What is a GWT Module?

According to Google, “A GWT ‘module’ is simply an encapsulation of functionality”. A module bundles together all the configuration needed to use use your code in a GWT project. While a GWT module shares some similarities with a Java package it is not the same thing. GWT modules follow the Java package dotted-path naming convention, e.g. most of the standard GWT modules are located under “com.google.gwt”. The naming convention is where the similarities end. If you want to add new classes, you can’t just add them anywhere.
GWT modules are defined using XML descriptor files ending with the .gwt.xml extension. The concept of a GWT module descriptor is similar to deployment descriptors in Tomcat and other application servers. The contents of the XML descriptor files are use to specify:
• Inherited modules
• An entry point application class name (optional)
• Source Path entries
• Public Path entries
Inherited Modules
Modules from which settings and code will be inherited.
Entry Point Classes
A module entry point is any class that implements the EntryPoint interface. When a module is loaded, each entry point class listed in the .gwt.xml file is instantiated and its onModuleLoad() method is called. It is possible to create a module with more than one entry point. It is also possible to create a module with no entry points. If you wish to make a re-usable GWT code library, you should package it as a GWT module that contains no entry point classes. This module could then be inherited by other GWT modules.
Source Path
Source Path entries can be used to specify which sub-packages contain translatable source code. Only files found on the source path are candidates to be translated into JavaScript. When modules are inherited by other modules, the source paths are combined so that each module will have access to the translatable source it requires. The client sub-package is included in the source path by default.
Public Path
Public Path entries can be used to specify which sub-packages are public. All the files found on the public path will be copied to the module’s output directory when your application is compiled. When modules are inherited by other modules, the public paths are combined so that each module will have access to the static resources it expects. The public sub-package is included in the public path by default.