Map JSON string array to List<String> using Jackson

Question

I have following JSON returned from server.

 String json = {
    "values": ["name","city","dob","zip"]
 };

I want to use ObjectMapper to return the List<String> values. Something like:

List<String> response = mapper.readValue(json, List.class)

I have tried several ways but none of them worked. Any help is appreciated.

---

Edit: I don't want additional wrapper objects. I want to straight away get the List<String> out.

Answer 1

The TypeFactory in Jackson allows to map directly to collections and other complex types:

String json = "[ \"abc\", \"def\" ]";
ObjectMapper mapper = new ObjectMapper();

List<String> list = mapper.readValue(json, TypeFactory.defaultInstance().constructCollectionType(List.class, String.class));

Answer 2

You could define a wrapper class as following:

public class Wrapper {

    private List<String> values;

    // Default constructor, getters and setters omitted
}

Then use:

Wrapper wrapper = mapper.readValue(json, Wrapper.class);
List<String> values = wrapper.getValues();

---

If you want to avoid a wrapper class, try the following:

JsonNode valuesNode = mapper.readTree(json).get("values");

List<String> values = new ArrayList<>();
for (JsonNode node : valuesNode) {
    values.add(node.asText());
}

Answer 3

There is another way you might be interested in, partly similar to accepted answer but can be written as one-liner (line breaks should help with understanding).

Input JSON:

{
    "values": ["name", "city", "dob", "zip"]
}

Code snippet:

String json = "{\"values\":[\"name\",\"city\",\"dob\",\"zip\"]}";
ObjectMapper mapper = new ObjectMapper();

List<String> list = Arrays.asList(
    mapper.convertValue(
        mapper.readTree(json).get("values"),
        String[].class
    )
);

list.forEach(System.out::println);

This code snippet outputs the following:

name
city
dob
zip

Please note that Arrays.asList() returns a list of fixed size because it is backed by a given array. To get a resizable list just wrap it like that:

List<String> resizableList = new ArrayList<>(
    Arrays.asList(new String[] {"a", "b", "c"})
);

---

Of course this solution can be adapted to more complex cases, not just Strings.

For example, for a given POJO User:

class User {
    private int id;
    private String name;

    public int getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    @Override
    public String toString() {
        return String.format(
            "[User = {id: %d, name: \"%s\"}]",
            id,
            name
        );
    }
}

and input JSON:

{
    "values": [{
            "id": 1,
            "name": "Alice"
        }, {
            "id": 2,
            "name": "Bob"
        }
    ]
}

following code snippet:

ObjectMapper mapper = new ObjectMapper();
String json = "{\"values\":[{\"id\":1,\"name\":\"Alice\"},{\"id\":2,\"name\":\"Bob\"}]}";

List<User> list = Arrays.asList(
    mapper.convertValue(
        mapper.readTree(json).get("values"),
        User[].class
    )
);

list.forEach(System.out::println);

yelds the following output:

[User = {id: 1, name: "Alice"}]
[User = {id: 2, name: "Bob"}]