struts2 零配置返回type=json出错

struts2 零配置返回type=json出错
@Action(value = "/image",results ={@Result(name="success",type="json",params = {"root","result" })})
tomcat启动解析@Action、、、时,type类型出错
错误：
The Result type [json] which is defined in the Result annotation on the class [class action.UploadAction] or determined by the file extension or is the default result type for the PackageConfig of the action,could not be found as a result-type defined for the Struts/XWork package [action#convention-default#] - [unknown location]
知道的就说吧