session应用

怼烎@ 2022-05-16 06:14 149阅读 0赞

*  用于保存用户登录信息
 *  用于防止表单重复提交（也可以通过客户端判断表单是否已经提交过来防止表单重复提交，但是不能防止刷新造成的重复提交），而在服务器端防止表单重复提交的原理：表单由服务器动态生成，与此同时生成一个与之对应的随机数，并将随机数连同表单一起发送至客户端，客户端提交表单时携带随机数，服务器比对随机数，若相同则提交成功否则不接受提交信息。

FormServlet:

package cn.itcast.form;
    
    import java.io.IOException;
    import java.io.PrintWriter;
    import java.security.MessageDigest;
    import java.security.NoSuchAlgorithmException;
    import java.util.Random;
    
    import javax.servlet.ServletException;
    import javax.servlet.http.HttpServlet;
    import javax.servlet.http.HttpServletRequest;
    import javax.servlet.http.HttpServletResponse;
    
    import sun.misc.BASE64Encoder;
    //产生表单
    public class FormServlet extends HttpServlet {
        
    
        public void doGet(HttpServletRequest request, HttpServletResponse response)
                throws ServletException, IOException {
    
            response.setContentType("text/html;charset=UTF-8");
            response.setCharacterEncoding("UTF-8");
            PrintWriter  out = response.getWriter();
    
    
            String token = TokenProcessor.getInstance().generateToken();
            request.getSession().setAttribute("token", token);
    
            out.print("<form action='/day07/servlet/FormSubmitServlet' method='post'>");
                out.print("<input type='hidden' name='token' value='"+token+"'>");
                out.print("<input type='text' name='username'>");
                out.print("<input type='submit' value='提交'>");
            out.print("</form>");
    
    
        }
    
        public void doPost(HttpServletRequest request, HttpServletResponse response)
                throws ServletException, IOException {
            doGet(request, response);
        }
    }
    
    class TokenProcessor{
        //1.  把构造方法私有
        //2.  自己产生一个类的对象
        //3.  定义一个方法返回上面产生的对象
    
        private TokenProcessor(){};
        public static final TokenProcessor instance = new TokenProcessor();
        public static TokenProcessor getInstance(){
            return instance;
        }
    
        public String generateToken(){
    
            //3843849384   9849238402840243802  983434
            String token = System.currentTimeMillis() + "" + new Random().nextInt(99999999);
    
            //数据指纹 数据摘要  md5
            try {
                MessageDigest md = MessageDigest.getInstance("md5"); 
                byte md5[] = md.digest(token.getBytes());   //128位  16【12，23，34，544543543543，】
    
                //base64编码    SABDSSDSD
                BASE64Encoder encoder = new BASE64Encoder();
                return encoder.encode(md5);
            } catch (NoSuchAlgorithmException e) {
                throw new RuntimeException(e);
            }
        }
    }

FormSubmitServlet:

package cn.itcast.form;
    
    import java.io.IOException;
    
    import javax.servlet.ServletException;
    import javax.servlet.http.HttpServlet;
    import javax.servlet.http.HttpServletRequest;
    import javax.servlet.http.HttpServletResponse;
    
    public class FormSubmitServlet extends HttpServlet {
        
    
        public void doGet(HttpServletRequest request, HttpServletResponse response)
                throws ServletException, IOException {
    
            boolean b = isToken(request);
            if(!b){
                //用户带过来的令牌无效，阻止提交
                System.out.println("你是重复提交！！");
                return;
            }
    
            //用户带过来的令牌有效，处理提交
            request.getSession().removeAttribute("token");
    
            String username = request.getParameter("username");
            //把用户提交的数据保存到数据库中
            System.out.println("处理提交请求，把" + username + "保存到数库中！！");
    
        }
    
        //判断用户带过来的令牌是否有效
        private synchronized boolean isToken(HttpServletRequest request) {
            String client_token = request.getParameter("token");
            if(client_token==null){
                return false;
            }
    
            String server_token = (String) request.getSession().getAttribute("token");
            if(server_token==null){
                return false;
            }
    
            if(!client_token.equals(server_token)){
                return false;
            }
    
            return true;
        }
    
        public void doPost(HttpServletRequest request, HttpServletResponse response)
                throws ServletException, IOException {
            doGet(request, response);
        }
    
    }