Jackson: How to make @JsonBackReference and a custom deserializer work at the same time?

Question

I have a JSON structure which contains maps of strings to object:

{
    "domains": {
        "ldap": {
            "users": {
                "fwalther": {
                    "firstName": "Fritz",
                    "lastName": "Walther"
                },
                // ...
            }
        }
    },
    // ...
}

I want to deserialize this structure to Domain and User objects using Jackson, and I want from each user have a back-reference to it's map key (which is the user ID) and to the Domain container. Either of this works, but it fails if I try to get both back-references at once.

Java classes with @JsonManagedReference and @JsonBackReference:

public class Domain {
    @JsonManagedReference
    @JsonDeserialize(contentUsing = UserDeserializer.class)
    private Map<String, User> users;

    public Map<String, User> getUsers() {
        return users;
    }
}

public class User {
    @JsonBackReference
    private Domain domain;

    String userId;

    private String firstName;
    private String lastName;

    // ... getters
}

Custom deserializer to get the map key:

public class UserDeserializer extends JsonDeserializer<User> {
    @Override
    public User deserialize(JsonParser p, DeserializationContext ctxt)
            throws IOException, JsonProcessingException {
        String key = p.getCurrentName();
        User result = p.readValueAs(User.class);

        result.userId = key;
        return result;
    }
}

Both mechanisms, i.e. the @JsonManagedReference/@JsonBackReference pair and @JsonDeserialize with the custom deserializer, work if I activate only one of them. But if I combine the mechanisms (as shown in the code above), I get the following exception when parsing the JSON:

java.lang.IllegalArgumentException: Cannot handle managed/back reference 'defaultReference': type: value deserializer of type org.example.UserDeserializer does not support them
    at com.fasterxml.jackson.databind.JsonDeserializer.findBackReference(JsonDeserializer.java:366) ~[jackson-databind-2.9.8.jar:2.9.8]
    at com.fasterxml.jackson.databind.deser.std.ContainerDeserializerBase.findBackReference(ContainerDeserializerBase.java:104) ~[jackson-databind-2.9.8.jar:2.9.8]
    at com.fasterxml.jackson.databind.deser.BeanDeserializerBase._resolveManagedReferenceProperty(BeanDeserializerBase.java:786) ~[jackson-databind-2.9.8.jar:2.9.8]
    at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.resolve(BeanDeserializerBase.java:487) ~[jackson-databind-2.9.8.jar:2.9.8]
...

Looking at the code where the exception is thrown, I see that I'd need to implement the findBackReference in my custom deserializer, but I have no clue how, and I couldn't find and information on this either. Any ideas?

Or are there other ways to get both the map key and back reference to the containing object?

Have you seen this answer? https://stackoverflow.com/questions/31319358/jsonmanagedreference-vs-jsonbackreference — Bor Laze, Apr 30 '19 at 17:39
No, I haven't, but this question is only about how to use `@JsonManagedReference` and `@JsonBackReference` in general. These annotation work by themselves - they just fail if I also add the `@JsonDeserialize` annotation. — oberlies, May 01 '19 at 09:36
For JSONs with multiple maps, it may also make sense to use a generic deserializer that can deserialize map keys into all properties with a certain annotation - see https://stackoverflow.com/a/55953495/1523648 — oberlies, May 02 '19 at 13:31

score 2 · Accepted Answer · edited Nov 06 '21 at 17:52

With the help of this answer, I found the solution: The custom deserializer needs to be based on the default deserializer, which implements the back references mechanism correctly.

This is a little more complicated than just inheriting from the right base class. Instead, you need to get hold of the (fully configured) default deserializer instance through a custom BeanDeserializerModifier, and then pass this instance to your subclass of DelegatingDeserializer:

public ObjectMapper getMapperWithCustomDeserializer() {
    ObjectMapper objectMapper = new ObjectMapper();

    SimpleModule module = new SimpleModule();
    module.setDeserializerModifier(new BeanDeserializerModifier() {
        @Override
        public JsonDeserializer<?> modifyDeserializer(DeserializationConfig config,
                    BeanDescription beanDesc, JsonDeserializer<?> defaultDeserializer) {
            if (beanDesc.getBeanClass() == User.class) {
                return new UserDeserializer(defaultDeserializer);
            } else {
                return defaultDeserializer;
            }
        }
    });
    objectMapper.registerModule(module);

    return objectMapper;
}

The custom deserializer then would need to look like this:

public class UserDeserializer extends DelegatingDeserializer {

    public UserDeserializer(JsonDeserializer<?> delegate) {
        super(delegate);
    }

    @Override
    protected JsonDeserializer<?> newDelegatingInstance(JsonDeserializer<?> newDelegate) {
        return new UserDeserializer(newDelegate);
    }

    @Override
    public User deserialize(JsonParser p, DeserializationContext ctxt)
            throws IOException {
        String key = p.getCurrentName();
        User result = (User) super.deserialize(p, ctxt);

        result.userId = key;
        return result;
    }
}

Finally, you need to remove the @JsonDeserialize annotation. Then, the custom deserializer and the @JsonBackReference should work.

Jackson: How to make @JsonBackReference and a custom deserializer work at the same time?

1 Answers1