How to compare JSON documents and return the differences with Jackson or Gson?

Reading the JSON documents as Maps and comparing them

You could read both JSON documents as Map<K, V>. See the below examples for Jackson and Gson:

ObjectMapper mapper = new ObjectMapper();
TypeReference<HashMap<String, Object>> type = 
    new TypeReference<HashMap<String, Object>>() {};

Map<String, Object> leftMap = mapper.readValue(leftJson, type);
Map<String, Object> rightMap = mapper.readValue(rightJson, type);

Gson gson = new Gson();
Type type = new TypeToken<Map<String, Object>>(){}.getType();

Map<String, Object> leftMap = gson.fromJson(leftJson, type);
Map<String, Object> rightMap = gson.fromJson(rightJson, type);

Then use Guava’s Maps.difference(Map<K, V>, Map<K, V>) to compare them. It returns a MapDifference<K, V> instance:

MapDifference<String, Object> difference = Maps.difference(leftMap, rightMap);

If you are not happy with the result, you can consider flattening the maps and then compare them. It will provide better comparison results especially for nested objects and arrays.

Creating flat Maps for the comparison

To flat the map, you can use:

public final class FlatMapUtil {

    private FlatMapUtil() {
        throw new AssertionError("No instances for you!");
    }

    public static Map<String, Object> flatten(Map<String, Object> map) {
        return map.entrySet().stream()
                .flatMap(FlatMapUtil::flatten)
                .collect(LinkedHashMap::new, (m, e) -> m.put("/" + e.getKey(), e.getValue()), LinkedHashMap::putAll);
    }

    private static Stream<Map.Entry<String, Object>> flatten(Map.Entry<String, Object> entry) {

        if (entry == null) {
            return Stream.empty();
        }

        if (entry.getValue() instanceof Map<?, ?>) {
            return ((Map<?, ?>) entry.getValue()).entrySet().stream()
                    .flatMap(e -> flatten(new AbstractMap.SimpleEntry<>(entry.getKey() + "/" + e.getKey(), e.getValue())));
        }

        if (entry.getValue() instanceof List<?>) {
            List<?> list = (List<?>) entry.getValue();
            return IntStream.range(0, list.size())
                    .mapToObj(i -> new AbstractMap.SimpleEntry<String, Object>(entry.getKey() + "/" + i, list.get(i)))
                    .flatMap(FlatMapUtil::flatten);
        }

        return Stream.of(entry);
    }
}

It uses the JSON Pointer notation defined in the RFC 6901 for the keys, so you can easily locate the values.

Example

Consider the following JSON documents:

{
  "name": {
    "first": "John",
    "last": "Doe"
  },
  "address": null,
  "birthday": "1980-01-01",
  "company": "Acme",
  "occupation": "Software engineer",
  "phones": [
    {
      "number": "000000000",
      "type": "home"
    },
    {
      "number": "999999999",
      "type": "mobile"
    }
  ]
}

{
  "name": {
    "first": "Jane",
    "last": "Doe",
    "nickname": "Jenny"
  },
  "birthday": "1990-01-01",
  "occupation": null,
  "phones": [
    {
      "number": "111111111",
      "type": "mobile"
    }
  ],
  "favorite": true,
  "groups": [
    "close-friends",
    "gym"
  ]
}

And the following code to compare them and show the differences:

Map<String, Object> leftFlatMap = FlatMapUtil.flatten(leftMap);
Map<String, Object> rightFlatMap = FlatMapUtil.flatten(rightMap);

MapDifference<String, Object> difference = Maps.difference(leftFlatMap, rightFlatMap);

System.out.println("Entries only on the left\n--------------------------");
difference.entriesOnlyOnLeft()
          .forEach((key, value) -> System.out.println(key + ": " + value));

System.out.println("\n\nEntries only on the right\n--------------------------");
difference.entriesOnlyOnRight()
          .forEach((key, value) -> System.out.println(key + ": " + value));

System.out.println("\n\nEntries differing\n--------------------------");
difference.entriesDiffering()
          .forEach((key, value) -> System.out.println(key + ": " + value));

It will produce the following output:

Entries only on the left
--------------------------
/address: null
/phones/1/number: 999999999
/phones/1/type: mobile
/company: Acme


Entries only on the right
--------------------------
/name/nickname: Jenny
/groups/0: close-friends
/groups/1: gym
/favorite: true


Entries differing
--------------------------
/birthday: (1980-01-01, 1990-01-01)
/occupation: (Software engineer, null)
/name/first: (John, Jane)
/phones/0/number: (000000000, 111111111)
/phones/0/type: (home, mobile)