Session-Servlet 防止表单重复提交 登录系统 代码

public class AdminLoginServlet extends HttpServlet {

private AdminService aService = new AdminServiceImpl();

public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
//post 怎么解决乱码
request.setCharacterEncoding("UTF-8");
String checkcode = request.getParameter("checkcode");
if(checkcode == ""){
request.setAttribute("msg", "验证码为空");
RequestDispatcher rd= request.getRequestDispatcher("/login.jsp");
rd.forward(request, response);
}else{
String ckcode = (String) request.getSession().getAttribute("ckcode");
if(checkcode.equals(ckcode)){
//request.getSession().getAttribute("checkcode");
String name = request.getParameter("uname");
String pass = request.getParameter("upass");

Admin entity = aService.checkLogin(name, pass);

if(entity==null){
//重定向
request.setAttribute("msg", "用户名或者密码错误");
RequestDispatcher rd= request.getRequestDispatcher("/login.jsp");
rd.forward(request, response);
//请求转发

}else{
System.out.println(entity.toString());
//利用session存入用户对象
request.getSession().setAttribute("admin", entity);
//转发到这个index.jsp页面中 疑问 index.jsp能获取到admin对象的值
//response.sendRedirect("./index.jsp");
//请求转发
RequestDispatcher rd= request.getRequestDispatcher("/index.jsp");
rd.forward(request, response);
}
}else{
request.setAttribute("msg", "验证码不正确");
RequestDispatcher rd= request.getRequestDispatcher("/login.jsp");
rd.forward(request, response);
}
}
}

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

}