limited to the code that can be realistically exercised without the need to run it inside Mule itself. As a rule of thumb, code that is Mule aware (for example, code that relies on the registry), will better be exercised with a functional test With this in mind, the following are good candidates for unit testing: *Custom transformers *Custom components *Custom expression evaluators *All the Spring beans that your Mule application will use. Typically, these beans come as part of a dependency JAR and are tested while being built, alleviating the need for re-retesting them in your Mule application project