Welcome to the lesson on Returning JSON Responses in Spring Boot. In the previous lesson, we covered the fundamentals of RESTful APIs and walked you through creating a simple controller to handle HTTP requests. Today, we’re going to dive a little deeper by focusing on returning JSON responses and the process known as serialization. By the end of this lesson, you'll understand how to properly structure your data and customize JSON responses in Spring Boot.

Before we dive into returning JSON responses, it’s essential to understand the concept of Kotlin data classes. A data class in Kotlin is a simple class that is designed to hold data. Kotlin provides a concise syntax to define data classes and automatically generates useful methods like toString, equals, hashCode, and copy.

Here's an example of a Recipe class that serves as a Kotlin data class:

Kotlin
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1package com.codesignal.models
2
3data class Recipe(
4    val ingredients: List<String>,
5    val instructions: List<String>
6)

Each time you send a GET request to a Spring Boot REST endpoint, the data is converted into JSON format to be transmitted over HTTP. This conversion process is known as serialization. Spring Boot utilizes a powerful library called Jackson to perform this serialization seamlessly.

Jackson takes your Kotlin objects and, through a series of configurations and rules, converts them into JSON. This conversion ensures that your client applications can easily read and process the data you serve through your API endpoints.

One of the great features of Jackson is its flexibility. You can easily customize the serialization process using Jackson annotations. This allows you to exclude specific fields, modify field names, and apply various other transformations.

Here’s an example showcasing how you can use Jackson annotations to tweak the serialization process:

Kotlin
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1package com.codesignal.models
2
3import com.fasterxml.jackson.annotation.JsonIgnore
4import com.fasterxml.jackson.annotation.JsonProperty
5
6data class Recipe(
7    @JsonProperty("recipe_ingredients") val ingredients: List<String>,
8    val instructions: List<String>,
9    @JsonIgnore val internalNotes: String?
10)

In this example:

• @JsonProperty("recipe_ingredients") changes the field name ingredients to recipe_ingredients in the JSON response.
• @JsonIgnore excludes the internalNotes field from being included in the JSON response.

Jackson provides various annotations to customize the serialization and deserialization process. Below is a list of some of these annotations, along with a brief description of where they can be used (on a field, method, or class) and what they do:

• @JsonInclude: Can be used on a class or field. Specifies the conditions under which properties are included in the JSON output (e.g., only non-null fields).

• @JsonFormat: Can be used on a field or method. Defines the format in which a property should be serialized (e.g., date formats).

• @JsonPropertyOrder: Can be used on a class. Sets the order in which properties will appear in the JSON output.

• @JsonIgnoreProperties: Can be used on a class. Ignores the specified properties during serialization and deserialization.

• @JsonProperty: Can be used on a field or method. Changes the property name in the JSON output.

• @JsonIgnore: Can be used on a field or method. Indicates that the field or method should be ignored during serialization and deserialization.

• @JsonCreator: Can be used on a constructor or factory method. Indicates a constructor or factory method to be used for creating an instance from JSON.

• @JsonValue: Can be used on a method. Indicates that the return value of this method will represent the single value to be serialized.

These annotations provide granular control over how your Kotlin objects are represented in JSON, enabling you to tailor the output to meet specific requirements.

In this lesson, you’ve learned how to use Kotlin data classes to structure your data, how serialization works with the Jackson library in Spring Boot, and how to customize the serialization process with Jackson annotations. These concepts are crucial for effectively managing and returning JSON responses in your RESTful APIs.