怎么在Struts2.0 （action）中响应Ajax（jQuery）请求？

/* 发送ajax请求 */
$.ajax({
type: "POST",
/* 请求的action */
url: "/user_checkusername.action",
data: "username=zhangsan",
dataType:'text',
success: function(data){
alert(data);
},
error: function(){
alert("出错了");
}
});

private InputStream inputStream;
public InputStream getResult()
{
return inputStream;
}
public String checkusername()
{
try
{
inputStream = new ByteArrayInputStream("要返回的字符串".getBytes("UTF-8"));
}
catch (UnsupportedEncodingException e)
{
// TODO Auto-generated catch block
e.printStackTrace();
}
return "isExist";
}

<package name="default" namespace="/" extends="struts-default">
<action name="user_*" class="buyerRegAction" method="{1}">
<result name="isExist" type="stream">
<param name="contentType">text/html</param>
<!-- 指定由getReuslt()方法返回输出结果的InputStream -->
<param name="inputName">result</param>
</result>
</action>
</package>