$(function(){
    $("#username").blur(function(){
        $.get("CheckUserServlet",{
            username:$("#username").val()
            },function(data,textStatus){
                if(data=="true"){
                    $("#msg").html("用户名已存在").css("color","#ff0000");
                    $(":submit").attr("disabled","disabled")
                }else{
                    $("#msg").html("恭喜您,可以注册").css("color","#0000ff");
                    $(":submit").attr("disabled","")
                }
            }
    });
});

提示错误如下：
Uncaught ReferenceError: $ is not defined