How do you create reactive endpoints in Spring WebFlux?

Table of Contents

• Introduction
  • What Are Reactive Endpoints in Spring WebFlux?
  • Creating Reactive Endpoints in Spring WebFlux
    • Step 1: Set Up the Project
    • Step 2: Create a Reactive Controller
    • Step 3: Running the Application
  • Advanced Example: Creating Reactive Endpoints with a Database
    • Step 1: Add MongoDB Dependency
    • Step 2: Define a Reactive Repository
    • Step 3: Create a Reactive Service
    • Step 4: Create Reactive Endpoints with MongoDB
  • Practical Example: Reactively Fetching a List of Products
• Conclusion

Introduction

In reactive programming, handling asynchronous data and building non-blocking web applications can significantly enhance performance and scalability. Spring WebFlux provides an effective way to create reactive endpoints that handle requests asynchronously using Mono and Flux, which represent reactive data streams. These reactive types allow you to process requests in a non-blocking manner, enabling the application to scale better under high concurrency scenarios.

Creating reactive endpoints in Spring WebFlux is straightforward and involves using annotations like @GetMapping, @PostMapping, and returning Mono or Flux instead of traditional objects. This allows Spring WebFlux to handle data asynchronously, without blocking threads, making your application more responsive and scalable.

In this guide, we'll walk through how to create reactive endpoints in Spring WebFlux using Mono and Flux.

What Are Reactive Endpoints in Spring WebFlux?

A reactive endpoint is a web endpoint in a Spring WebFlux application that processes requests asynchronously and returns data in a reactive stream format. Rather than returning standard data types (e.g., String, List, or Map), a reactive endpoint returns Mono<T> or Flux<T>:

• Mono: Represents a reactive stream that can emit 0 or 1 item.
• Flux: Represents a reactive stream that can emit 0 to N items.

These types enable Spring WebFlux to handle web requests in a non-blocking and event-driven manner, optimizing server resources by freeing up threads that would typically be blocked while waiting for I/O operations to complete.

Creating Reactive Endpoints in Spring WebFlux

Step 1: Set Up the Project

To create reactive endpoints in Spring WebFlux, first, ensure that your Spring Boot project has the necessary dependencies.

Add the spring-boot-starter-webflux dependency to your pom.xml (Maven) or build.gradle (Gradle):

Maven:

Gradle:

This will include Spring WebFlux and all required libraries to enable the creation of reactive web applications.

Step 2: Create a Reactive Controller

A reactive controller in Spring WebFlux uses annotations like @RestController and returns reactive data types (Mono and Flux) from the endpoint methods. Here's how you can create a simple reactive controller with Mono and Flux:

In this example:

• The greet() endpoint returns a single string wrapped in a Mono.
• The getNumbers() endpoint returns a stream of integers wrapped in a Flux.
• The getProductById() endpoint retrieves a Product object based on a path variable and wraps it in a Mono.

Step 3: Running the Application

When you run the Spring Boot application, the endpoints defined in the ReactiveController will be exposed, and Spring WebFlux will handle them asynchronously. The responses will be returned as reactive streams, which means the server will not block waiting for responses but will allow other requests to be processed concurrently.

Advanced Example: Creating Reactive Endpoints with a Database

You can also integrate reactive endpoints with reactive databases like MongoDB or R2DBC (for relational databases). Here's an example using Spring Data MongoDB to fetch products from a MongoDB database reactively.

Step 1: Add MongoDB Dependency

Add the Spring Data MongoDB dependency to your project:

Maven:

Gradle:

Step 2: Define a Reactive Repository

Spring Data MongoDB allows you to create a reactive repository interface that handles non-blocking database operations. Here’s how you can define a repository for Product:

Step 3: Create a Reactive Service

Next, you’ll create a service that interacts with the database through the reactive repository:

Step 4: Create Reactive Endpoints with MongoDB

Finally, you can create reactive endpoints in your controller to fetch data from MongoDB:

Practical Example: Reactively Fetching a List of Products

Assume you have a Product class with fields id and name, and a database that stores products. Here’s how a complete example might look:

The above code represents a Product document stored in MongoDB. The ProductController exposes endpoints that return products in a reactive manner. For example, the /products endpoint would return a stream of products.

Conclusion

Creating reactive endpoints in Spring WebFlux allows you to handle web requests asynchronously and efficiently. By using Mono and Flux, you can return asynchronous data streams from your endpoints, ensuring that your application remains non-blocking and can scale effectively to handle many concurrent requests.

Whether you're building simple APIs or integrating with reactive databases, Spring WebFlux makes it easy to create reactive web applications. By embracing this approach, you can significantly enhance the performance and responsiveness of your application, especially under heavy load.

Reactive programming with Spring WebFlux provides powerful features like backpressure handling, asynchronous processing, and efficient memory usage. If you're building modern web services that require high concurrency, WebFlux is a perfect fit.