How do you create a custom converter in Spring Boot?

Table of Contents

• Introduction
• What Is an HTTP Message Converter in Spring Boot?
• Steps to Create and Register a Custom Converter in Spring Boot
  • Step 1: Create a Custom Converter
    • Example: Custom String to Product Converter
  • Step 2: Register the Custom Converter
    • Example: Registering the Custom Converter
  • Step 3: Use the Custom Converter in Controller Methods
    • Example: Using the Custom Converter in a Controller
  • Step 4: Handling Custom Serialization and Deserialization (Optional)
    • Example: Custom HTTP Message Converter
    • Registering the Custom HTTP Message Converter
• Conclusion

Introduction

In Spring Boot, a custom converter allows you to transform data between different formats (such as JSON, XML, or custom objects) during HTTP requests and responses. Spring Boot uses HttpMessageConverter to handle the serialization and deserialization process. While Spring Boot provides several built-in converters, you can create your own to handle custom data types or specific needs in your application. This guide walks you through the process of creating and registering a custom converter in Spring Boot.

What Is an HTTP Message Converter in Spring Boot?

An HTTP Message Converter in Spring Boot is used to convert incoming HTTP requests to Java objects (deserialization) and outgoing Java objects to HTTP responses (serialization). Spring Boot automatically configures default converters like MappingJackson2HttpMessageConverter for JSON and Jaxb2RootElementHttpMessageConverter for XML.

However, when you need to convert custom data types or implement specific behavior, you can create and register your own custom converter.

Steps to Create and Register a Custom Converter in Spring Boot

Step 1: Create a Custom Converter

A custom converter in Spring Boot implements the Converter<S, T> interface from Spring’s core library. You need to implement the convert(S source) method to specify how to convert the source data type (S) to the target data type (T).

For example, let’s create a converter that converts a string to a custom Product object.

Example: Custom String to Product Converter

In this example:

• The StringToProductConverter class implements the Converter<String, Product> interface.
• The convert() method takes a string (e.g., "1:ProductName") and returns a Product object with the ID and name parsed from the string.

Step 2: Register the Custom Converter

Once you've created your custom converter, you need to register it with Spring Boot’s ConversionService. This can be done in a @Configuration class by adding a ConversionService bean.

Example: Registering the Custom Converter

In this example:

• The DefaultFormattingConversionService is used to manage conversions.
• The addConverter() method registers the StringToProductConverter.

Step 3: Use the Custom Converter in Controller Methods

Once registered, the custom converter will automatically be applied when Spring Boot binds HTTP request parameters to method arguments in your controllers.

Example: Using the Custom Converter in a Controller

In this example:

• The getProduct method expects a productInfo query parameter in the format "id:name", which is automatically converted to a Product object using the StringToProductConverter.
• For instance, a request like /product?productInfo=1:ProductName will return a Product object with ID 1 and name ProductName.

Step 4: Handling Custom Serialization and Deserialization (Optional)

In some cases, you might need to customize how data is serialized (converted from Java objects to JSON) or deserialized (converted from JSON to Java objects). You can achieve this by creating a custom HttpMessageConverter.

Example: Custom HTTP Message Converter

In this example:

• The CustomHttpMessageConverter extends AbstractHttpMessageConverter<Product> to handle the custom serialization and deserialization of the Product class.
• It uses ObjectMapper to process JSON data for the Product object.

You can register this custom converter in your Spring Boot application by adding it to the list of HttpMessageConverter beans.

Registering the Custom HTTP Message Converter

Conclusion

Creating custom converters in Spring Boot allows you to handle complex data transformations that are not covered by default converters. Whether you need to convert a specific data type, handle custom serialization/deserialization, or modify the binding of request parameters, implementing a custom converter offers flexibility and control. With the @Configuration class to register your converter and the Spring Boot infrastructure handling the rest, your application can seamlessly integrate custom conversions into its HTTP request and response workflows.