Typically these classes are used during compilation of your GWT app. Most of them are generator classes and utility classes used by these generators. Generators are used to generate additional client side code for your app, e.g. code that implements your GWT RPC service if you have one. As compilation is done in a normal JVM its fine to have references to BufferedImage in these classes. I assume that BufferedImage will be used to generate image bundles (a big image containing many small images used in ClientBundles).