一、HttpServlet概述
HttpServlet类是GenericServlet的子类，它提供了对HTTP请求的特殊支持，所以通常我们都会通过继承HttpServlet来完成自定义的Servlet。
二、HttpServlet覆盖了service()方法
HttpServlet类中提供了service(HttpServletRequest,HttpServletResponse)方法，这个方法是HttpServlet自己的方法，不是从Servlet继承来的。
在HttpServlet的service(ServletRequest,ServletResponse)方法中会把ServletRequest和ServletResponse强转成HttpServletRequest和HttpServletResponse，然后调用service(HttpServletRequest,HttpServletResponse)方法，这说明子类可以去覆盖service(HttpServletRequest,HttpServletResponse)方法即可，这就不用自己去强转请求和响应对象了。
其实子类也不用去覆盖service(HttpServletRequest,HttpServletResponse)方法，因为HttpServlet还要做另一步简化操作，下面会介绍。
HttpServlet.java
代码语言：javascript复制public abstract class HttpServlet extends GenericServlet {
protected void service(HttpServletRequest req,
HttpServletResponse resp)
throws ServletException, IOException {
//根据request对象是GET请求还是POST请求，然后去调用
//doGet方法或者doPost方法
/*
if(req.getMethod=="GET"){
doGet(req,resp);
}
else{
doPost(req,resp);
}
*/
}
@Override
public void service(ServletRequest req, ServletResponse res)
throws ServletException, IOException {
HttpServletRequest request;
HttpServletResponse response;
try {
request = (HttpServletRequest) req;
response = (HttpServletResponse) res;
} catch (ClassCastException e) {
throw new ServletException("non-HTTP request or response");
}
service(request, response);
}
……
}三、doGet()和doPost()
在HttpServlet的service(HttpServletRequest,HttpServletResponse)方法会去判断当前请求是GET还是POST，如果是GET请求，那么会去调用本类的doGet()方法，如果是POST请求会去调用doPost()方法，这说明我们在子类中去覆盖doGet()或doPost()方法即可。
代码语言：javascript复制public class AServlet extends HttpServlet {
public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
System.out.println("hello doGet()...");
}
}public class BServlet extends HttpServlet {
public void doPost(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
System.out.println("hello doPost()...");
}
}
