Specifying `Enum` value serialization using `@JsonProperty`

[allenchen1154]

Currently, if I want to deserialize an enum with a value that isn't its Enum.name(), I can do either

public enum TestEnum {
    VALUE_ONE("value1");

    private String valueInJson;

    private TestEnum(String valueInJson) {
        this.valueInJson = valueInJson;
    }

    @JsonCreator
    public static TestEnum getEnumFromValue(String value) {
        for (TestEnum testEnum : values()) {
            if (testEnum.valueInJson.equals(value)) {
                return testEnum;
            }
        }
        throw new IllegalArgumentException();
    }
}

or, using DeserializationFeature.READ_ENUMS_USING_TO_STRING,

public enum TestEnum {
    VALUE_ONE("value1");

    private String valueInJson;

    private TestEnum(String valueInJson) {
        this.valueInJson = valueInJson;
    }

    @Override
    public String toString() {
        return valueInJson;
    }
}

This seems like a lot of boilerplate - is there a simpler way to do this, similar to how Gson handles it?

public enum TestEnum {
    @SerializedName("value1")
    VALUE_ONE
}

It's both more concise and handles both serialization and deserialization.

[cowtowncoder]

Interesting idea. No, currently there isn't, but I think we should be able to repurpose something.
For example, @JsonProperty could work; ideally I try to avoid addition of new single-use annotations.

[allenchen1154]

Agreed that repurposing @JsonProperty would be good, since it matches the annotation's description as

used to indicate external property name, name used in data format

[cowtowncoder]

Ugh. While a reasonable idea, I don't know how to implement this -- Java enums are a nasty kludge under the hood, so I don't quite know how to locate annotations that hide in there. It is doable, but not a quick fix as I hoped it would be.

FWIW, to solve it for specific app, trick is to override AnnotationIntrospector.findEnumValue() (of JacksonAnnotationIntrospector, usually), and can probably hard-code it for specific use.
It's only difficult to do in general way.

[allenchen1154]

Would something like this work?

@Override
public String findEnumValue(Enum<?> value) { 
    JsonProperty jsonPropertyAnnotation = value.getClass().getField(value.name()).getAnnotation(JsonProperty.class);
    if (jsonPropertyAnnotation != null && !jsonPropertyAnnotation.value().equals(JsonProperty.USE_DEFAULT_NAME)) {
        return jsonPropertyAnnotation.value();
    } else {
        return value.name();
    }
}

Source for reflection code: http://stackoverflow.com/a/7260009

[cowtowncoder]

Sounds like that could work. One issue is that accessing annotation directly does not allow processing of mix-in annotations; but trying to make that work would be significant hassle.

[cowtowncoder]

I think it just might be possible to use AnnotatedClass to get mix-ins too, without tons of trouble.
But I think a new introspection method (or override) is needed in AnnotationIntrospector, to not only pass Enum value but also associated accessor to give access to annotations.

[cowtowncoder]

On second thought: looking at how enums are actually implemented, values are static fields of the declaring class (of that type or sub-type as needed). That is fine as far as it goes, except that existing functionality for resolving annotations only deals with member fields; and of static things only static factory methods are considered. Defining mix-ins for enums might be bit inconenient as well.

So given all of this, I don't see an issue with going with @allenchen1154's suggestion.

[Raniz85]

This functionality should be documented somewhere in @JsonProperty, currently there's no information that it can be used for this purpose in there.

[cowtowncoder]

@Raniz85 appreciate your suggesting this (makes sense): could you please file a new issue for jackson-annotations since that's where javadocs are? Due to number of reports things sometimes fall through the cracks, especially comments in closed issues (we do go over open issues regularly). Thanks!

[Raniz85]

Done here

[cowtowncoder]

Thanks!