@Override
public void doAtLaunch() {
    super.doAtLaunch();

    //register the routinefunction
    RoutineRegistryFunctionRegistry.register(new ReverseStringRoutineReverseStringFunction("myReverseString"));

    //register the CF descriptor
    customFieldDescriptorRegistry.register("com.atlassian.jira.plugin.system.customfieldtypes:project", //this is the real key of the CF
            new AbstractCustomFieldDescriptorFactory(
                    "example.project.cfdf", //internal identification. Must be unique in the SIL system
                    "Project Key to String", //short description, followed by a longer one
                    "Extract the project key using GenericValue or project") {

				// Descriptors are created when needed, and some of them might be contextual. This is why you register here a creator of a
			    // descriptor and not the real descriptor
			    @Override
                public FieldDescriptor createDescriptor(Issue issue, CustomField cf) {
                    return new ProjectCFDescriptor(projectManager);
                }
            }
    );
}

@Override
public void doAtStop() {
    //first, make sure that super is called, even if it has an exception here
    try {
        //unregister the routinefunction
        RoutineRegistryFunctionRegistry.unregister("myReverseString");
        
        //unregister the CF descriptor
        customFieldDescriptorRegistry.unregister("com.atlassian.jira.plugin.system.customfieldtypes:project");  //this is the real key of the CF
    } catch(Exception e) {
        LOG.error("Failed to unregister!", e);
    }
    
    super.doAtStop();
}