How do you create a reactive REST controller in Spring WebFlux?

Table of Contents

• Introduction
  • 1. Setting Up Spring WebFlux for Reactive Programming
    • Example: Add Dependencies in pom.xml (Maven)
  • 2. Creating a Reactive REST Controller
    • Example: A Reactive Controller for a User Resource
  • 3. Creating the Service Layer
    • Example: Reactive Service for Fetching User Data
  • 4. Handling Reactive Database Access
    • Example: Reactive MongoDB Repository
  • 5. Testing a Reactive Controller
    • Example: Testing the Reactive Controller
  • 6. Error Handling in Reactive Controllers
    • Example: Handling Errors in the Controller
• Conclusion

Introduction

Building reactive applications with Spring WebFlux allows you to create non-blocking, highly scalable services that can efficiently handle many concurrent requests. A reactive REST controller in Spring WebFlux returns asynchronous results using reactive types like Mono and Flux, enabling the application to serve many requests without blocking resources.

In this guide, we’ll walk through the process of creating a reactive REST controller in Spring WebFlux, demonstrate how to use Mono and Flux, and explore how to build non-blocking, asynchronous APIs for your application.

1. Setting Up Spring WebFlux for Reactive Programming

To begin with Spring WebFlux, you need a basic Spring Boot application with the spring-boot-starter-webflux dependency, which enables support for reactive programming.

Example: Add Dependencies in pom.xml (Maven)

xml

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<dependencies>    <!-- Spring WebFlux Starter for reactive programming -->    <dependency>        <groupId>org.springframework.boot</groupId>        <artifactId>spring-boot-starter-webflux</artifactId>    </dependency>    <!-- Optionally add a reactive database dependency, such as MongoDB or R2DBC -->    <dependency>        <groupId>org.springframework.boot</groupId>        <artifactId>spring-boot-starter-data-mongodb-reactive</artifactId>    </dependency> </dependencies>

For Gradle, the dependencies would look like this:

Once you’ve added the dependencies and created a Spring Boot application, Spring Boot will automatically configure WebFlux for you.

2. Creating a Reactive REST Controller

In Spring WebFlux, you can create REST endpoints using reactive types like Mono and Flux. A **Mono** represents a sequence of 0 or 1 item (asynchronous response), while a **Flux** represents a sequence of 0 to N items (asynchronous stream).

Let’s see how to create a basic reactive REST controller that returns both a single item (using Mono) and a stream of items (using Flux).

Example: A Reactive Controller for a User Resource

In this example:

• The getUser method returns a Mono<User>, which represents a single asynchronous response. The user is fetched based on an id passed via the URL.
• The getAllUsers method returns a Flux<User>, representing a stream of User objects, which can be processed asynchronously, one by one.

3. Creating the Service Layer

The service layer typically contains the business logic and data retrieval logic. You can use reactive repositories (e.g., ReactiveMongoRepository) to fetch data from reactive databases such as MongoDB or R2DBC.

Example: Reactive Service for Fetching User Data

In this example:

• getUserById(String id) returns a Mono<User>, which asynchronously fetches a single user from the database (or any other data source).
• getAllUsers() returns a Flux<User>, which asynchronously fetches all users.

4. Handling Reactive Database Access

For database operations, we can use reactive repositories that return Mono or Flux. If you are using MongoDB, for example, Spring Data provides ReactiveMongoRepository for reactive access to MongoDB.

Example: Reactive MongoDB Repository

In this repository:

• findByUsername(String username) returns a Mono<User>, which is used when searching for a user by a unique identifier (like a username).
• findAllByAgeGreaterThan(int age) returns a Flux<User>, which is used for retrieving a stream of users that match a given condition.

5. Testing a Reactive Controller

Testing reactive controllers can be done using WebTestClient, a reactive alternative to MockMvc. With WebTestClient, you can send HTTP requests and validate the responses asynchronously.

Example: Testing the Reactive Controller

In this test:

• testGetUser tests the getUser endpoint by sending a GET request to /users/1 and validating the response.
• testGetAllUsers tests the getAllUsers endpoint by sending a GET request to /users and checking the list of users returned.

6. Error Handling in Reactive Controllers

In reactive applications, you can handle errors in a non-blocking way using operators like onErrorResume, onErrorReturn, and doOnError.

Example: Handling Errors in the Controller

In this example, if the user is not found, a custom UserNotFoundException is thrown, which can be handled globally with an exception handler.

Conclusion

Creating a reactive REST controller in Spring WebFlux is straightforward, thanks to the powerful abstractions provided by Mono and Flux. By using these types, you can build non-blocking, asynchronous REST APIs that scale efficiently and handle high loads with ease. The reactive approach also integrates well with reactive data sources, such as MongoDB or R2DBC, enabling a fully reactive application from the frontend to the database.

With the help of reactive controllers and the WebTestClient, you can build, test, and maintain robust, non-blocking RESTful services that make optimal use of system resources.