Flattening multidimensional JSON in JAVA

I have a JSON which looks like this,

{
  "users": [
    {
      "displayName": "Sharad Dutta",
      "givenName": "",
      "surname": "",
      "extension_user_type": "user",
      "identities": [
        {
          "signInType": "emailAddress",
          "issuerAssignedId": "kkr007@gmail.com"
        }
      ],
      "extension_timezone": "VET",
      "extension_locale": "en-GB",
      "extension_tenant": "EG12345"
    },   
    {
      "displayName": "Sharad Dutta",
      "givenName": "",
      "surname": "",
      "extension_user_type": "user",
      "identities": [
        {
          "signInType": "emailAddress",
          "issuerAssignedId": "kkr007@gmail.com"
        }
      ],
      "extension_timezone": "VET",
      "extension_locale": "en-GB",
      "extension_tenant": "EG12345"
    }
  ]
}

I am writing a Java code where I am parsing this Multidimensional JSON to FLAT JSON. If you look closely, the JSON is wrapped in a "users" wrapper and then has couple of users as objects. For each user, there is a field called "identifiers" which is again a wrapper.

I want to flat this JSON, I have written a code but it is leaving a JSON blob for identifiers, for non nested it is working fine

JSONObject output;
        try {
            output = new JSONObject(userJsonAsString);
            JSONArray docs = output.getJSONArray("users");
            System.out.println(docs);

This is giving me below output, however i still have to flat inner wrapper "identifiers"

[
  {
    "extension_timezone": "VET",
    "extension_tenant": "EG12345",
    "extension_locale": "en-GB",
    "identities": [
      {
        "signInType": "emailAddress",
        "issuerAssignedId": "pdhongade007@gmail.com"
      }
    ],
    "displayName": "Sharad Dutta",
    "surname": "",
    "givenName": "",
    "extension_user_type": "user"
  },
  {
    "extension_timezone": "VET",
    "extension_tenant": "EG12345",
    "extension_locale": "en-GB",
    "identities": [
      {
        "signInType": "userName",
        "issuerAssignedId": "pdhongade007"
      }
    ],
    "displayName": "Wayne Rooney",
    "surname": "Rooney",
    "givenName": "Wayne",
    "extension_user_type": "user"
  }
]

This is what I need,

{
    "extension_timezone": "VET",
    "extension_tenant": "EG12345",
    "extension_locale": "en-GB",
    "signInType": "emailAddress",
    "issuerAssignedId": "pdhongade007@gmail.com",
    "displayName": "Sharad Dutta",
    "surname": "",
    "givenName": "",
    "extension_user_type": "user"
  }

I have to then parse this to CSV, which I know how, I just need to flat this further. Any help will be appreciated. I tried looking around, but a lot of them were using external dependencies.

//UPDATE

{
  "extension_timezone": "VET",
  "extension_tenant": "EG12345",
  "extension_locale": "en-GB",
  "signInType": "userName",
  "displayName": "Wayne Rooney",
  "surname": "Rooney",
  "givenName": "Wayne",
  "issuerAssignedId": "pdhongade007",
  "extension_user_type": "user"
}

When I tried @Jakub solution, I am getting the flat JSON, however it is not iterating for all the users. Just one!(I am guessing the last user only)

You can traverse the json and just store the "leafs" in map.
Note: array of primitives will turn into the last value in the array with this approach, but that's what you described

Something like this:

void flatJson() throws JSONException {
    JSONObject object = new JSONObject(userJsonAsString); // this is your input
    Map<String, Object> flatKeyValue = new HashMap<>();
    readValues(object, flatKeyValue);
    System.out.println(new JSONObject(flatKeyValue)); // this is flat
}

void readValues(JSONObject object, Map<String, Object> json) throws JSONException {
    for (Iterator it = object.keys(); it.hasNext(); ) {
        String key = (String) it.next();
        Object next = object.get(key);
        readValue(json, key, next);
    }
}

void readValue(Map<String, Object> json, String key, Object next) throws JSONException {
    if (next instanceof JSONArray) {
        JSONArray array = (JSONArray) next;
        for (int i = 0; i < array.length(); ++i) {
            readValue(json, key, array.opt(i));
        }
    } else if (next instanceof JSONObject) {
        readValues((JSONObject) next, json);
    } else {
        json.put(key, next);
    }
}

Please try the below approach, this will give you a comma separated format for both user and identifier (flat file per se),

 public static void main(String[] args) throws JSONException, ParseException {
String userJsonFile = "path to your JSON";
final StringBuilder sBuild = new StringBuilder();
final StringBuilder sBuild2 = new StringBuilder();
try {
String userJsonAsString = convert your JSON to string and store in var;
} catch (Exception e1) {
e1.printStackTrace();
}
JSONParser jsonParser = new JSONParser();
JSONObject output = (JSONObject) jsonParser.parse(userJsonAsString);
try {
JSONArray docs = (JSONArray) output.get("users");
Iterator<Object> iterator = docs.iterator();
while (iterator.hasNext()) {
JSONObject userEleObj = (JSONObject)iterator.next();
JSONArray nestedIdArray = (JSONArray) userEleObj.get("identities");
Iterator<Object> nestIter = nestedIdArray.iterator();
while (nestIter.hasNext()) {
JSONObject identityEleObj = (JSONObject)nestIter.next(); 
identityEleObj.keySet().stream().forEach(key -> sBuild2.append(identityEleObj.get(key) + ","));
userEleObj.keySet().stream().forEach(key -> {
if (StringUtils.equals((CharSequence) key, "identities")) {
sBuild.append(sBuild2.toString());
sBuild2.replace(0, sBuild2.length(), "");
} else {
sBuild.append(userEleObj.get(key) + ","); 
}
});
}
sBuild.replace(sBuild.lastIndexOf(","), sBuild.length(), "\n");  
}
System.out.println(sBuild);
} catch (Exception e) {
e.printStackTrace();
}
}