handle events. Widgets publish events which can be subscribed to, when the event occurs all the subscribers will be notified.

How to GWT applications talk to the server?
GWT RPC makes it easy for the client and server to pass Java objects back and forth over HTTP. When used properly, RPCs give you the opportunity to move all of your UI logic to the client, resulting in greatly improved performance, reduced bandwidth, reduced web server load, and a pleasantly fluid user experience

"Don't Click the Back Button"- The History
GWT's history API provides a means for you to access the browser's history stack and control what happens when the back or reload button is clicked. This way, even if you click the back button you can still return to the same state of the app.

Some aspects of the API are: the historyToken parameter is the current history state so each item in the history stack is called a token; HistoryListener is triggered when the back or forward button is clicked, you would implement this to get notified when there is a change to the browser history; you can create new history items on the stack -- for instance, History.newItem(token); hyperlink class provides a link to a different state of the app where it creates a new history item without reloading the page.

Internationalization
GWT provides a way for you to internationalize your applications and libraries. With the I18N package you can translate your apps into other languages.

To create localised messages, you need to implement the Messages interface and then add the corresponding localised properties files. .properties files store the translated strings.

Controlling the look of widgets with CSS

With GWT you can control the look of your widgets using CSS and keep the code and presentation separate. Every widget has a style name which is associated with a CSS rule.

To change the font size of all your buttons, as demonstrated here, you would do the following:

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

Optimised Permutations

GWT compiler constructs multiple compilations based on all possible permutations and records them in a file -- suffix.cache.html

The end-user only downloads one optimized compilation for their particular circumstances. For instance, a compilation for Firefox 3 and UK English. The performance is optimised, because the user is downloading only what is relevant to them.

JavaScript Native Interface (JSNI)

JSNI allows you to incorporate JavaScript into your Java code with the use of the native keyword. The JavaScript code in JSNI methods is encapsulated inside a comment block. It starts with /*-{ and ends with }-*/ and is inserted between the parameter list and the ending semicolon.

What's new in GWT 1.5?

Some of the features in the release candidate for 1.5 are:

• Java 5 support (enums, annotations, autoboxing etc)
  
• An improved compiler to make apps faster
  
• Additions to the UI library such as widget animations
  
• DOM API -- to make DOM development easier
  
• Improvements to internationalization
  
• Accessibility Support 
  
• Improvements to JRE Emulation Library