Styjun

I want to convert all keys of JSON string/object to UPPERCASE in Java. The JSON can be nested.

I tried setting FieldNamingPolicy.UPPER_CAMEL_CASE in GsonBuilder but I guess that just works for String to JAVA Object and not for String to String.

 String payload = ""key" : "key1" : "value1","key2" : "value2"";
 GsonBuilder gsonBuilder = new GsonBuilder();
 gsonBuilder.registerTypeAdapterFactory(myCustomTypeAdapterFactory);
 gsonBuilder.setFieldNamingPolicy(FieldNamingPolicy.UPPER_CAMEL_CASE);
 Gson gson = gsonBuilder.create();
 Map mapDeserialized = gson.fromJson(payload, Map.class);

 System.out.println("Map " + mapDeserialized);

There are other solutions through JACKSON with custom TypeAdapterFactory but those only work for one level and not for nested.

"key" :
"key1" : "value1",
"key2" : "value2"


to

"KEY" :
"KEY1" : "value1",
"KEY2" : "value2"


As you said FieldNamingPolicy is applied only for bean fields not for map keys. However UPPER_CAMEL_CASE is not what you want, it is camel case with first letter capitalized (SometingLikeThis). You have to implement your own deserializer that would do that for your:

import com.google.gson.*;
import com.google.gson.reflect.TypeToken;

import java.lang.reflect.Type;
import java.util.HashMap;
import java.util.Map;

public class UpperCaseAdapter implements JsonSerializer<Map<String, Object>>, JsonDeserializer<Map<String, Object>> 
 public static final Type TYPE = new TypeToken<Map<String, Object>>() .getType();

 @Override
 public JsonElement serialize(Map<String, Object> src, Type typeOfSrc, JsonSerializationContext context) 
 // TODO implement serialization if needed
 return null;
 

 @Override
 public Map<String, Object> deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException 
 Map<String, Object> map = new HashMap<>();
 for (Map.Entry<String, JsonElement> entry : json.getAsJsonObject().entrySet()) 
 Object value = null;
 if (entry.getValue().isJsonPrimitive()) 
 value = entry.getValue().getAsString();
 else if (entry.getValue().isJsonObject()) 
 value = context.deserialize(entry.getValue(), TYPE); // deserialize the object using the same type
 else if (entry.getValue().isJsonArray()) 
 // TODO implement deserailization of array
 else if (entry.getValue().isJsonNull()) 
 // skip nulls
 continue;
 
 map.put(entry.getKey().toUpperCase(), value); //toUpperCase() is what we want
 
 return map;
 

you can use the adapter then:

 String payload = ""key" : "key1" : "value1","key2" : "value2", "key3": "value"";
 Gson gson = new GsonBuilder()
 .registerTypeAdapter(UpperCaseAdapter.TYPE, new UpperCaseAdapter())
 .create();
 Map<String, Object> mapDeserialized = gson.fromJson(payload, UpperCaseAdapter.TYPE);

 System.out.println("Map " + mapDeserialized);

and the output is:
Map KEY3=value, KEY=KEY2=value2, KEY1=value1