问在动态Web项目中检查/处理用户名不可用(已作为主键使用的用户名)错误

EN

<form action="register1.jsp" method="post">
                    <div class="firstname">
                        <label for="firstname"></label> <input type="text"
                            placeholder="My first name is" name="firstname" required>
                    </div>
                    <div class="lastname">
                        <label for="lastname"></label> <input type="text"
                            placeholder="My last name is" name="lastname" required>
                    </div>
                    <div class="email">
                        <label for="email"></label> <input type="email"
                            placeholder="My e-mail ID is" name="email_id" required>
                    </div>
                    <div class="username">
                        <label for="username"></label> <input type="text"
                            placeholder="My username is" name="username" required>
                    </div>
                    <div class="pwd">
                        <label for="pwd"></label> <input type="password"
                            placeholder="My password is" name="pwd" required>
                    </div>
                    <div class="submit">
                        <input type="submit" value="Register" id="form_button" />
                    </div>
                </form>

<%@ page import="java.sql.*"%>
<%@ page import="javax.sql.*"%>
<%@page import="java.sql.ResultSet"%>
<%@page import="java.sql.Statement"%>
<%@page import="java.sql.Connection"%>
<%
String firstname=request.getParameter("firstname");
String lastname=request.getParameter("lastname");
String username=request.getParameter("username");
session.putValue("username",username);
String pwd=request.getParameter("pwd");
String email_id=request.getParameter("email_id");

Class.forName("com.mysql.jdbc.Driver");
Connection con= DriverManager.getConnection("jdbc:mysql://localhost/cinushi_university","CinthiyaSingh", "Cinthiya@098");
Statement st=con.createStatement();
ResultSet rs;
int i=st.executeUpdate("insert into users values('"+ username+ "','"+pwd+ "','"+firstname+ "','"+ lastname+ "','"+ email_id+ "')");
if (i > 0) {
    response.sendRedirect("login.jsp");
} else {
    request.setAttribute("errorMessage", "Already Existing Username/Email");
    request.getRequestDispatcher("Register.jsp").forward(request, response);

}%>