How do you implement custom serializers and deserializers in Jackson?

Table of Contents

• Introduction
• Custom Serializers in Jackson
  • Example: Custom Serializer for a Date Object
    • Step 1: Create a Custom Serializer
    • Step 2: Register the Custom Serializer
      • Option 1: Register the Serializer with @JsonSerialize on a Field
      • Option 2: Register the Serializer Globally in ObjectMapper
• Custom Deserializers in Jackson
  • Example: Custom Deserializer for Date Object
    • Step 1: Create a Custom Deserializer
    • Step 2: Register the Custom Deserializer
      • Option 1: Register the Deserializer with @JsonDeserialize on a Field
      • Option 2: Register the Deserializer Globally in ObjectMapper
• Conclusion

Introduction

Jackson provides a powerful mechanism for converting Java objects to and from JSON. While Jackson's default serialization and deserialization mechanisms are sufficient for most use cases, sometimes you need more control over how certain Java objects are converted to JSON or how JSON is converted into Java objects. In such cases, Jackson allows you to implement custom serializers and custom deserializers.

Custom serializers and deserializers are particularly useful when:

• You need to change the way an object is serialized into JSON (e.g., formatting, filtering fields).
• You want to change how incoming JSON is deserialized into Java objects (e.g., complex transformations, conditional parsing).
• You are dealing with third-party libraries or legacy systems where the default Jackson behavior doesn't match the required format.

In this guide, we will walk through how to implement custom serializers and deserializers in Jackson.

Custom Serializers in Jackson

A custom serializer in Jackson allows you to control how a Java object is converted into its JSON representation. To implement a custom serializer, you need to:

1. Extend JsonSerializer<T>.
2. Implement the serialize() method, where you specify how to serialize the object.

Example: Custom Serializer for a Date Object

Let’s say we want to serialize a Date object in a custom format, say "dd-MM-yyyy".

Step 1: Create a Custom Serializer

In the above code, CustomDateSerializer extends JsonSerializer<Date>. It overrides the serialize() method, where we format the Date object to a custom format and write it as a string.

Step 2: Register the Custom Serializer

Now, we need to register the custom serializer with Jackson. You can either register it globally or on a field-by-field basis.

Option 1: Register the Serializer with @JsonSerialize on a Field

With the @JsonSerialize annotation, we specify that the birthDate field should be serialized using the CustomDateSerializer.

Option 2: Register the Serializer Globally in ObjectMapper

You can register the custom serializer globally in the ObjectMapper if you want all Date objects to be serialized in this way.

Here, we create a custom SimpleModule, add our custom serializer for the Date class, and register it with the ObjectMapper.

Custom Deserializers in Jackson

A custom deserializer in Jackson is used when you need to control how JSON is deserialized into a Java object. To implement a custom deserializer, you need to:

1. Extend JsonDeserializer<T>.
2. Implement the deserialize() method, where you specify how to transform JSON into a Java object.

Example: Custom Deserializer for Date Object

Let’s say we need to deserialize a Date string in the format "dd-MM-yyyy" into a Date object.

Step 1: Create a Custom Deserializer

Here, CustomDateDeserializer extends JsonDeserializer<Date>. The deserialize() method reads a JSON string and converts it into a Date object using the specified date format.

Step 2: Register the Custom Deserializer

Just like with the custom serializer, you can register the custom deserializer on a field-by-field basis or globally.

Option 1: Register the Deserializer with @JsonDeserialize on a Field

In this example, the birthDate field will be deserialized using the CustomDateDeserializer.

Option 2: Register the Deserializer Globally in ObjectMapper

In this example, the custom deserializer for Date is registered globally in the ObjectMapper.

Conclusion

Custom serializers and deserializers in Jackson provide a powerful way to control how Java objects are converted to JSON and how JSON data is mapped to Java objects. Whether you're working with specific fields, global configurations, or need complex transformations, Jackson gives you the flexibility to define your own custom logic.

• Custom serializers allow you to define how Java objects should be represented in JSON, which is especially useful when working with complex or non-standard formats.
• Custom deserializers let you control how incoming JSON is converted back into Java objects, giving you full control over the parsing logic.

By implementing custom serializers and deserializers, you can handle advanced use cases and ensure your data is processed correctly according to the specific needs of your application.