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 ?

Just guessing - looks like it numbers your enum fields in order when serializing, KG = 1, GM = 2, etc. Does it work if you use these numbers as your enum initializers i.e use KG(1); GM(2); LBS(3); OZ(4); in the definition of enum Unit ? — Graham Griffiths, Aug 29 '13 at 08:43
Your question sorted my issue. I was looking for the annotations `@JsonValue` and `@JsonCreator`. — Hola Soy Edu Feliz Navidad, Aug 22 '19 at 14:20
Very true; added `@JsonValue` on the getValue/OverriddenToString that gets value and `@JsonCreator` on the matchFromString custom methods that matches on both EnumName/EnumValue! — ravikanth, Sep 18 '19 at 14:25

score 6 · Accepted Answer · answered Aug 29 '13 at 08:32

6

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)

answered Aug 29 '13 at 08:32

Hari Menon

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I think the string in the question is how it serialized the JSON - so not directly under control to change it – Graham Griffiths Aug 29 '13 at 08:50
Raze2Dust: My mistake. Yes I meant Unit instead of WeightMeasurementUnit. – chetan Aug 29 '13 at 17:25
I have little control over how JSON is serialized. However I want to make sure that I am not doing anything wrong with the enum before I can say that JSON is serialized wrong. Thanks. – chetan Aug 29 '13 at 17:27
@Raze2Dust: that was the problem. The code which serialized JSON that way was using the wrong mapping for ObjectMapper. I just changed my Unit enum to override toString and annotated it with (at)JsonValue – chetan Aug 30 '13 at 04:11

luckyhandler · Answer 2 · 2015-12-29T10:29:18.170

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;
    }
}

value is not defined in Unit enum class at the end – stallion May 03 '21 at 13:03 — stallion, May 03 '21 at 13:03

Deserialize java enum from JSON

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