Java中的表单填充验证

我确实使用servlet签入，它可以工作

如何验证表格填写？ 例如，如果用户名已经注册，是否将用户重新发送到.jsp文件

对不起，英语不好

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

    // fetch the username that was sent in the request
    String username = request.getParameter("username");

    // TODO: verify if the username is taken in the database

    // based on the results set the value
    request.setAttribute("isUsernameTaken", "true");

    RequestDispatcher dispatcher = getServletContext().getRequestDispatcher("/register.jsp");

    dispatcher.forward(request, response);      
}