How to Access the Default Jackson Deserializer within a Custom Deserializer?

Accessing Default Deserializer in Jackson Custom Deserializer

When customizing deserialization in Jackson, there may be scenarios where you need to leverage the default deserialization behavior before applying custom logic. To address this requirement, here's a comprehensive guide to accessing the default deserializer from within a custom deserializer.

Consider the following scenario:

public class UserEventDeserializer extends StdDeserializer<user> {

    public UserEventDeserializer() {
        super(User.class);
    }

    @Override
    public User deserialize(JsonParser jp, DeserializationContext ctxt)
        throws IOException, JsonProcessingException {

        User deserializedUser = null;
        try {
            deserializedUser = super.deserialize(jp, ctxt, new User());
        } catch (UnsupportedOperationException e) {
            // Access default Jackson deserializer here
        }
        // Perform custom logic on deserializedUser...

        return deserializedUser;
    }
}</user>

Solution: BeanDeserializerModifier

The recommended approach to access the default deserializer is through a BeanDeserializerModifier. This allows you to modify the default behavior for specific bean classes, in this case, User.

public class UserDeserializerModifier extends BeanDeserializerModifier {

    @Override
    public JsonDeserializer> modifyDeserializer(DeserializationConfig config, BeanDescription beanDesc, JsonDeserializer> deserializer) {
        if (beanDesc.getBeanClass() == User.class) {
            return new JsonDeserializer(); // Default deserializer
        }
        return super.modifyDeserializer(config, beanDesc, deserializer);
    }
}

Example Usage

To use the custom modifier, register it with a SimpleModule and configure it on your ObjectMapper:

SimpleModule module = new SimpleModule();
module.setDeserializerModifier(new UserDeserializerModifier());

ObjectMapper mapper = new ObjectMapper();
mapper.registerModule(module);

Note: Implementing ResolvableDeserializer may be necessary to avoid JsonMappingException when using this approach.

Alternative Approaches

While BeanDeserializerModifier is a reliable and flexible solution, here are some alternative approaches:

• Overriding AnnotationIntrospector: This involves extending AnnotationIntrospector and modifying the behavior for annotations like @Transactional. However, it can be more complex and may not always work as expected.
• Using JsonDeserializerBuilder: This approach requires accessing the application context from Spring and can be less straightforward than BeanDeserializerModifier.

Conclusion

By leveraging BeanDeserializerModifier, you can effectively access and utilize the default Jackson deserializer within your custom deserializer, allowing for flexible and efficient deserialization behavior.

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