Jackson JsonNode to typed Collection

Question

What is the proper way to convert a Jackson JsonNode to a java collection?

If it were a json string I could use ObjectMapper.readValue(String, TypeReference) but for a JsonNode the only options are ObjectMapper.treeToValue(TreeNode, Class) which wouldn't give a typed collection, or ObjectMapper.convertValue(Object, JavaType) which feels wrong on account of its accepting any POJO for conversion.

Is there another "correct" way or is it one of these?

Toneless

Answer 1

Acquire an ObjectReader with ObjectMapper#readerFor(TypeReference) using a TypeReference describing the typed collection you want. Then use ObjectReader#readValue(JsonNode) to parse the JsonNode (presumably an ArrayNode).

For example, to get a List<String> out of a JSON array containing only JSON strings

ObjectMapper mapper = new ObjectMapper();
// example JsonNode
JsonNode arrayNode = mapper.createArrayNode().add("one").add("two");
// acquire reader for the right type
ObjectReader reader = mapper.readerFor(new TypeReference<List<String>>() {
});
// use it
List<String> list = reader.readValue(arrayNode);

Childbed

In older Jackson versions (2.5 and before), there is no readerFor() method. Use the reader() method instead. – Hanukkah

Answer 2

If an Iterator is more useful...

...you can also use the elements() method of ArrayNode. Example see below.

sample.json

{
    "first": [
        "Some string ...",
        "Some string ..."
    ],
    "second": [
        "Some string ..."
    ]
}

So, the List<String> is inside one of the JsonNodes.

Java

When you convert that inner node to an ArrayNode you can use the elements() method, which returns an Iterator of JsonNodes.

File file = new File("src/test/resources/sample.json");
ObjectMapper mapper = new ObjectMapper();
JsonNode jsonNode = mapper.readTree(file);
ArrayNode arrayNode = (ArrayNode) jsonNode.get("first");
Iterator<JsonNode> itr = arrayNode.elements();
// and to get the string from one of the elements, use for example...
itr.next().asText();

New to Jackson Object Mapper?

I like this tutorial: https://www.baeldung.com/jackson-object-mapper-tutorial

Update:

You can also use .iterator() method of ArrayNode. It is the same:

Same as calling .elements(); implemented so that convenience "for-each" loop can be used for looping over elements of JSON Array constructs.

from the javadocs of com.fasterxml.jackson.core:jackson-databind:2.11.0

Crescent

is there any way to one-line the iteration of the array into a List<>? I m using a for loop to iterate over the JsonNode/ArrayNode and adding it to a list, but I was hoping for something that I could just ArrayNode.asList() or something – Psychognosis

A two-line solution is described in the answer above this one: https://mcmap.net/q/395177/-jackson-jsonnode-to-typed-collection You first need a reader, then you can read into your List. ObjectReader reader = mapper.readerFor(new TypeReference<List<String>>() { }); List<String> list = reader.readValue(arrayNode); – Crescent

Answer 3

The ObjectMapper.convertValue() function is convenient and type-aware. It can perform a wide array of conversions between tree nodes and Java types/collections, and vice-versa.

An example of how you might use it:

List<String> list = new ArrayList<>();
list.add("one");
list.add("two");

Map<String,List<String>> hashMap = new HashMap<>();
hashMap.put("myList", list);

ObjectMapper mapper = new ObjectMapper();
ObjectNode objectNode = mapper.convertValue(hashMap, ObjectNode.class);
Map<String,List<String>> hashMap2 = mapper.convertValue(objectNode, new TypeReference<Map<String, List<String>>>() {});

Strongbox