防止表单重复提交

<script type="text/javascript">
        //方式一
        var iscommitted = false;
        function dosubmit(){
            if(!iscommitted){
                iscommitted = true;
                return true;
            }else{
                return false;
            }
            
        }
        //方式二
        function dosubmit(){
            document.getElementById("submit").disabled = ‘disabled‘;
            return true;
        }
        
</script>

//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{  //工具类产生表单标识号
    //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);
        }
    }
}

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);
    }

}