Jackson enum Serializing and DeSerializer

Question

The serializer / deserializer solution pointed out by @xbakesx is an excellent one if you wish to completely decouple your enum class from its JSON representation.

Alternatively, if you prefer a self-contained solution, an implementation based on @JsonCreator and @JsonValue annotations would be more convenient.

So leveraging on the example by @Stanley the following is a complete self-contained solution (Java 6, Jackson 1.9):

public enum DeviceScheduleFormat {

    Weekday,
    EvenOdd,
    Interval;

    private static Map<String, DeviceScheduleFormat> namesMap = new HashMap<String, DeviceScheduleFormat>(3);

    static {
        namesMap.put("weekday", Weekday);
        namesMap.put("even-odd", EvenOdd);
        namesMap.put("interval", Interval);
    }

    @JsonCreator
    public static DeviceScheduleFormat forValue(String value) {
        return namesMap.get(StringUtils.lowerCase(value));
    }

    @JsonValue
    public String toValue() {
        for (Entry<String, DeviceScheduleFormat> entry : namesMap.entrySet()) {
            if (entry.getValue() == this)
                return entry.getKey();
        }

        return null; // or fail
    }
}

Note that as of this commit in June 2015 (Jackson 2.6.2 and above) you can now simply write:

public enum Event {
    @JsonProperty("forgot password")
    FORGOT_PASSWORD;
}

The behavior is documented here: https://fasterxml.github.io/jackson-annotations/javadoc/2.11/com/fasterxml/jackson/annotation/JsonProperty.html

Starting with Jackson 2.6 this annotation may also be used to change serialization of Enum like so:

 public enum MyEnum {
      @JsonProperty("theFirstValue") THE_FIRST_VALUE,
      @JsonProperty("another_value") ANOTHER_VALUE;
 }

as an alternative to using JsonValue annotation.

You should create a static factory method which takes single argument and annotate it with @JsonCreator (available since Jackson 1.2)

@JsonCreator
public static Event forValue(String value) { ... }

Read more about JsonCreator annotation here.

Actual Answer:

The default deserializer for enums uses .name() to deserialize, so it's not using the @JsonValue. So as @OldCurmudgeon pointed out, you'd need to pass in {"event": "FORGOT_PASSWORD"} to match the .name() value.

An other option (assuming you want the write and read json values to be the same)...

More Info:

There is (yet) another way to manage the serialization and deserialization process with Jackson. You can specify these annotations to use your own custom serializer and deserializer:

@JsonSerialize(using = MySerializer.class)
@JsonDeserialize(using = MyDeserializer.class)
public final class MyClass {
    ...
}

Then you have to write MySerializer and MyDeserializer which look like this:

MySerializer

public final class MySerializer extends JsonSerializer<MyClass>
{
    @Override
    public void serialize(final MyClass yourClassHere, final JsonGenerator gen, final SerializerProvider serializer) throws IOException, JsonProcessingException
    {
        // here you'd write data to the stream with gen.write...() methods
    }

}

MyDeserializer

public final class MyDeserializer extends org.codehaus.jackson.map.JsonDeserializer<MyClass>
{
    @Override
    public MyClass deserialize(final JsonParser parser, final DeserializationContext context) throws IOException, JsonProcessingException
    {
        // then you'd do something like parser.getInt() or whatever to pull data off the parser
        return null;
    }

}

Last little bit, particularly for doing this to an enum JsonEnum that serializes with the method getYourValue(), your serializer and deserializer might look like this:

public void serialize(final JsonEnum enumValue, final JsonGenerator gen, final SerializerProvider serializer) throws IOException, JsonProcessingException
{
    gen.writeString(enumValue.getYourValue());
}

public JsonEnum deserialize(final JsonParser parser, final DeserializationContext context) throws IOException, JsonProcessingException
{
    final String jsonValue = parser.getText();
    for (final JsonEnum enumValue : JsonEnum.values())
    {
        if (enumValue.getYourValue().equals(jsonValue))
        {
            return enumValue;
        }
    }
    return null;
}

I've found a very nice and concise solution, especially useful when you cannot modify enum classes as it was in my case. Then you should provide a custom ObjectMapper with a certain feature enabled. Those features are available since Jackson 1.6. So you only need to write toString() method in your enum.

public class CustomObjectMapper extends ObjectMapper {
    @PostConstruct
    public void customConfiguration() {
        // Uses Enum.toString() for serialization of an Enum
        this.enable(WRITE_ENUMS_USING_TO_STRING);
        // Uses Enum.toString() for deserialization of an Enum
        this.enable(READ_ENUMS_USING_TO_STRING);
    }
}

There are more enum-related features available, see here:

https://github.com/FasterXML/jackson-databind/wiki/Serialization-Features https://github.com/FasterXML/jackson-databind/wiki/Deserialization-Features