How to efficiently map a org.json.JSONObject to a POJO?

Question

This question must have been asked before, but I couldn't find it.

I'm using a 3rd party library to retrieve data in JSON format. The library offers the data to me as a org.json.JSONObject. I want to map this JSONObject to a POJO (Plain Old Java Object) for simpler access/code.

For mapping, I currently use the ObjectMapper from the Jackson library in this way:

JSONObject jsonObject = //...
ObjectMapper mapper = new ObjectMapper();
MyPojoClass myPojo = mapper.readValue(jsonObject.toString(), MyPojoClass.class);

To my understanding, the above code can be optimized significantly, because currently the data in the JSONObject, which is already parsed, is again fed into a serialization-deserialization chain with the JSONObject.toString() method and then to the ObjectMapper.

I want to avoid these two conversions (toString() and parsing). Is there a way to use the JSONObject to map its data directly to a POJO?

Answer 1

Since you have an abstract representation of some JSON data (an org.json.JSONObject object) and you're planning to use the Jackson library - that has its own abstract representation of JSON data (com.fasterxml.jackson.databind.JsonNode) - then a conversion from one representation to the other would save you from the parse-serialize-parse process. So, instead of using the readValue method that accepts a String, you'd use this version that accepts a JsonParser:

JSONObject jsonObject = //...
JsonNode jsonNode = convertJsonFormat(jsonObject);
ObjectMapper mapper = new ObjectMapper();
MyPojoClass myPojo = mapper.readValue(new TreeTraversingParser(jsonNode), MyPojoClass.class);

JSON is a very simple format, so it should not be hard to create the convertJsonFormat by hand. Here's my attempt:

static JsonNode convertJsonFormat(JSONObject json) {
    ObjectNode ret = JsonNodeFactory.instance.objectNode();

    @SuppressWarnings("unchecked")
    Iterator<String> iterator = json.keys();
    for (; iterator.hasNext();) {
        String key = iterator.next();
        Object value;
        try {
            value = json.get(key);
        } catch (JSONException e) {
            throw new RuntimeException(e);
        }
        if (json.isNull(key))
            ret.putNull(key);
        else if (value instanceof String)
            ret.put(key, (String) value);
        else if (value instanceof Integer)
            ret.put(key, (Integer) value);
        else if (value instanceof Long)
            ret.put(key, (Long) value);
        else if (value instanceof Double)
            ret.put(key, (Double) value);
        else if (value instanceof Boolean)
            ret.put(key, (Boolean) value);
        else if (value instanceof JSONObject)
            ret.put(key, convertJsonFormat((JSONObject) value));
        else if (value instanceof JSONArray)
            ret.put(key, convertJsonFormat((JSONArray) value));
        else
            throw new RuntimeException("not prepared for converting instance of class " + value.getClass());
    }
    return ret;
}

static JsonNode convertJsonFormat(JSONArray json) {
    ArrayNode ret = JsonNodeFactory.instance.arrayNode();
    for (int i = 0; i < json.length(); i++) {
        Object value;
        try {
            value = json.get(i);
        } catch (JSONException e) {
            throw new RuntimeException(e);
        }
        if (json.isNull(i))
            ret.addNull();
        else if (value instanceof String)
            ret.add((String) value);
        else if (value instanceof Integer)
            ret.add((Integer) value);
        else if (value instanceof Long)
            ret.add((Long) value);
        else if (value instanceof Double)
            ret.add((Double) value);
        else if (value instanceof Boolean)
            ret.add((Boolean) value);
        else if (value instanceof JSONObject)
            ret.add(convertJsonFormat((JSONObject) value));
        else if (value instanceof JSONArray)
            ret.add(convertJsonFormat((JSONArray) value));
        else
            throw new RuntimeException("not prepared for converting instance of class " + value.getClass());
    }
    return ret;
}

Note that, while the Jackson's JsonNode can represent some extra types (such as BigInteger, Decimal, etc) they are not necessary since the code above covers everything that JSONObject can represent.