Using Jackson with Map

Question

I want to know if there is some way to get the following deserializing properly:

public class MyClass {
     Map<String, Serializable> map;
     OtherObject object;
     [...]
}

ObjectMapper mapper = new ObjectMapper();
MyClass instance = mapper.readValue(someJson, MyClass);

Currently, I'm trying and I get an exception

com.fasterxml.jackson.databind.JsonMappingException: Can not construct instance of java.io.Serializable: abstract types either need to be mapped to concrete types, have custom deserializer, or contain additional type information

I understand that, I was hoping for a way to manipulate the ObjectMapper such that it can handle this sort of object. — cjamesm, Nov 13 '18 at 17:28
`Serializable` is a polymorphic type. This requires some extra work in Jackson. See for example: https://stackoverflow.com/questions/30362446/deserialize-json-with-jackson-into-polymorphic-types-a-complete-example-is-giv — Robert, Nov 13 '18 at 17:29

score 1 · Answer 1 · answered Oct 16 '20 at 07:40

1

As of v2.10.0, Jackson supports naive deserialization of 'Serializable' values, i.e., it will consider 'Serializable' the same as 'Object'. See this Github issue for more details.

So update Jackson to 2.10.0+ and try.

answered Oct 16 '20 at 07:40

Sivaraam Subramanian

26
1
8

YMMV. I changed a field from `Object` to `Serializable`. My problem was about JSON serialization with regard to `ObjectMapper.enableDefaultTyping(DefaultTyping.JAVA_LANG_OBJECT)`. I wanted the type info for `Serializable` field to be included (just like `Object`), but updating to v2.10.0 does *not* change the behavior here. As other posts suggested, I ended up changing my field back to `Object` instead. – quietmint Feb 08 '22 at 22:25

RichW · Answer 2 · 2018-11-13T19:34:38.733

You can change your data class's Map property declaration to not reference Serializable:

public class MyClass {
     Map<String, ?> map;
     OtherObject object;
     [...]
}

This will affect the signature of the Map's getter and setter as well. This does loosen type-checking, however, and the the map's values could be any subclass of Object instead of implementations of Serializable. If that's an important detail, you could do post-serialization validation of the map values.

Or you can configure a custom Deserializer, as those in your question's comments have suggested. It's just a matter of where you want to add the code, and that depends on how widespread this case is in your application.

Using Jackson with Map

2 Answers2