By making it a singleton, we do need an explicit Setup operation. However, it means that we can refer to the settings in client code that doesn’t have access to a request or to the ServletContext.
Other refactorings to simplify the logic or make it more scalable.
We get a warning on any attempt to use anything that may be affected by DeveloperSettings, because DeveloperSetting tries to use ConfigurationProperties to find its properties file. We could avoid this by requiring the use of a ConfigurationPropertiesStub, but it seems to make more sense to routinely create a DeveloperSettingsStub on each ServletContextStub.
It's OK for these stub classes to implement deprecated methods in the interfaces, as long as they declare those methods with @Override and @Deprecated.
