@JsonTypeInfo property added twice to generated JSON: add support for JsonType.As.EXISTING_PROPERTY

[heapifyman]

Hi,

I have an abstract super class annotated with

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.PROPERTY, property = "type")

where type is an existing property of the class.

The full class hierarchy looks like this:

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.PROPERTY, property = "type")
@JsonSubTypes({
                      @JsonSubTypes.Type(value = SubClassA.class, name = "a"),
                      @JsonSubTypes.Type(value = SubClassB.class, name = "b")
              })
public abstract class AbstractSuperClass {
    @Transient
    private String type;

    protected AbstractSuperClass() {
        setType(getTypeValue());
    }
    protected abstract String getTypeValue();

    // getters and setters
}

public class SubClassA extends AbstractSuperClass {
    private double a;

    public SubClassA(double a) {
        this.a = a;
    }

    // getters and setters

    @Override
    protected String getTypeValue() {
        return "a";
    }
}

public class SubClassB extends AbstractSuperClass {

    private int b;

    public SubClassB(int b) {
        this.b = b;
    }

    // getters and setters

    @Override
    protected String getTypeValue() {
        return "b";
    }
}

Now when a SubClass of this class gets serialised, the type is added twice to the generated JSON. E.g. the JSON for a SubClassB would look like this:

{"type":"b","type":"b","b":42}

For the most part this does not seem to be an issue, but some clients (e.g. Postman) refuse this as Malformed JSON.

I know I could add e.g. @JsonTypeId or @JsonIgnore but in that case, type will not be included in generated JSON when using a List<AbstractSuperClass>, which is what I definitely need.

I assume that include = JsonTypeInfo.As.EXISTING_PROPERTY would solve my problem but this results in java.lang.IllegalStateException: Do not know how to construct standard type serializer for inclusion type: EXISTING_PROPERTY

[cowtowncoder]

I think there's another issue opened for this. And yes, it is a bug; and also yes, EXISTING_PROPERTY, when implemented should solve it. Unfortunately I haven't had time to dig deep into doing just that.

[fleebytes]

I also have a need for JsonType.As.EXISTING_PROPERTY to be implemented. I have done this locally, but would like to know how extensive the test cases need to be before I commit to my fork and issue a pull request. Thanks!

[cowtowncoder]

@fleebytes I think a fork & Pull Request is a good way -- we can help with tweaks, and testing. Usually the first thing is to ensure that none of existing tests start failing, but here it may not help a lot, given that EXISTING_PROPERTY is not being used. So new tests are needed, perhaps based on existing tests for creator-functionality.
But I can help with tests too, if there's at a test or two to verify happy case, including round-trip (serialize, then deserialize back) test.

For what it is worth, this is a REALLY important feature, and should go in 2.5. I have been hoping to find time to tackle it myself, after resolving other somewhat related problems (wrt visibility of type info).
So all help is much appreciated.

[fleebytes]

@cowtowncoder I've taken a stab at writing the tests. There is round-trip serialization for three scenarios: (1) sub-classes each have the type property (no reference in base class), (2) sub-classes each have getType() method implemented but no explicit property, and (3) base class has abstract getType() method that enforces sub-classes to have that method. All three of them have working round trip test cases. Let me know if there is anything else I can do to help complete adding this feature to jackson-databind. Thanks!

[ouaibsky]

Hi

Can you tell me in which version this fix has been released ?
Thx
Christophe

[cowtowncoder]

Original fix was in 2.5.0. There is one related fix (#222) that will be included in 2.6.0 (and 2.6.0-rc3 if such is released before 2.6.0 final).