In the question What is an efficient way to implement a singleton pattern in Java? the answer with the most upvotes says, to use a Enum for implementing a singleton.
Tha
You shouldn't care about the actual instance(s) of your servlets - lifecycle management is handled by the servlet container according to the Servlet specification contract to which you have agreed. If it makes sense to implement parts of you server-side functionality as a singleton, then go ahead and do that any way you like and use it from your servlet.