by com.google.gwt.dev.GWTComplier, passing it the location of your module definition file along with some other parameters. It will use the JRE emulation library to parse the code. A module is a set of related Java classes and files accompanied by a single configuration file. The module definition typically includes an entry point, which is a class that excutes when the application starts.
The compiler starts with the entry-point class, following dependencies required to compile the Java code. The GWT does not compile everything in the modules, it only includes what is being used.