Merge/Extend JSON Objects using Gson in Java

Question

Oftentimes, I have a need to merge two JSON Objects (similar to the way jQuery's $.extend() works). However, the Gson library has no built in functionality and they have said they won't implement it.

Doing something like:

private void merge(JsonObject firstObj, JsonObject secondObj){
    for(String keyInSecondObj : secondObj.entrySet().keySet()) {
      if(!firstObj.has(keyInSecondObj )){
        firstObj.add(secondMap.get(keyInSecondObj));
    }
}

Is too simple because it doesn't handle recursively merging JsonObjects, doesn't handle conflicts when the key exists in both maps, and has no special handling for non-primitive values such as Arrays.

I failed to find any pre-built solutions to do this. I would prefer to use something that has been thoroughly tested instead of writing my own method, but it must be Gson (not Jackson or other).

Edit: I ended up writing my own implementation as have added as an answer to this question

This question is not a duplicate because it's not using Gson (or Java for that matter).

Answer 1

Here's my first attempt at writing my own static merge method. Feel free to poke holes in it.

import com.google.gson.JsonArray;
import com.google.gson.JsonElement;
import com.google.gson.JsonObject;
import java.util.Map;

public class GsonTools {

    public static enum ConflictStrategy {

        THROW_EXCEPTION, PREFER_FIRST_OBJ, PREFER_SECOND_OBJ, PREFER_NON_NULL;
    }

    public static class JsonObjectExtensionConflictException extends Exception {

        public JsonObjectExtensionConflictException(String message) {
            super(message);
        }

    }

    public static void extendJsonObject(JsonObject destinationObject, ConflictStrategy conflictResolutionStrategy, JsonObject ... objs) 
            throws JsonObjectExtensionConflictException {
        for (JsonObject obj : objs) {
            extendJsonObject(destinationObject, obj, conflictResolutionStrategy);
        }
    }

    private static void extendJsonObject(JsonObject leftObj, JsonObject rightObj, ConflictStrategy conflictStrategy) 
            throws JsonObjectExtensionConflictException {
        for (Map.Entry<String, JsonElement> rightEntry : rightObj.entrySet()) {
            String rightKey = rightEntry.getKey();
            JsonElement rightVal = rightEntry.getValue();
            if (leftObj.has(rightKey)) {
                //conflict                
                JsonElement leftVal = leftObj.get(rightKey);
                if (leftVal.isJsonArray() && rightVal.isJsonArray()) {
                    JsonArray leftArr = leftVal.getAsJsonArray();
                    JsonArray rightArr = rightVal.getAsJsonArray();
                    //concat the arrays -- there cannot be a conflict in an array, it's just a collection of stuff
                    for (int i = 0; i < rightArr.size(); i++) {
                        leftArr.add(rightArr.get(i));
                    }
                } else if (leftVal.isJsonObject() && rightVal.isJsonObject()) {
                    //recursive merging
                    extendJsonObject(leftVal.getAsJsonObject(), rightVal.getAsJsonObject(), conflictStrategy);
                } else {//not both arrays or objects, normal merge with conflict resolution
                    handleMergeConflict(rightKey, leftObj, leftVal, rightVal, conflictStrategy);
                }
            } else {//no conflict, add to the object
                leftObj.add(rightKey, rightVal);
            }
        }
    }

    private static void handleMergeConflict(String key, JsonObject leftObj, JsonElement leftVal, JsonElement rightVal, ConflictStrategy conflictStrategy) 
            throws JsonObjectExtensionConflictException {
        {
            switch (conflictStrategy) {
                case PREFER_FIRST_OBJ:
                    break;//do nothing, the right val gets thrown out
                case PREFER_SECOND_OBJ:
                    leftObj.add(key, rightVal);//right side auto-wins, replace left val with its val
                    break;
                case PREFER_NON_NULL:
                    //check if right side is not null, and left side is null, in which case we use the right val
                    if (leftVal.isJsonNull() && !rightVal.isJsonNull()) {
                        leftObj.add(key, rightVal);
                    }//else do nothing since either the left value is non-null or the right value is null
                    break;
                case THROW_EXCEPTION:
                    throw new JsonObjectExtensionConflictException("Key " + key + " exists in both objects and the conflict resolution strategy is " + conflictStrategy);
                default:
                    throw new UnsupportedOperationException("The conflict strategy " + conflictStrategy + " is unknown and cannot be processed");
            }
        }
    }
}

Answer 2

You can use

  Map firstObject = new GSON().fromJson(json1, HashMap.class);
  Map secondObject = new GSON().fromJson(json2, HashMap.class);

// merge Map firstObject and secondObject as you want, see this post

  String resultJson = new GSON().toJson(resultMap);