如何在不知道子类的情况下通过实现的接口反序列化枚举

Question

I am writing a custom spring boot starter that provides a uniform error response class for all repositories that will add this starter. It also provides the corresponding exception handler. The problem is, that this error response needs an error code which might differ between all those repositories, using my starter. So the solution would be to create an error response with an error code that is an interface. Other repositories can then create enums implementing this interface to achieve the desired behavior. It is written in kotlin

interface BaseErrorCode {
    val message: String
}

class ErrorResponse {
    val customMessage: String,
    val errorCode: BaseErrorCode,
    val timestamp: OffsetDateTime
}

Now, in another repository, I use this starter, get access to the classes above and create my error code enum:

enum class MyCustomErrorCodes : BaseErrorCode {
    FOO
}

Now, I can throw an exception and through the handler, this json will be produced:

{
    "customMessage": "My message",
    "errorCode": "FOO",
    "timestamp": "2023-01-27T12:15:31.7730645+01:00"
}

So serializing works absolutely fine.

However, when deserializing the ErrorResponse in my integration-tests, I get the following exception:

com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `my.package.BaseErrorCode` (no Creators, like default constructor, exist): abstract types either need to be mapped to concrete types, have custom deserializer, or contain additional type information
 at [Source: (String)"{"customMessage":"My message","errorCode":"FOO","timestamp":"2023-01-27T12:12:06.4932227+01:00"}"; line: 1, column: 54] (through reference chain: my.package.ErrorResponse["errorCode"])

Approach 1 I know a solution where you write a custom deserializer. But this could not be placed in the spring boot starter because I need to know the implementing classes of that ´BaseErrorCode`-interface.

Approach 2 Using @JsonSubType , I can also tell Jackson how to handle this interface. But I do need some information about implementing enums as well, which I do not have in my starter

Is there any way to deserialize this error response

• only by modifying the starter, not each repository
• without knowing the classes that implement BaseErrorCode , BUT knowing that it will ALWAYS be an enum?

Answer 1

I think the answer lies in using intentionally dissimilar objects. I have not tested, but I think:

• follow what you have with serialization, and note that you are not serializing the whole enum rather an instance of enum which conforms to BaseErrorCode .
• on deserialization you make a concrete implementation of BaseErrorCode (this can be private to the Starter) that you deserialize the JSON in to. (Adding @JsonIgnoreProperties(ignoreUnknown = true) to guard against an unknown subclass sending more data than you can deal with).