How do you return JSON responses in a Spring application?

Table of Contents

• Introduction
• How Spring Converts Java Objects to JSON
  • Using @RestController to Return JSON
  • Using @ResponseBody to Return JSON
• Practical Example of Returning JSON in Spring
  • 1. Creating a Spring Boot Project
  • 2. Defining a Model Class
  • 3. Creating a REST Controller
  • 4. Testing the API
• Handling Custom JSON Responses
• Conclusion

Introduction

In Spring applications, particularly when building RESTful APIs, it's common to return data in JSON format. Spring Boot makes it easy to handle JSON responses by automatically converting Java objects to JSON. This functionality is made seamless through the use of the @RestController annotation, along with built-in message converters like Jackson that handle JSON serialization and deserialization.

How Spring Converts Java Objects to JSON

Spring Boot uses Jackson (a JSON processing library) by default to automatically convert Java objects into JSON responses. This conversion is enabled when you use annotations like @RestController or @ResponseBody. Here’s how Spring processes an HTTP request and returns a JSON response:

1. Controller Method: The method in a controller returns a Java object.
2. Message Converter: The message converter (Jackson, by default) serializes this object to JSON.
3. HTTP Response: The serialized JSON is sent back to the client as an HTTP response.

Using @RestController to Return JSON

To return JSON in a Spring Boot application, annotate your class with @RestController. This tells Spring to automatically convert the response to JSON.

Example:

In this example:

• The getUser() method returns a User object.
• Spring automatically converts this object to JSON using Jackson and sends it as the HTTP response.

Response (JSON):

Using @ResponseBody to Return JSON

In traditional Spring MVC, you can use the @ResponseBody annotation to indicate that the return value of a method should be written directly to the HTTP response body (and converted to JSON if necessary).

Although @RestController includes @ResponseBody functionality, using @ResponseBody explicitly is another option when you don’t need a full REST controller.

Practical Example of Returning JSON in Spring

1. Creating a Spring Boot Project

You can create a Spring Boot project with the Spring Web dependency, which enables you to build web applications and RESTful services. Use Spring Initializr or Spring Tool Suite (STS) to generate the project.

2. Defining a Model Class

3. Creating a REST Controller

• Here, the getUsers() method returns a list of User objects.
• Spring automatically converts this list to JSON and returns it to the client.

4. Testing the API

You can test the API using tools like Postman or cURL by sending a GET request to /api/users. You should see the following JSON response:

Handling Custom JSON Responses

You can customize the JSON responses by using Jackson annotations like @JsonIgnore and @JsonProperty. For example:

This will change the JSON output to:

You can also exclude certain fields from the response using @JsonIgnore.

Conclusion

Returning JSON responses in a Spring Boot application is straightforward, thanks to built-in support for JSON serialization via the Jackson library. By using @RestController or @ResponseBody, Spring automatically converts Java objects to JSON and sends them back as HTTP responses. This makes it easy to develop RESTful services that communicate with clients via standard HTTP methods and JSON.