A SIP servlet is a Java programming language class that extends the javax.servlet.sip.SipServlet class, optionally overriding SipServlet's methods. These methods correspond to the SIP protocol's requests, and are named doRequest where Request is a SIP request name. For example, the doRegister method will respond to incoming SIP REGISTER requests. See SIP Requests for a list of all request methods.
SipServlet also defines several response methods: doProvisionalResponse for SIP 100 series responses; doSuccessResponse for SIP 200 series responses; doRedirectResponse for SIP 300 series responses; and doErrorResponse for SIP 400, 500, and 600 series responses. See SIP Responses for more information about SIP responses.
All the response methods in SipServlet are empty, and a typical SIP servlet will override these methods. All the other request methods defined in SipServlet will reject any incoming corresponding SIP requests with a SIP 500 error (server error) response if the request method is not overridden.