request,session,cookie用来保存数据的例子

共用界面

<html>
<head>
    <title>Title</title>
</head>
<body>
    <form action="/login" method="post">  //action的内容根据实验对象不同需要修改
        <table>
            <tr>
                <th>用户名:</th>
                <td><input type="text" name="username"></td>
            </tr>
            <tr>
                <th>密码:</th>
                <td><input type="text" name="password"></td>
            </tr>
            <tr>
                <td><input type="submit" value="登录"></td>
                <td><input type="submit" value="重置"> </td>
            </tr>
        </table>
    </form>
</body>
</html>

一.request

LoginServlet

@WebServlet("/login")
public class LoginServlet extends HttpServlet {
    private String myusername="admin";
    private String mypassword="123123";
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
          String username=  req.getParameter("username");
          String password=req.getParameter("password");

          if(username.equals(myusername) &&password.equals(mypassword)){
              req.setAttribute("username",username);    //request存信息的方法
              req.getRequestDispatcher("welcome.jsp").forward(req,resp);
          }else{
              resp.sendRedirect("login.jsp");
          }
    }
}

LogoutServlet

@WebServlet("/logout")
public class LogoutServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.sendRedirect("login.jsp");
    }
}

welcome.jsp

hello
<%=request.getAttribute("username")%> // request 读取信息的方法
<a href="/logout"></a>

结果

可以看到启动welcome时用户信息已经没了

原因：request只能在一次请求响应中存在，运行welcome.jsp时已经是另外一次响应了

二.Session

SessionLoginServlet

@WebServlet("/session_login")
public class SessionLoginServlet extends HttpServlet {
    private String myusername="admin";
    private String mypassword="123123";

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String username=  req.getParameter("username");
        String password=req.getParameter("password");

        if(username.equals(myusername) &&password.equals(mypassword)){
            HttpSession session=req.getSession();       //创建Session的方法
            session.setAttribute("username",username);  //Session存储信息的方法
            resp.sendRedirect("session_welcome.jsp");   
        }else{
            resp.sendRedirect("session_login.jsp");
        }
}
}

SessionLogoutServlet

   protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        HttpSession session =req.getSession();
        session.invalidate();  //session销毁自己的方法
    }

session_welcome.jsp

hello<%=session.getAttribute("username")%> //session获取信息的方法
<a href="/session_logout">退出登录</a>

结果

怎样刷新都还有信息再那

当把服务器停止再启动时

可以看到信息已经不在了

原因：

session的信息保存在服务器端，当服务器停止了则信息没了

三.Cookie

CookieLoginServlet

@WebServlet("/cookie_login")
public class CookieLoginServlet extends HttpServlet {
    private String myusername="admin";
    private String mypassword="123123";
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String username=  req.getParameter("username");
        String password=req.getParameter("password");

        if(username.equals(myusername) &&password.equals(mypassword)){
            Cookie cookie=new Cookie("username",username);          //创建cookie
            resp.addCookie(cookie);                                 //将变动的cookie从服务器响应给客户端
            resp.sendRedirect("cookie_welcome.jsp");
        }else{
            resp.sendRedirect("cookie_login.jsp");
        }
    }
}

CookieLogoutServlet

  protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        Cookie[]cookies=req.getCookies();            //获取到cookie数组
        for(Cookie cookie:cookies){                  //用foreach找到我们传的cookie 
　　　　　　　　if(cookie.getName().equals("username")){
                cookie.setMaxAge(0);　　　　　　　　　　//设置cookie的生命周期为0
            }
        }
}

cookie_welcome.jsp

<%
Cookie []cookies=request.getCookies();
for(Cookie cookie:cookies){
    if(cookie.getName().equals("username")){
        out.write(cookie.getValue());
    }
}
%>
<a href="/cookie_logout">退出登录</a>

结果

怎么刷新都在

将服务器停止再启动

发现仍然还在

原因：

cookie不因为服务器的停止就不在，它保存在浏览器中