Deserialize java enum from JSON

Question

We use Jackson 1.9.1 to serialize and deserialize JSON request response strings to/from Java objects. Primitive Java types, collection types, and custom objects are (de)serialized without issues. However, I have a problem trying to deserialize JSON string into java enum. JSON string is serialized like so:

"wt":{"wt":100.5,"unit":{"LBS":3}}

Java type for wt is like so:

public class Weight {

    protected double weight;
    protected Unit unit;
}

I referred to this, this, and this on SO and came up with enum for weight units like so:

public enum Unit {

    KG("kg"),
    GM("gm"),
    LBS("lbs"),
    OZ("oz");

    private String value;  
    private WeightMeasurementUnit(String value) { this.value = value; }

    @JsonValue
    public String getValue() { return this.value; }

    @JsonCreator
    public static Unit create(String val) {
        Unit[] units = Unit.values();
        for (Unit unit : units) {
            if (unit.getValue().equals(val)) {
                return unit;
            }
        }
        return LBS;
    }
}

The problem is, when ever I try to deserialize above mentioned JSON I get this error saying: "Unrecognized field "LBS" (Class a.b.c.d.Weight), not marked as ignorable" Exception stacktrace is like so:

Caused by: org.codehaus.jackson.map.exc.UnrecognizedPropertyException: Unrecognized field "LBS" (Class a.b.c.d.Weight), not marked as ignorable
 at [Source: java.io.ByteArrayInputStream@20172017; line: 1, column: 464] (through reference chain: a.b.c.d.MyRequest["blah"]->a.b.c.d.AnotherType["wt"]->a.b.c.d.Weight["LBS"])
    at org.codehaus.jackson.map.exc.UnrecognizedPropertyException.from(UnrecognizedPropertyException.java:53)
    at org.codehaus.jackson.map.deser.StdDeserializationContext.unknownFieldException(StdDeserializationContext.java:267)
    at org.codehaus.jackson.map.deser.std.StdDeserializer.reportUnknownProperty(StdDeserializer.java:673)
    at org.codehaus.jackson.map.deser.std.StdDeserializer.handleUnknownProperty(StdDeserializer.java:659)
    at org.codehaus.jackson.map.deser.BeanDeserializer.handleUnknownProperty(BeanDeserializer.java:1365)

...

My questions are: Is the serialized JSON string for enum seem correct ? What else should I include (or annotate) for the enum to be properly deserialized ?

Answer 1

I am assuming that in the public enum Unit code block, you mean Unit instead of WeightMeasurementUnit.

The Weight class has only a weight and a unit, so if you pass {"wt":100.5,"unit":"lbs"}, it should work, because a unit is just a unit without value. So there is no way for the deserializer to parse {"LBS":3} as a Unit. What is the 3 for?

Another problem is that your value is "lbs" whereas you are passing "LBS". So either you need to standardise or you need to use unit.getValue().equalsIgnoreCase(val)

Answer 2

I would suggest you update your jackson version to 2.7.0-rc2 (and probably also before) and then configure the ObjectMapper as follows:

private ObjectMapper createObjectMapper() {
    final ObjectMapper mapper = new ObjectMapper();
    // enable toString method of enums to return the value to be mapped
    mapper.enable(SerializationFeature.WRITE_ENUMS_USING_TO_STRING);
    mapper.enable(DeserializationFeature.READ_ENUMS_USING_TO_STRING);
    return mapper;
}

In your enum you just have to override the toString() method:

public enum Unit {
    KG,
    GM,
    LBS,
    OZ;

    // UPDATE: implicitly already the default so override not needed in this case
    @Override
    public String toString() {
        return name();
    }
}

You don't need any annotations or custom deserializers. This would be the way to map a simple enum to a json and vice-versa.

If your enum should be mapped from a special string you have to add a value field and a Constructor which assigns this field and return the value in the toString method.

public enum Unit {
    KG("kilogram"),
    GM("gram"),
    LBS("blah"),
    OZ("anything");

    Unit(final String value) {
        this.value = value;
    }

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