swfupload struts2的问题

struts2 配置文件 <package name="json" extends="json-default">
<action name="uploadFiles" class="com.dmp.cms.actions.FileAction" method="uploadFiles">
<result name="success" type="json">
<param name="includeProperties">success</param>
</result>
<result name="input" type="json">
<param name="includeProperties">success</param>
</result>
</action>
</package> action的代码
        
                  public String uploadFiles() {
      System.out.println(getMyUpload()+"_____________+______________________");
             String rootPath = this.getRequest().getSession().getServletContext().getRealPath("/");
             System.out.println("rootPath="+rootPath);
             rootPath += ROOT;
             String sp = rootPath + getPath();
             System.out.println("sp="+sp);
             FileUtils.mkDirectory(sp);
             System.out.println("sp="+sp);
             try {
                     FileUtils.upload(getMyUploadFileName(), sp, getMyUpload());
                     this.success = true;
             } catch (RuntimeException e) {
                     e.printStackTrace();
             }
             System.out.println(success);
             return SUCCESS;
     }为什么会报下面这个错误  WARN - Caught OgnlException while setting property 'includeProperties' on type  'com.googlecode.jsonplugin.JSONResult'.   ognl.NoSuchPropertyException: com.googlecode.jsonplugin.JSONResult.includeProperties
at ognl.ObjectPropertyAccessor.setProperty(ObjectPropertyAccessor.java:132)
at com.opensymphony.xwork2.util.OgnlValueStack$ObjectAccessor.setProperty(OgnlValueStack.java:68)
at ognl.OgnlRuntime.setProperty(OgnlRuntime.java:1656)
at ognl.ASTProperty.setValueBody(ASTProperty.java:101)
at ognl.SimpleNode.evaluateSetValueBody(SimpleNode.java:177)
at ognl.SimpleNode.setValue(SimpleNode.java:246)