Question

I’m just starting using the Jackson JSON library. Jackson is a very powerful library, but it has a terribly extensive API. A lot of things can be done in multiple ways. This makes it hard to find your way in Jackson – how to know what is the correct/best way of doing things?

Why would I use this solution:

String json = "{\"a\":2, \"b\":\"a string\", \"c\": [6.7, 6, 5.6, 8.0]}";
ObjectMapper mapper = new ObjectMapper();
JsonNode node = mapper.readValue(json, JsonNode.class);
if (node.isObject()) {
    ObjectNode obj = mapper.convertValue(node, ObjectNode.class);
    if (obj.has("a")) {
        System.out.println("a=" + obj.get("a").asDouble());
    }
}

Over a solution like this:

String json = "{\"a\":2, \"b\":\"a string\", \"c\": [6.7, 6, 5.6, 8.0]}";
ObjectMapper mapper = new ObjectMapper();
JsonNode node = mapper.readTree(json);
if (node.isObject()) {
    ObjectNode obj = (ObjectNode) node;
    if (obj.has("a")) {
        System.out.println("a=" + obj.get("a").asDouble());
    }
}     

Or over solutions that I came across using JsonFactory and JsonParser and maybe even more options…

It seems to mee that mapper.readValue is most generic and can be used in a lot of cases: read to JsonNode, ObjectNode, ArrayNode, PoJo, etc. So why would I want to use mapper.readTree?

And what is the best way to convert a JsonNode to an ObjectNode? Just cast to ObjectNode? Or use something like mapper.convertValue?

Answer 1

readValue() can be used for any and all types, including JsonNode. readTree() only works for JsonNode (tree model); and is added for convenience.

Note that you NEVER want to use your first example: it is equivalent to writing out your node as JSON, then reading it back — just cast it.