Abstract class used as a base for context retriever implementations that retrieve the security context from a principal made available through some environmental object.
This implementation supports both HttpServletRequest and EJBContext environmental objects but subclasses can add additional support by overriding the extractContext and extractPrincipal methods. This class has special support for Principal objects that implement the ContextRetrieverPrincipal interface. Often it will be sufficient for subclasses to declare themselves, without adding any methods at all.