AJAX出现乱码了怎么回事

1、jsp 
  <script type="text/javascript">
var xmlHttp;
function creatXMLHttp() {
var username =document.getElementById("Username").value;
if (window.XMLHttpRequest) {
xmlHttp = new XMLHttpRequest();
} else {
xmlHttp = new ActiveXobject("Microsoft.XMLHTTP");
}
}
function UpdateUser(username) {
creatXMLHttp();
xmlHttp.open("POST", "../UpdateSelectAdmin.asp?Username=" + username);
xmlhttp.setRequestHeader( "Content-Type", "text/html;charset=GBK" );
xmlHttp.onreadystatechange = UpdateUserCallback;
xmlHttp.send(null);
}
function UpdateUserCallback() {
if (xmlHttp.readyState == 4) {
if (xmlHttp.status == 200) {
var text = xmlHttp.responseText;
document.getElementById("txtHint").innerHTML = text;
}
}
}
</script>

out.print("<input type='hidden' name='Username' value="+ login.getUsername() + ">");

2、 response.setContentType("text/html");
PrintWriter out = response.getWriter();
String username=request.getParameter("Username");
out.print(username);
找了很多案例都无法解决。求帮忙啊~

ajax里
username 是中文吗?
username=encodeURI(username);
username=encodeURI(username); //最重要的部分,两次调用encodeURI ,就是编码两次

servlet
String name=request.getParameter("username");
name=URLDecoder.decode(name,"utf8"); //post 传递的时候,一定是用utf8编码的,url 自己可以设定
response.setContentType("text/html;charset=utf-8") 

function onSendInfo(reqId){
    if(confirm("是否确认发送站内短信?")){
        var friendList = document.getElementById("friendList");
        var msgContent = document.getElementById("textarea2");
        if(onValidate(friendList,msgContent)){
            return;
        }
        var usernames=onSelect(friendList);
        var params="?reqId="+reqId
                +"&msgContent="+encodeURI($("#textarea2").val())
                +"&sendMember="+encodeURI(usernames)
                +"&msgTitle="+encodeURI($("#textarea").val());
        $.ajax({    
            type: "post",    
            url: "motorcade_alliance_send_message.jspx"+params,
            dataType: "json",/*这句可用可不用，没有影响*/
            contentType: "application/json; charset=utf-8",    
            success: function (data) {
                if(data.status==0){
                    alert('发送站内短信成功!');
                }else if(data.status==1){
                    alert(data.errors);
                }
            }, 
            beforeSend: function(XMLHttpRequest){  
                XMLHttpRequest.setRequestHeader("RequestType", "ajax");  
            },  
            error: function (XMLHttpRequest, textStatus, errorThrown) {    
            alert(errorThrown);    
            }    
        });
    }
}

原因是编码不统一