转发和重定向的区别

• 请求转发：浏览器只发送一次请求，URL不变。
  • 实现原理：由ServletContext在后端实现，会去调用/error对应的servlet的doGet/doPost方法。（通过方法调用实现）

this.getServletContext().getRequestDispatcher("/error").forward(req,resp);

// 常用的是request
req.getRequestDispatcher("/error").forward(req, resp);

• 重定向：浏览器发送两次请求，URL改变。
  • index.jsp：提交之后，访问/login（Web.xml中映射为RedirectServlet）

    • 
      
      Hello World!
      
      
          用户名： 
      
          密码： 
      
          
      
      
      
      

  • RedirectServlet：处理来自form表单的请求。（第二个代码段就是重定向的原理）

    • public class RedirectServlet extends HttpServlet {
          @Override
          protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
              String username = req.getParameter("username");
              String password = req.getParameter("password");
      
              System.out.println(username + ":" +password);
      
              resp.sendRedirect("/response/success.jsp");
          }
      
          @Override
          protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
              this.doGet(req, resp);
          }
      }

      • public class RedirectServlet extends HttpServlet {
            @Override
            protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
                String username = req.getParameter("username");
                String password = req.getParameter("password");
        
                System.out.println(username + ":" +password);
        
        //        resp.sendRedirect("/response/success.jsp");
        
                resp.setStatus(HttpServletResponse.SC_MOVED_TEMPORARILY);
                resp.setHeader("Location", "/response/success.jsp");
            }
        
            @Override
            protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
                this.doGet(req, resp);
            }
        }

  • 实现原理：由Tomcat解析HttpServletResponse设置的Status Code和Location。浏览器发现状态码是302时，就知道是重定向，然后重新向Location给定的URL发送请求（相当于JS的window.location）