How do you configure WebFlux in a Spring Boot application?

Table of Contents

• Introduction
  • 1. Setting Up Spring Boot with WebFlux
    • Step 1: Add Dependencies
    • Step 2: Application Properties
  • 2. Create a Reactive Controller with WebFlux
    • Step 3: Create a Reactive Controller
  • 3. Testing the Application
    • Step 4: Run the Application
    • Step 5: Verify the Reactive Endpoint
  • 4. Configuring the Reactive Web Server
    • Example: Switching to Jetty
  • 5. Configuring Reactive Database Access (Optional)
  • 6. Customizing WebFlux Configuration
    • Example: Customizing WebFlux Configuration
• Conclusion

Introduction

Spring WebFlux is a framework for building reactive, non-blocking, and asynchronous web applications in Java. It is a part of the Spring Framework and provides an alternative to the traditional Spring MVC for building scalable web services and APIs. With Spring WebFlux, you can develop applications that handle large numbers of concurrent connections efficiently.

In this guide, we’ll walk through how to configure Spring WebFlux in a Spring Boot application, set up the necessary dependencies, and create a simple reactive controller to get you started.

1. Setting Up Spring Boot with WebFlux

To get started with Spring WebFlux in a Spring Boot application, you’ll need to include the appropriate dependencies in your project. WebFlux is available as part of the Spring Boot starter-webflux dependency.

Step 1: Add Dependencies

If you’re using Maven, add the spring-boot-starter-webflux dependency in your pom.xml:

For Gradle, add the dependency in your build.gradle:

This will include all necessary components for WebFlux, such as Reactor, which is used for reactive programming in Spring.

Step 2: Application Properties

For most applications, no specific configuration is needed in the application.properties or application.yml for WebFlux to work, as it will use sensible defaults. However, you can customize the server, port, or other settings related to WebFlux.

For example, if you want to run the application on a custom port, you can specify the port in application.properties:

If you need to configure the reactive server (e.g., using Netty instead of the default Tomcat), you can add the following property:

2. Create a Reactive Controller with WebFlux

Once WebFlux is set up, you can start creating reactive controllers. A reactive controller is similar to a traditional Spring MVC controller but returns a **Mono** or **Flux** object instead of a direct value or view.

• Mono: Represents a stream of 0 or 1 item.
• Flux: Represents a stream of 0 to N items.

Here’s a basic example of a reactive controller in Spring WebFlux.

Step 3: Create a Reactive Controller

In this example:

• The hello() method returns a Mono<String>, which will emit the string "Hello, Spring WebFlux!".
• When a request is made to /hello, Spring WebFlux handles it asynchronously and returns the response without blocking.

3. Testing the Application

Step 4: Run the Application

You can now run your Spring Boot application using the mvn spring-boot:run (for Maven) or gradle bootRun (for Gradle) command, or by running the main class with the @SpringBootApplication annotation.

Step 5: Verify the Reactive Endpoint

Once your application is running, open a browser or use an HTTP client (like Postman or curl) to test the /hello endpoint:

You should receive the response:

This shows that your Spring Boot application is working with Spring WebFlux to handle reactive endpoints.

4. Configuring the Reactive Web Server

Spring Boot provides support for both blocking and non-blocking web servers. By default, when you add the spring-boot-starter-webflux dependency, Netty is used as the non-blocking server. You can also use Undertow or Jetty as the reactive web server.

If you need to switch from the default Netty server to another server like Jetty, you can add the corresponding dependency.

Example: Switching to Jetty

Once the Jetty starter is added, Spring Boot will use Jetty instead of Netty as the reactive server.

5. Configuring Reactive Database Access (Optional)

If your application needs to connect to a reactive database (e.g., MongoDB, R2DBC), you can add the corresponding starter and configure the connection properties.

For reactive MongoDB, you would add the dependency like this:

Then, configure the MongoDB connection in application.properties:

Similarly, for R2DBC, you would add the spring-boot-starter-data-r2dbc dependency and configure the connection.

6. Customizing WebFlux Configuration

Spring WebFlux also allows you to customize its configuration via the WebFluxConfigurer interface. This is useful for modifying the request handling behavior, setting up custom filters, or enabling CORS.

Example: Customizing WebFlux Configuration

Conclusion

Configuring Spring WebFlux in a Spring Boot application is straightforward, especially when you use Spring Boot's auto-configuration features. After adding the necessary dependencies (like spring-boot-starter-webflux), you can start building reactive controllers that return Mono or Flux types, representing asynchronous responses. This makes it easy to create scalable, non-blocking, and reactive web applications in Spring Boot.

By using Netty or another non-blocking server and implementing reactive databases like MongoDB or R2DBC, you can fully leverage the power of reactive programming in your Spring Boot applications. With Spring WebFlux, your app will be well-suited for handling many concurrent users without sacrificing performance.