What is GWT?

Writing browser based AJAX (Asynchronous JavaScript and XML) web applications can be very tedious and requires in depth knowledge of both JavaScript, and myriad browser quirks and incompatibilities. The Google Web Toolkit (GWT) is an open source framework that allows Java developers to easily develop AJAX web applications without learning the ins and outs of JavaScript and browser development.
GWT provides a Java API that lets you build component based GUIs while avoiding JavaScript, and abstracting the HTTP protocol and underlying browser DOM model. All of this achieved using the GWT compiler, which does not generate Java bytecode, rather it generates JavaScript! The GWT compiler takes your client side Java code and generates JavaScript. A compiled GWT application consists of fragments of HTML, XML and JavaScript. If you want to have your web application communicate with a web server, GWT has you covered as well. GWT has a Remote Procedure Call (RPC) mechanism that makes it easy for the client and server to pass Java objects back and forth.
GWT also helps to enables developers to effectively test and debug their applications without first compiling them into JavaScript, and deploying the applications to a web-server. GWT allows applications to be run in what is called “Hosted Mode”. In “Hosted Mode”, a JVM executes your GWT application code as Java bytecode inside of an embedded browser window. Running GWT applications in “Hosted Mode” makes debugging your GWT applications very easy. Once you have tested your GWT application in “Hosted Mode”, you can compile your Java source code to JavaScript and deploy your application. GWT applications that have been deployed are said to be running in “Web Mode”.
So, in short, you write your application’s client and server side pieces in Java, and the GWT compiler will convert your client Java classes into browser compliant JavaScript and HTML. You deploy this JavaScript and HTML to your web servers, so end users will only see the web mode version of your application, while the server side classes can be deployed on Tomcat or any Servlet container of your choosing.
The goal of this tutorial is to help Java developers with little knowledge of web development learn how to develop web applications using GWT. Once you are done with this tutorial you should have the skills to build complex and fully functional GWT applications.