Sailfin自带的例子CallSetup的实现过程

1. 首先，UAC（可以是X-lite等）向RegistrarServlet注册，RegistrarServlet继承SipServlet，doRegister(SipServletRequest request)对注册请求进行出来，其间，会对expires头进行判断请求是否过期，如果获得contact头，则response会发送200 ok给contact头，否则会发给from头，最后持久化to ,contact, expires到Registration。

2. 注册完后，用户通过浏览器即可浏览已经注册的信息，ListWebServlet负责显示所有注册的用户信息，它继承自HttpServlet，通过持久层获得所有注册信息然后现实在页面上。

3. 任意选择两个注册的用户来作为呼叫双方，发送请求消息，B2BCallServlet处理请求，进行双方的呼叫建立。方法如下，此处sip结合在httpservlet里了：
SipApplicationSession as = sf.createApplicationSession();
          //Just to make it simpler to understand 0 will be called A 1 B
          String callA = registrations.get(0).getContact();
          String callB = registrations.get(1).getContact();
         
          Address to = sf.createAddress(callB);
          Address from = sf.createAddress(callA);
         
          SipServletRequest sipReq = sf.createRequest(as, "INVITE", from, to);

         logger.log(Level.FINE, "ListWebServlet sipRequest = " + sipReq.toString());
         sipReq.setAttribute("CALL","INITIAL");
        
         // set servlet to invoke by reponse
         SipSession s = sipReq.getSession();
         s.setHandler("b2b");

4. B2BCallServlet的doRequest(SipServletRequest req)处理接收到的请求