用jquery写的校验用户名

$(function(){
        $("input[name='username']").blur(function(){
            var uname = $(this).val();
            console.log(uname);
            console.log(uname=="");
            if (uname=="") {
                $("#span1").html("用户名不能为空!");
            }else{
                $.post(
                        "${pageContext.request.contextPath}/userAction_UserExists",
                        {"uname":uname},
                        function(data){
                            $("#span1").html(data);
                        },
                        'json'
                    );
            }
        });
    });

后端java代码:

private String uname;
    public String UserExists() throws IOException {
        log.info(uname);
        if ("admin".equals(uname)) {
            String s = "[{"info":"用户名不存在!"}]";
            String json = JSONArray.fromObject(s).toString();
            ServletActionContext.getResponse().getWriter().write(json);
        }else {
            
        }
        return null;
    }

    public void setUname(String uname) {
        this.uname = uname;
    }

只给出了js代码和java代码,html代码太乱了不好意思往上贴