How to create generic method for ObjectNode put-CodePudding

I have a use case where I need to support multiple types of values to be inserted into the ObjectNode (of Jackson ObjectMapper), however when I try to use generics it has errors Cannot resolve method 'put(java.lang.String, T)'

    public static <T> JsonNode of(String key, T value) {
        ObjectMapper objectMapper = new ObjectMapper();
        ObjectNode root = objectMapper.createObjectNode();
        root.put(key, value); // <== error shows up on this line


        return root;
}

The underlying ObjectNode does support many types for the value, e.g. <String, Boolean>, <String, String>, <String, Integer>, etc.

For full documentation Jackson

What's the correct way to do this? Thanks.

CodePudding user response：

The problem is that through type erasure, the compiler will produce the method below for your generic method.

    public static JsonNode of(String key, Object value) {
        ObjectMapper objectMapper = new ObjectMapper();
        ObjectNode root = objectMapper.createObjectNode();
        root.put(key, value); // <== error shows up on this line
        return root;
}

Per Jackson documentation, there's no such method put(String key, Object value).

Instead of calling the method of(String key, T value), I would just do the following:

  ObjectNode root = new ObjectMapper().createObjectNode();
  root.put(key, value);

Or you could write write several overloaded methods of(...).

CodePudding user response：

There is another approach that works for any json object:

Map<String, Object> map = new ObjectMapper().readValue(json, new TypeReference<HashMap<String,Object>>() {});

The object in the value can be any value object (String, Integer, etc), another Map<String, Object> as a nested object or a List<Object>/List<Map<String, Object>> and so on down to any depth.

CodePudding user response：

If you look at the signature of the ObjectNode#put method you will see that

public JsonNode put(String fieldName, JsonNode value)

the second argument is actually bound to a specific type and it is JsonNode , not just any T (much like your method allows).

So, because of the type erasure - the compiler has to ensure you provided a compatible type at compile time and I suggest you to put a suitable bound on the method, for example:

public static <T extends JsonNode> JsonNode of(String key, T value) {
    ObjectMapper objectMapper = new ObjectMapper();
    ObjectNode root = objectMapper.createObjectNode();
    root.set(key, value);

    return root;
}

making sure your method accepts only JsonNode or its descendants.

Small note 1: it is not a good thing to instantiate ObjectMapper every time the method gets called as it is a bit expensive object to create, better to make it a field.

Small note 2: The method ObjectNode#put gets deprecated since version 2.4, and it is recommended to use ObjectNode#set instead.