Jackson deserializing into Map with an Enum Key, POJO Value

Question

I am trying to deserialize JSON into a Java POJO using Jackson. Without giving away confidential information, here is an example stack trace when ObjectMapper's deserialization fails:

org.codehaus.jackson.map.JsonMappingException: Can not construct Map key of type com.example.MyEnum from String "coins": not a valid representation: Can not construct Map key of type com.example.MyEnum from String "coins": not one of values for Enum class

My JSON looks like this:

"foo": {
    "coins": null,
    ...
}

And the class I want to deserialize into has this field:

private Map<MyEnum, MyPojo> foo;

And my enum type looks like this:

public enum MyEnum {
    COINS("coins"),
    ...
}

I do realize that I am trying to deserialize a null value. But I believe this should still work: the result of the deserialization should be equivalent to having a Map with foo.put(MyEnum.COINS, null), which is indeed a valid Java instruction. Help is much appreciated, thanks in advance.

Answer 1

In addition to one good solution presented (factory method), there are 2 other ways:

• If MyEnum.toString() would return coins, you can make Jackson use toString() over name() with ObjectMapper.enable(DeserializationFeature.READ_ENUMS_USING_TO_STRING)
• You could add some other method to return id to use, and mark that method with @JsonValue annotation (you can actually use that on toString() as well, instead of enabling the above feature) -- if that annotation exists, the value returned by that method is used as the id.