Jackson - Deserialize Interface to enum

Question

I have an interface Event and multiple enums that implement this interface (UserEvent, BusinessEvent, etc).

I want to deserialize the following json data:

{
  "event" : "SIGNUP"
}

To this bean:

public class Input
{ 
   private Event event;

   public Event getEvent() {..}
   public void setEvent(Event event) {..}
}

public enum UserEvent implements Event
{
    SIGNUP;
}

Here, i'd like event to be deserialized to UserEvent.SIGNUP.

How can I accomplish this? Reading up on @JsonTypeInfo seems to indicate that an additional type attribute would be needed, but in this case, there's just one string which maps directly to an enum value.

Are all of implementations of `Event` known at build time or they can change during runtime? — rkosegi, Jan 20 '17 at 14:17
What if two enum classes define a constant with the same name? — Maurice Perry, Jan 20 '17 at 14:19

Vlad Bochenin · Accepted Answer · 2021-04-01T14:48:24.380

You are using Event interface for field event in Input and jackson doesn't know anything about UserEvent as implementation of this interface.

You can use custom JsonDeserializer to get value:

public interface Event {
}

public static class Input
{
    private Event event;

    @JsonDeserialize(using = EventDeserializer.class)
    public Event getEvent() {
        return event;
    }

    public void setEvent(Event event) {
        this.event = event;
    }
}

public enum UserEvent implements Event
{
    SIGNUP;
}

public static class EventDeserializer  extends StdDeserializer<Event> {

    protected EventDeserializer() {
        super(Event.class);
    }

    @Override
    public Event deserialize(JsonParser p, DeserializationContext ctxt) throws IOException, JsonProcessingException {
        return UserEvent.valueOf(p.getText());
    }
}

@Test
public void converEnum() throws Exception {
    ObjectMapper objectMapper = new ObjectMapper();
    Input input = objectMapper.readValue("{\n" +
            "  \"event\" : \"SIGNUP\"\n" +
            "}", Input.class);

    Assert.assertThat(input.getEvent(), Matchers.is(UserEvent.SIGNUP));
}

Does this solution back you into the corner of only being able to use a single concrete implementation for the `Event` field? Since you annotate the getter for the `Event` field with the `UserEventDeserializer`, what happens if the concrete value in the Event field is a `BusinessEvent`? — mfunaro, Mar 31 '21 at 17:50
@mfunaro I don't think so. Even if example is just for UserEvent nothing will stop you to write more sofisticated code with possible enums values in 'deserilize' method. — Vlad Bochenin, Apr 01 '21 at 14:47

score 2 · Answer 2 · answered Jan 20 '17 at 14:24

2

For a single enum the jsonschema2pojo-maven-plugin generates code like the following:

@JsonCreator
public static Foo fromValue(String value) {
    Foo constant = CONSTANTS.get(value);
    if (constant == null) {
        throw new IllegalArgumentException(value);
    } else {
        return constant;
    }
}

I guess you could write a factory method which is annotated with @JsonCreator and somehow decides which enum to choose. I'm not sure if it matters much where you put this method.

answered Jan 20 '17 at 14:24

Puce

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Where would I specify this method - on the interface itself? – Ali Jan 20 '17 at 14:24
As I said, I don't know if it matters much. If you use Java SE 8 (recommended) then the interface might be an option. – Puce Jan 20 '17 at 14:26
I'm on Java 8. But defining it on the interface isn't working - doesn't seem to be getting called – Ali Jan 20 '17 at 14:29
Using jackson 2.7.6 – Ali Jan 20 '17 at 14:30

score 2 · Answer 3 · answered Nov 15 '20 at 21:10

Having a similar task I ended up creating a custom deserializer:

public class IFooDeserializer extends StdDeserializer<IFoo> {

    public IFooDeserializer() {
        this(null);
    }

    public IFooDeserializer(Class<?> vc) {
        super(vc);
    }

    @Override
    public IFoo deserialize(JsonParser p, DeserializationContext ctxt) throws IOException, JsonProcessingException {
        String value = p.getCodec().readValue(p, String.class);
        return Stream.of(GoodFoo.values(), BadFoo.values())
                .flatMap(Stream::of)
                .filter(el -> el.name().equals(value))
                .findAny()
                .orElseThrow(() -> new RuntimeException("Could not deserialize foo: " + value));
    }
}

Jackson - Deserialize Interface to enum

3 Answers3