GSON: How to move fields to parent object

问题

I'm using Google GSON to transform my Java object to JSON.

Currently I'm having the following structure:

"Step": {
  "start_name": "Start",
  "end_name": "End",
  "data": {
    "duration": {
      "value": 292,
      "text": "4 min."
    },
    "distance": {
       "value": 1009.0,
       "text": "1 km"
    },
    "location": {
       "lat": 59.0000,
       "lng": 9.0000,
       "alt": 0.0
    }
  }
}

Currently a Duration object is inside a Data object. I would like to skip the Data object and move the Duration object to the Step object, like this:

"Step": {
  "start_name": "Start",
  "end_name": "End",
  "duration": {
    "value": 292,
    "text": "4 min."
  },
  "distance": {
     "value": 1009.0,
     "text": "1 km"
  },
  "location": {
     "lat": 59.0000,
     "lng": 9.0000,
     "alt": 0.0
  }
}

How can I do this using GSON?

EDIT: I've tried to use a TypeAdapter to modify the Step.class, but in the write-method I'm not able to add my duration object to the JsonWriter.

回答1:

You can probably do this by writing, and then registering a custom serializer for Step, and making sure inside it you work with Duration etc, instead of Data.

// registering your custom serializer:
GsonBuilder builder = new GsonBuilder ();
builder.registerTypeAdapter (Step.class, new StepSerializer ());
Gson gson = builder.create ();
// now use 'gson' to do all the work

The code for the custom serializer below, I'm writing off the top of my head. It misses exception handling, and might not compile, and does slow things like create instances of Gson repeatedly. But it represents the kind of thing you'll want to do:

class StepSerializer implements JsonSerializer<Step>
{
  public JsonElement serialize (Step src,
                                Type typeOfSrc,
                                JsonSerializationContext context)
    {
      Gson gson = new Gson ();
      /* Whenever Step is serialized,
      serialize the contained Data correctly.  */
      JsonObject step = new JsonObject ();
      step.add ("start_name", gson.toJsonTree (src.start_name);
      step.add ("end_name",   gson.toJsonTree (src.end_name);

      /* Notice how I'm digging 2 levels deep into 'data.' but adding
      JSON elements 1 level deep into 'step' itself.  */
      step.add ("duration",   gson.toJsonTree (src.data.duration);
      step.add ("distance",   gson.toJsonTree (src.data.distance);
      step.add ("location",   gson.toJsonTree (src.data.location);

      return step;
    }
}

回答2:

I don't think there is beautifull way to do it in gson. Maybe get java object (Map) from initial json, remove data, put duration and serialize to json:

Map initial = gson.fromJson(initialJson);

// Replace data with duration in this map
Map converted = ...

String convertedJson = gson.toJson(converted);

回答3:

Ran into the same problem. The answer from @ArjunShunkar pointed me in the right direction. I fixed it writing a custom Serializer, but slightly different:

public class StepSerializer implements JsonSerializer<Step> {
    @Override
    public JsonElement serialize(Step src, Type typeOfSrc, JsonSerializationContext context) {
        JsonObject step = new JsonObject();
        step.add ("start_name", context.serialize(src.start_name);
        step.add ("end_name",   context.serialize(src.end_name);

        JsonObject data = context.serialize(src.data).getAsJsonObject();
        data.entrySet().forEach(entry -> {
            step.add(entry.getKey(), entry.getValue());
        });

        return step;
    }
}

This could be improved further, the "start_name" and "end_name" props are still hardcoded. Could be removed by going over the entrySet for the root object and excluding 'data' there, leaving only the element that needs to be unwrapped hardcoded.

来源：https://stackoverflow.com/questions/10898914/gson-how-to-move-fields-to-parent-object

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java

json

gson