Why HttpClient Skips Null Parameters and How to Fix It in Java

I was testing an API with HttpClient and encountered an endpoint that returns system resources, each tagged with a version. The intended logic is: on the first request, send an empty version to get the latest data; on subsequent requests, send the stored version to check for updates. I wrote a method that saves the versions from the first response and reuses them on later calls.

During testing I discovered that the first request, where I passed null for the version fields, did not actually include those parameters in the JSON payload. Investigation revealed that JSONObject.put(key, null) silently removes the key, so the parameter is omitted from the request.

/**
 * Get static system resources
 *
 * First call stores each version, subsequent calls reuse them
 */
public JSONObject getResource() {
    JSONObject response = null;
    String url = urls.getString("static_config");
    JSONObject args = getParams(token);
    args.put("audio_country", "");
    args.put("audio_label", "");
    args.put("game_user_number", "");
    args.put("banner", "");
    args.put("video_open_time", "");
    args.put("tip", "");
    args.put("objective", objectiveVersion);
    args.put("startup_page", startupVersion);
    args.put("game", gameVersion);
    args.put("app", appVersion);
    args.put("gift", giftVersion);
    args.put("app_media_config", videoVersion);
    HttpGet httpGet = getHttpGet(url, args);
    output(args.size());
    response = getHttpResponseEntityByJson(httpGet);
    if (response.containsKey("dataInfo") && objectiveVersion.isEmpty()) {
        JSONObject dataInfo = response.getJSONObject("dataInfo");
        objectiveVersion = dataInfo.getJSONObject("objective").getString("version");
        startupVersion = dataInfo.getJSONObject("startup_page").getString("version");
        gameVersion = dataInfo.getJSONObject("game").getString("version");
        appVersion = dataInfo.getJSONObject("app").getString("version");
        giftVersion = dataInfo.getJSONObject("gift").getString("version");
        videoVersion = dataInfo.getJSONObject("video_open_time").getString("version");
    }
    return response;
}

The problem stemmed from the version fields being declared as String members with default value null. When null is passed to JSONObject.put, the key is dropped, so the server receives no version information and cannot return the expected data.

public String objectiveVersion = ""; // purpose page version
public String startupVersion = "";   // startup page version
public String gameVersion = "";      // game version
public String appVersion = "";       // app update config version
public String giftVersion = "";     // gift version
public String videoVersion = "";    // video version

Changing the initialization from null to an empty string ( "") ensures that the keys are present in the JSON payload, even if their values are empty, allowing the API to process the request correctly and return the required version data.