How Can I Leverage Default Deserialization Behavior Within a Custom Jackson Deserializer?

Custom Deserialization with Default Behavior in Jackson

When implementing custom deserializer in Jackson, it may be necessary to utilize the default deserializer to pre-populate an object before applying custom logic. This article addresses the challenge of accessing the default deserializer from a custom one.

To achieve this, create a BeanDeserializerModifier and register it using a SimpleModule. Here's an example:

public class UserEventDeserializer extends StdDeserializer<User> implements ResolvableDeserializer {

    private static final long serialVersionUID = 7923585097068641765L;

    private final JsonDeserializer<?> defaultDeserializer;

    public UserEventDeserializer(JsonDeserializer<?> defaultDeserializer) {
        super(User.class);
        this.defaultDeserializer = defaultDeserializer;
    }

    @Override
    public User deserialize(JsonParser jp, DeserializationContext ctxt)
            throws IOException, JsonProcessingException {
        User deserializedUser = (User) defaultDeserializer.deserialize(jp, ctxt);

        // Special logic

        return deserializedUser;
    }

    @Override
    public void resolve(DeserializationContext ctxt) throws JsonMappingException {
        ((ResolvableDeserializer) defaultDeserializer).resolve(ctxt);
    }

    public static void main(String[] args) throws JsonParseException, JsonMappingException, IOException {
        SimpleModule module = new SimpleModule();
        module.setDeserializerModifier(new BeanDeserializerModifier() {
            @Override
            public JsonDeserializer<?> modifyDeserializer(DeserializationConfig config, BeanDescription beanDesc, JsonDeserializer<?> deserializer) {
                if (beanDesc.getBeanClass() == User.class)
                    return new UserEventDeserializer(deserializer);
                return deserializer;
            }
        });

        ObjectMapper mapper = new ObjectMapper();
        mapper.registerModule(module);
        User user = mapper.readValue(new File("test.json"), User.class);
    }
}

In this example:

1. UserEventDeserializer extends both StdDeserializer and ResolvableDeserializer.
2. It takes the default deserializer as a constructor argument.
3. The deserialize method first deserializes using the default deserializer, then applies custom logic to the resulting object.
4. It implements ResolvableDeserializer to avoid mapping exceptions.
5. A BeanDeserializerModifier is used to replace the default deserializer for User with the custom deserializer.

By registering this module with an ObjectMapper, the custom deserialization logic can be used while still benefiting from the default behavior for other classes.

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