How to iterate all subnodes of a json object?

Question

I want to iterate through all nodes of a json object, and write out a plain key-value map, as follows:

{
    "name": [{
        "first": "John",
        "last": "Doe",
        "items": [{
                "name": "firstitem",
                "stock": 12
            },
            {
                "name": "2nditem",
                "stock": 23
            }
        ]
    }],
    "company": "John Company"
}

Should result in:

name-first-1=John
name-last-1=Doe
name-items-name-1-1=firstitem (meaning the list index is always appended at the end of the name)
name-items-name-1-2=2nditem
company=John Company

This is how to get the json string as a json object:

ObjectMapper mapper = new ObjectMapper(); //using jackson
JsonNode root = mapper.readTree(json);
//TODO how loop all nodes and subnodes, and always get their key + value?

But how can I now iterate through all nodes and extract their key and content?

Possible duplicate of [How to deserialize JSON into flat, Map-like structure?](https://stackoverflow.com/questions/20355261/how-to-deserialize-json-into-flat-map-like-structure) — membersound, Feb 06 '18 at 14:29

Daniel Taub · Accepted Answer · 2018-02-26T23:10:34.187

This will work for you :

ObjectMapper mapper = new ObjectMapper();
JsonNode root = mapper.readTree(json);
Map<String, String> map = new HashMap<>();
addKeys("", root, map, new ArrayList<>());

map.entrySet()
     .forEach(System.out::println);

private void addKeys(String currentPath, JsonNode jsonNode, Map<String, String> map, List<Integer> suffix) {
    if (jsonNode.isObject()) {
        ObjectNode objectNode = (ObjectNode) jsonNode;
        Iterator<Map.Entry<String, JsonNode>> iter = objectNode.fields();
        String pathPrefix = currentPath.isEmpty() ? "" : currentPath + "-";

        while (iter.hasNext()) {
            Map.Entry<String, JsonNode> entry = iter.next();
            addKeys(pathPrefix + entry.getKey(), entry.getValue(), map, suffix);
        }
    } else if (jsonNode.isArray()) {
        ArrayNode arrayNode = (ArrayNode) jsonNode;

        for (int i = 0; i < arrayNode.size(); i++) {
            suffix.add(i + 1);
            addKeys(currentPath, arrayNode.get(i), map, suffix);

            if (i + 1 <arrayNode.size()){
                suffix.remove(arrayNode.size() - 1);
            }
        }

    } else if (jsonNode.isValueNode()) {
        if (currentPath.contains("-")) {
            for (int i = 0; i < suffix.size(); i++) {
                currentPath += "-" + suffix.get(i);
            }

            suffix = new ArrayList<>();
        }

        ValueNode valueNode = (ValueNode) jsonNode;
        map.put(currentPath, valueNode.asText());
    }
}

For the json you gave the output will be :

name-items-name-1-2=2nditem
name-items-name-1-1=firstitem
name-items-stock-1-1=12
name-first-1=John
name-items-stock-1-2=23
company=John Company
name-last-1=Doe

Thomas Weller · Answer 2 · 2020-10-30T08:34:26.157

23

elements() gives you an iterator for subnodes and fields() gives you the properties.

With that, you can code a recursive function that walks through all nodes.

edited Oct 30 '20 at 08:34

answered Feb 06 '18 at 11:57

Thomas Weller

55,411
20
125
222

7

... and you just need to code the traverser to be recursive. – Stephen C Feb 06 '18 at 12:46
What's difference between `elements()` versus `iterator()` ? – daparic Oct 30 '20 at 05:52
1

@eigenfield: [`iterator()`](http://fasterxml.github.io/jackson-databind/javadoc/2.2/com/fasterxml/jackson/databind/JsonNode.html#iterator()) sounds pretty similar. Maybe even better suited. I have not tried that. – Thomas Weller Oct 30 '20 at 08:35

score 10 · Answer 3 · answered Feb 06 '18 at 14:48

Here is working sample, input is String

public static void main(String[] args) throws IOException {
    JsonNode node = om.readTree(input);
    LOG.info(node.toString());
    process("", node);
}

private static void process(String prefix, JsonNode currentNode) {
    if (currentNode.isArray()) {
        ArrayNode arrayNode = (ArrayNode) currentNode;
        Iterator<JsonNode> node = arrayNode.elements();
        int index = 1;
        while (node.hasNext()) {
            process(!prefix.isEmpty() ? prefix + "-" + index : String.valueOf(index), node.next());
            index += 1;
        }
    }
    else if (currentNode.isObject()) {
        currentNode.fields().forEachRemaining(entry -> process(!prefix.isEmpty() ? prefix + "-" + entry.getKey() : entry.getKey(), entry.getValue()));
    }
    else {
        LOG.info(prefix + ": " + currentNode.toString());
    }
}

Nice. But for nested lists, that won't put all lists' indexes at the end of the key? (see my example were name + items are two nested lists, and both indexes should appear at the end). — membersound, Feb 06 '18 at 14:53
well, this code just shows a sample how recursively iterate over json node. You can change prefix forming algorithm. — Boris Chistov, Feb 06 '18 at 14:58

score 4 · Answer 4 · answered Feb 06 '18 at 12:17

4

You can convert JSON object to HashMap so you will get key and value pairs

here i use GSON library

code snippet

Gson gson = new Gson();
Type type = new TypeToken<Map<String, String>>(){}.getType();
Map<String,String> map = gson.fromJson(json, type);

so you can iterate this map for your purpose.

answered Feb 06 '18 at 12:17

Dhiraj Pandit

87
5

4

Well but how are nested maps handled, if the result is a `Map`? – membersound Feb 06 '18 at 12:20

How to iterate all subnodes of a json object?

4 Answers4

Linked