How Do I Call The Default Deserializer From A Custom Deserializer In Jackson

java

custom-deserializer

jackson-api

deserialization-context

byAlex Kataev·Feb 11, 2025

You can call the base deserializer or the default deserializer from a custom deserializer by using the ctxt.findRootValueDeserializer(). This mechanism follows the DRY principle and helps you avoid recursion. Implement it like this:

public class MyCustomDeserializer extends JsonDeserializer<MyClass> {
    @Override
    public MyClass deserialize(JsonParser jp, DeserializationContext ctxt) throws IOException {
        // "I love it when a plan comes together!" - Hannibal, A-Team
        return (MyClass) ctxt.findRootValueDeserializer(ctxt.constructType(MyClass.class))

                              // Handoff to Jackson’s built-in handling
                             .deserialize(jp, ctxt); 
    }
}

This code ensures that Jackson's built-in handling completes the default deserialization process when your custom logic defers to it.

Detailed answer and understanding the implementing process

Making your custom deserializer play nice

Our goal is to make a custom deserializer that knows when to hand over to the default one for the heavy weight processing. How then do we go about that?

Step 1: Implement `ResolvableDeserializer`

The ResolvableDeserializer helps regulate the interaction between your custom deserializer and the context it's operating within.

Step 2: Apply the `BeanDeserializerModifier`

This allows you to make changes to how Jackson handles specific types without rebuilding deserialization from scratch.

Step 3: Register your deserializer with a `SimpleModule`

Jackson's ObjectMapper is then able to handle your custom module seamlessly. Pretty neat, huh?

Step 4: Take advantage of `DeserializationContext`

The readValue method from this context permits you to switch back to default behavior swiftly.

Extra tips to get you ahead

Try the ObjectMapper readValue hack : ObjectMapper can effectively delegate deserialization of a node directly.
Use post-process actions: Consider applying your logic after Jackson has handled initialization and base translation.
Dependency injection: Hand over the default deserializer as an inbuilt reference straight to your custom deserializer for conditional alteration of processing.
JSON tree navigation: Sometimes, allowing Jackson to navigate the tree either before or after you apply your special logic would be the best step forward.

Avoiding recursion and other sticky situations

Warning: a step wrongly taken in custom deserialization might make you fall into infinite recursive calls or strange unexpected behavior. It's crucial to make sure your custom logic is a neat addition to Jackson's default operations and not an interference!

Dealing with Default deserializers

Knowing when to hand over

Understand when to hand off the task to Jackson's built-in deserializers. This is key in maintaining Jackson's native performance and consistency.

Resisting the urge to reinvent the wheel

The default Jackson mechanisms are optimized for general use. It would be a waste of time rewriting existing logic. Where possible, use the base deserializer.

Keeping your custom deserializer clean

Ensure your custom deserializer does not distort the result by introducing weird side-effects that could alter the expected outcome of the default deserialization process.

Integration and co-existence

You can integrate your custom deserializer neatly with default Jackson deserializers using annotations and Jackson's API offerings.

By sticking to these simple tips, you can harness your unique additions to the existing functionalities while respecting the fine-tuned foundation of Jackson.

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