<script type="text/javascript">
    $(function(){
        
        //为输入框绑定事件
        $("#username").blur(function(){
            //1、失去焦点获得输入框的内容
            var usernameInput = $(this).val();
            //2、去服务端校验该用户名是否存在---ajax
            $.post(
                "${pageContext.request.contextPath}/checkUsername",
                {"username":usernameInput},
                function(data){
                    var isExist = data.isExist;
                    //3、根据返回的isExist动态的显示信息
                    var usernameInfo = "";
                    if(isExist){
                        //该用户存在
                        usernameInfo = "该用户名已经存在";
                        $("#usernameInfo").css("color","red");
                    }else{
                        usernameInfo = "该用户可以使用"
                        $("#usernameInfo").css("color","green");
                    }
                    $("#usernameInfo").html(usernameInfo);
                    
                },
                "json"
            );
            
            
        });

        
    });
</script>

public class CheckUsernameServlet extends HttpServlet {

    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        
        //获得要校验的用户名
        String username = request.getParameter("username");
        
        //传递username到service
        UserService service = new UserService();
        boolean isExist = false;
        try {
            isExist = service.checkUsername(username);
        } catch (SQLException e) {
            e.printStackTrace();
        }
        //isExist是Boolean形的，所以不用加引号。
        //如果写回去的是一个String类型格式如下
        //"{\"namet\":\""+name+"\"}"
        response.getWriter().write("{\"isExist\":"+isExist+"}");
        
    }

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

public boolean checkUsername(String username) throws SQLException {
        
        UserDao dao = new UserDao();
        Long isExist = dao.checkUsername(username);
        
        return isExist>0?true:false;
    }

public class UserDao {

    public Long checkUsername(String username) throws SQLException {
        QueryRunner runner = new QueryRunner(DataSourceUtils.getDataSource());
        String sql = "select count(*) from user where username=?";
        Long query = (Long) runner.query(sql, new ScalarHandler(), username);
        return query;
    }

}