Handling UnresolvedForwardReference by pre-processing illegal values

[austalakov]

I have a Spring Boot API that uses Longs as IDs for objects. It also uses JsonIdentityInfo in combination with JsonIdentityReference. We also have a client application that erroneously sends 0 as default value of a Long, as opposed to null. This was not a problem until we upgraded from Spring Boot 2.0.0 to 2.3.3 (and thus Jackson 2.11.2). Now, the following JSON results in a UnresolvedForwardReference

{
  "id": 0,
  "fooObjects": [
    {
      "id": 0,
      "fooParameters": [
        {
          "id": 0,
          "fooObject": 0
        }
      ],
      "fooCollection": 0
    }
  ]
}

Where the error is:

Unresolved forward references for: 
 at [Source: (PushbackInputStream); line: 1, column: 6161]Object id [0] (for `com.foo.entities.FooObjectEntity`) at [Source: (PushbackInputStream); line: 1, column: 462]

Obviously, the problem should be fixed on the client side, but that is quite difficult in our situation, so I am looking if there is something we can do on the API side with Jackson.

I have tried the following:

1. A CustomContextualLongDeserializer that returns null instead of 0, if in the right context. This unfortunately causes a NullPointerException because of the return null statement here and that being called from here. It seems like a custom deserializer is too late in the chain to do a replacement with null.
2. A LongToLongConverter (same deal, returns null instead of 0) used in a JsonDeserialize annotation on the object fields marked as @JsonIdentityReference(alwaysAsId = true). This seems to work in some contexts and resolves the UnresolvedForwardReference problem for the above JSON, but when deserializing the same object in other contexts, it causes the following error instead:

Problem deserializing property 'fooCollection' (expected type: [simple type, class com.foo.entities.FooCollectionEntity]; actual type: `java.lang.Long`)

Any suggestions for a general approach here?

[cowtowncoder]

I cannot think of a good approach offhand, but one challenge is that description itself does not quite give enough information to reproduce the problem (specifically, class definitions missing). I don't know if knowing that would necessarily help, but it might be easier to see where possible additional configurability would help.

Use of DeserializationFeature.FAIL_ON_UNRESOLVED_OBJECT_IDS could help work around the issue in some cases, but I suspect there is some need to be able to customize various aspects of Object Id resolution and/or lookups.

It might be worth checking if use of @JsonIdentityInfo(resolver=) would allow more custom handling -- and one could be applied by sub-classing of JacksonAnnotationIntrospector, method findObjectIdInfo() if necessary (i.e. if you cannot change annotation itself).

[austalakov]

Thanks for taking a look. We do already use a @JsonIdentityInfo(resolver=)- the resolver returns null when the ID is 0 and that is exacly what causes the UnresolvedForwardReference which seems to originate here.

I will try and create a small reproducible example if I get the time. I was hoping there was some "pre-processing" hook we could use in Jackson as a workaround to our problem, but regardless, the best solution to the problem is on the client-side so we will probably have to proceed with that.

[cowtowncoder]

Unfortunately I don't think there are enough hooks yet, all things considered, and I think more extension points will be needed. But due to timing (we are about to release 2.12.0 within a week), additions will, even in the best case, take time.

A reproduction would be useful for future work, if you have time, and much appreciated.

[cowtowncoder]

Changed code to allow resolution as null if (and only if) DeserializationFeature.FAIL_ON_UNRESOLVED_OBJECT_IDS is disabled, in case described.