Servlet的体系

进行HTTP服务操作时，Servet所使用的类

接口

Serlvet

Servlet对象的根接口，声明了提供Servlet服务所需要用到方法

ServletConfig

Serlvet配置对象接口，声明了获取Servlet配置参数的方法

ServletRequest 

请求的接口，声明了请求操作的常用方法

ServletResponse

响应的接口，声明了响应的常用方法

抽象类

GenericServlet

实现了Servlet和ServletConfig接口，重载了init方法，新增对application对象的访问方法

 public ServletContext getServletContext()
  {
    return getServletConfig().getServletContext();
  }
 public void init()
    throws ServletException
  {}

HttpServlet

继承了GenericServlet，重载了使用HttpServletRequst、HttpServletResponse接口使用参数的service方法，扩展了doGet和doPost方法实现业务功能

protected void doGet(HttpServletRequest req, HttpServletResponse resp)
    throws ServletException, IOException
  {
    String protocol = req.getProtocol();
    String msg = lStrings.getString("http.method_get_not_supported");
    if (protocol.endsWith("1.1")) {
      resp.sendError(405, msg);
    } else {
      resp.sendError(400, msg);
    }
  }
dopost
 protected void doPost(HttpServletRequest req, HttpServletResponse resp)
    throws ServletException, IOException
  {
    String protocol = req.getProtocol();
    String msg = lStrings.getString("http.method_post_not_supported");
    if (protocol.endsWith("1.1")) {
      resp.sendError(405, msg);
    } else {
      resp.sendError(400, msg);
    }
  }

 protected void service(HttpServletRequest req, HttpServletResponse resp)
    throws ServletException, IOException
  {
    String method = req.getMethod();
    if (method.equals("GET"))
    {
      long lastModified = getLastModified(req);
      if (lastModified == -1L)
      {
        doGet(req, resp);
      }
      else
      {
        long ifModifiedSince;
        try
        {
          ifModifiedSince = req.getDateHeader("If-Modified-Since");
        }
        catch (IllegalArgumentException iae)
        {
          long ifModifiedSince;
          ifModifiedSince = -1L;
        }
        if (ifModifiedSince < lastModified / 1000L * 1000L)
        {
          maybeSetLastModified(resp, lastModified);
          doGet(req, resp);
        }
        else
        {
          resp.setStatus(304);
        }
      }
    }
    else if (method.equals("HEAD"))
    {
      long lastModified = getLastModified(req);
      maybeSetLastModified(resp, lastModified);
      doHead(req, resp);
    }
    else if (method.equals("POST"))
    {
      doPost(req, resp);
    }
    else if (method.equals("PUT"))
    {
      doPut(req, resp);
    }
    else if (method.equals("DELETE"))
    {
      doDelete(req, resp);
    }
    else if (method.equals("OPTIONS"))
    {
      doOptions(req, resp);
    }
    else if (method.equals("TRACE"))
    {
      doTrace(req, resp);
    }
    else
    {
      String errMsg = lStrings.getString("http.method_not_implemented");
      Object[] errArgs = new Object[1];
      errArgs[0] = method;
      errMsg = MessageFormat.format(errMsg, errArgs);
      
      resp.sendError(501, errMsg);
    }
  }