使用Jackson Json序列化无状态对象时发生异常

Question

I have been using the jackson implementation of the json protocol in my own little project, and it has gone just fine a while now until I decided (for the first time) to serialize a stateless object.

I know that might sound weird, why would I want to send a stateless object? What I serialize is requests for a server, and this particular one conatins no fields, just code for an instruction on the server side. My model can take any ClientRequest implementation and call it's perform() method. I want it to work even though the request comes without fields.

Code looks like this:

public class GetWallInputsRequest implements ClientRequest<List<WallInput>>
{
   @JsonCreator public GetWallInputsRequest()
   {
   }

   @Override public ServerResponse<List<WallInput>> perform()
   {
      return new WallMessageResponse( Wall.WALL.getInputs() );
   }
}

I get JsonMappingException: No serializer found for class GetWallInputsRequest. Google does not help me, which makes me wonder if I'm just being stupid. Sadly I don't see a way out of this.

I solved it after a lot of brute force attempting different things. And by solved it I mean not figured it out but made it work. By adding the line:

@JsonAutoDetect(getterVisibility=JsonAutoDetect.Visibility.NONE)

above the class declaration it seems to work out. Why this is necessary I don't know, but now It sends an empty json string instead of crashing.

Answer 1

The documentation says

Value that indicates that no access modifiers are auto-detectable: this can be used to explicitly disable auto-detection for specified types.

Since your class doesn't include any explicit notations to tell Jackson that there's a field or method to serialize, it determines that there is indeed nothing to look for. Without this, I presume, it's going to expect something , as suggested in the documentation quoted.

http://jackson.codehaus.org/1.9.0/javadoc/org/codehaus/jackson/annotate/JsonAutoDetect.Visibility.html