Jackson - @JsonValue annotated field cannot be deserialized

Question

I have these two classes:

Foo.java

@Data
@NoArgsConstructor
@AllArgsConstructor
public class Foo {

    private ListWrapper data;

}

ListWrapper.java

@Data
@NoArgsConstructor
@AllArgsConstructor
public class ListWrapper {

    @JsonValue
    private List<String> value;

}

I am using the following method to test serialization:

public void testSerialization() throws JsonProcessingException {
    Foo foo = new Foo();
    foo.setData(new ListWrapper(new ArrayList<String>() {
        {
            add("foo");
            add("bar");
        }
    }));
    ObjectMapper mapper = new ObjectMapper();
    System.out.println(mapper.writeValueAsString(foo));
}

The output is {"data":["foo","bar"]}. Now I am trying to deserialize this json string with the following method:

public void testDeserialization() throws IOException {
    String json = "{\"data\":[\"foo\",\"bar\"]}";
    ObjectMapper mapper = new ObjectMapper();
    System.out.println(mapper.readValue(json, Foo.class));
}

But this does not work. I am getting the following exception:

com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot deserialize instance of `ListWrapper` out of START_ARRAY token
 at [Source: (String)"{"data":["foo","bar"]}"; line: 1, column: 9] (through reference chain: Foo["data"])

I have no clue why this is happening. Can anyone give me a hint? I am using Jackson 2.9.9.

Answer 1

The exception is thrown by Jackson object mapper as it is expecting Object type for data attribute, but Array was found instead.

@JsonValue annotation is used for marshalling. The analogous annotation for unmarshalling is @JsonCreator.

Annotate your ListWrapper constructor

@JsonCreator
public ListWrapper(List<String> value) {
    this.value = value;
}