Session防止客户端表单提交

　　这个防止表单提交，struts也是这样写。

　　通过一个servlet来生成一个表单，下面这个用于生成

    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        TokenProcessor tok = TokenProcessor.newInstance();
        String token = tok.generateToken();
        request.getSession().setAttribute("token", token);
        request.getRequestDispatcher("/form.jsp").forward(request, response);
    }
    


    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        doGet(request,response);
    }

}

/**
 * @author du
 * 单例,用单例减少TokenProcessor被实例的次数，已达到
 * 返回的字符串相同机率的减少
 */
class TokenProcessor{ //token : 令牌
    
    /*
     * 1.构造函数私有
     * 2.本身new一个实例
     * 3.提供对外访问方法
     * */
    private TokenProcessor(){}
    private static final TokenProcessor instance = new TokenProcessor();
    public static TokenProcessor newInstance(){
        return instance;
    }
    public String generateToken(){
        
        String token = System.currentTimeMillis()+new Random().nextInt()+ "";
        //得到数据的摘要，相当于指纹，一共128位，java提供MessageDigest
        //用的算法是md5
        try {
            MessageDigest md =  MessageDigest.getInstance("md5");
            byte[] md5 = md.digest(token.getBytes());
            
            //base64编码：返回的是铭文字符串，键盘能看得见的字符
            //                它是把每三个字节变成4个字节,(每个字节8位，base64每次取6位，并且在前面添加两位'00')
            BASE64Encoder base = new BASE64Encoder();
            return base.encode(md5);
        } catch (NoSuchAlgorithmException e) {
            throw new RuntimeException(e);
        }
    }
}

　　jsp页面

<form action="/day03/servlet/DoFormServlet">
        <input type="hidden" value="${token }" name = "token"/>
         用户名：<input type="text" value="" name="username"/>
        <input type="submit" value="提交"/>
    </form>

　　处理请求Servlet

         public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        
        //检查token是否符合
        boolean flag = checkToken(request);
        
        if(!flag){
            System.out.println("请不要重复提交！！！");
            return;
        }
        
        request.getSession().removeAttribute("token"); //清除
        System.out.println("向数据库添加一条记录");
        
    }

    private boolean checkToken(HttpServletRequest request) {
        String token = request.getParameter("token");
        String check = (String) request.getSession().getAttribute("token");
        if(token==null){
            return false;
        }
        if(check==null){
            return false;
        }
        if(!token.equals(check)){
            return false;
        }
        
        return true;
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        doGet(request, response);
    }