Why does HttpServlet implement Serializable?
In my understanding of Servlet, the Servlet will be instantiated by the Container, its init() method will be called once, and the servlet will live like a singleton until the JVM shuts down.
I do not expect my servlet to be serialized, since it will be constructed new when the app server recovers or is starts up normally. The servlet should hold no session-specific members, so it does not make sense for it to be written to disk and re-instantiated. Is there a practical use for this?
My concerns are, that I put some non-serializable fields within there and then my app will mysteriously fail in a production environment where a different sort of session replication will take place.
Answer
Technically, I believe the servlet container is allowed to "passivate" the servlet object to disk, in a similar way that EJB session beans can be. So you're correct to ask the question if your app will fail due to non-serializable fields.
In practise, I've never heard of a container doing this, so it's really just legacy baggage from the bad old days of early J2EE. I wouldn't worry about it.