如何将自定义对象传递给app引擎端点

I'm using Google App Engine Endpoints. Docs says that only primitive types like String, int and so on are supported. I want to pass an object. I read about the @ApiTransform annotation so I thought to use to convert my object from/to String object, but I'm not sure about the "scope" of this kind of annotation. Is it the right goal? Or the transform api is designed for another goal? I didn't find any example or code about a real use of this annotation.I'm using Google App Engine Endpoints. Docs say