Deploy this jar file along with the vaadin.jar if you are experiencing problems such as java.lang.NoClassDefFoundError: com/google/gwt/user/client/ui/HasWidgets or similar when deploying a Vaadin application. Typically the exception is caused by class loaders (too) eagerly loading the class that a @ClientWidget annotation refers to. This compatibility jar provides dummy implementations of these classes. Do not use this jar during development. This jar should never be on the same classpath as gwt-user.jar/gwt-dev.jar. Note! This jar is only needed for Vaadin 6. Vaadin 7 uses a @Connect annotation on the client side to avoid this problem.