Converting Nested json into dot notation json

Question

I have a service from where I get a json string response like as shown below

{
  "id": "123",
  "name": "John"
}

I consume the rest call using HttpClient and converts the json string to Map<String, String> like as shown below.

String url= "http://www.mocky.io/v2/5979c2f5110000f4029edc93";
HttpClient client = HttpClientBuilder.create().build();
HttpGet httpGet = new HttpGet(url);
httpGet.setHeader("Content-Type", "application/json");
HttpResponse httpresponse = client.execute(httpGet);
String response = EntityUtils.toString(httpresponse.getEntity());

ObjectMapper mapper = new ObjectMapper();
Map<String, String> map = mapper.readValue(response, new TypeReference<Map<String, String>>(){});

The conversion from json string to HashMap is working fine, but actually my requirement was sometimes there can be some nested json within the main json, for example in the below json I am having an additional address key which is again a nested json having city and town details.

{
  "id": "123",
  "name": "John",
  "address": {
    "city": "Chennai",
    "town": "Guindy"
  }
}

If any nested json comes I need the make the json like as shown below

{
  "id": "123",
  "name": "John",
  "address.city": "Chennai",
  "address.town": "Guindy"
}

Currently I am using jackson library, but open to any other library which will give me this feature out of box

Can anyone help me by giving some suggestion on this.

Answer 1

Here is a recursive method that will flatten a nested Map with any depth to the desired dot notation. You can pass it to Jackson's ObjectMapper to get the desired json output:

@SuppressWarnings("unchecked")
public static Map<String, String> flatMap(String parentKey, Map<String, Object> nestedMap)
{
    Map<String, String> flatMap = new HashMap<>();
    String prefixKey = parentKey != null ? parentKey + "." : "";
    for (Map.Entry<String, Object> entry : nestedMap.entrySet()) {
        if (entry.getValue() instanceof String) {
            flatMap.put(prefixKey + entry.getKey(), (String)entry.getValue());
        }
        if (entry.getValue() instanceof Map) {
            flatMap.putAll(flatMap(prefixKey + entry.getKey(), (Map<String, Object>)entry.getValue()));
        }
    }
    return flatMap;
}

Usage:

mapper.writeValue(System.out, flatMap(null, nestedMap));