The SIP Servlet API is a part of JAIN APIs and being standardized as JSR116 of JCP (Java Community Process). The SIP Servlet API version 1.0 was published in February, 2003.
Note: In this document, the term “SIP Servlet” is used to represent the API, and “SIP servlet” is used to represent an application created with the API. J2EE provides Java Servlet that is a main technology of building Web applications. Although Java Servlet is used only to develop HTTP protocol-based applications on a Web application server, it basically has functions as a generic API for server applications. SIP Servlet is defined as the generic servlet API with SIP-specific functions added.

SIP Servlets are very similar to HTTP Servlets, and HTTP servlet developers will quickly adapt to the programming model. The service level defined by both HTTP and SIP Servlets is very similar, and you can easily design applications that support both HTTP and SIP. Listing 1 shows an example of a simple SIP servlet.
Listing 1-1 List 1: SimpleSIPServlet.java
package com.bea.example.simple;
import java.io.IOException;
import javax.servlet.*;
import javax.servlet.sip.*;
public class SimpleSIPServlet extends SipServlet {
protected void doMessage(SipServletRequest req)
throws ServletException, IOException
{
SipServletResponse res = req.createResponse(200);
res.send();
}
}

The above example shows a simple SIP servlet that sends back a 200 OK response to the SIP MESSAGE request. As you can see from the list, SIP Servlet and HTTP Servlet have many things in common:
1. Servlets must inherit the base class provided by the API. HTTP servlets must inherit HttpServlet, and SIP servlets must inherit SipServlet.
2. Methods doXxx must be overridden and implemented. HTTP servlets have doGet/doPost methods corresponding to GET/POST methods. Similarly, SIP servlets have doXxx methods corresponding to the method name (in the above example, the MESSAGE method). Application developers override and implement necessary methods.
3. The lifecycle and management method (init, destroy) of SIP Servlet are exactly the same as HTTP Servlet. Manipulation of sessions and attributes is also the same.
4. Although not appeared in the API, there is a deployment descriptor called sip.xml for a SIP servlet, which corresponds to web.xml. Application developers and service managers can edit this file to configure applications using multiple SIP servlets.

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