struts2中action实现ModelDriven后无法返回json的解决方法

public class deviceaction extends abstractaction implements modeldriven<device> {
    private static log log = logfactory.getlog(deviceaction.class);
    private device device=new device(); //只能json化 模型驱动的bean
    private string result; //无法返回reslut的json值

   
    public string getresult(){
        return result;
    }
    public void setresult(string result){
        this.result=result
    }
    public devicedto getmodel() {
        return device;
    }
    public device getdevice() {
        return device;
    }
    public void setdevice(device device) {
        this.device = device;
    }
}            

<action name="querypu"
            class="com.jxj.bss.web.actions.deviceaction"
            method="querypu">
            <interceptor-ref name="islogin"></interceptor-ref>
            <result type="json">
                <param name="includeproperties">
                    resultcode
                </param>
            </result>
        </action>

<action name="querypu"
            class="com.jxj.bss.web.actions.deviceaction"
            method="querypu">
            <interceptor-ref name="islogin"></interceptor-ref>
            <result type="json">
                <param name="root">action</param>
                <param name="includeproperties">
                    resultcode
                </param>
            </result>
        </action>