Flatten a JSON string to make the key including each level key value to Map using Gson or Jackson

Question

I have an enhanced question regarding Flatten a JSON string to Map using Gson or Jackson.

My scenario included duplicated keys, so the solution in the above questio

Answer 1

You can get the JSON as JsonNode and go through all fields recursively and add key and value field to a Map. When a value is an object instead of string you can add the field name to List to be joined with periods when a string is finally encountered. First create (for readability) a separate method that add Json fields to a Map:

void mapAppender(Map result, Entry node, List names) {
    names.add(node.getKey());
    if (node.getValue().isTextual()) {
        String name = names.stream().collect(joining("."));
        result.put(name, node.getValue().asText());
    } else {
        node.getValue().fields()
            .forEachRemaining(nested -> mapAppender(result, nested, new ArrayList<>(names)));
    }
}

and use it like this:

ObjectMapper mapper = new ObjectMapper();
Map result = new HashMap<>();
mapper.readTree(json).fields()
    .forEachRemaining(node -> mapAppender(result, node, new ArrayList()));

Where fields() returns an Iterator. Beware of StackOverflowErrors and perhaps low performance for deeply nested JSON.