CODEWITHSUNDEEP
javajsonserializationjackson
Pedro Henrique
Pedro Henrique

Reputation: 191

How to deserialize abstract types in Jackson without modifying the interface?

I'm working on a Java application that utilizes the Jackson library for JSON deserialization. I have an abstract class called LayoutBlock that represents a block in Slack's message layout. The application uses a custom LayoutBlockDeserializer to deserialize JSON data into specific subclasses of LayoutBlock based on the "type" property.

However, when running the test, I encounter the following exception:

com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `com.slack.api.model.block.LayoutBlock` (no Creators, like default constructor, exist): abstract types either need to be mapped to concrete types, have custom deserializer, or contain additional type information

Due to limitations in the project, I don't have access to modify the LayoutBlock interface to add any Jackson annotations such as @JsonTypeInfo or @JsonSubTypes. How can I resolve this issue and successfully deserialize JSON into the appropriate subclasses of LayoutBlock without modifying the LayoutBlock interface?

Here's an overview of my code:

LayoutBlockDeserializer:

public class LayoutBlockDeserializer extends JsonDeserializer<LayoutBlock> {
// ... implementation details
}

Slack Module:

public class SlackModule extends SimpleModule {

   @Override
   public void setupModule(SetupContext context) {
      addDeserializer(LayoutBlock.class, new 
      LayoutBlockDeserializer());
   }
}

Test:

@Test
void test() throws JsonProcessingException {
   ObjectMapper mapper = new ObjectMapper().registerModule(new SlackModule());
   mapper.readValue("any-json-here", LayoutBlock.class);
}

I would greatly appreciate any insights or alternative approaches that can help me handle the deserialization of abstract types in Jackson without modifying the LayoutBlock interface or adding annotations to it. Thank you in advance!

Upvotes: 0

Views: 1156

Answers (1)

gdomo
gdomo

Reputation: 2052

You could solve your problem with a mixin:

@JsonDeserialize(using = LayoutBlockDeserializer.class)
public class MixInForLayoutBlock {}

Use it via mapper.addMixIn(LayoutBlock.class, MixInForLayoutBlock.class); Don't forget to annotate LayoutBlock implementations with @JsonDeserialize(as = LayoutBlockImpl.class) (or also use mixin).

If you struggle with an deserializer implementation, here is a great answer.

Upvotes: 0

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