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

Table of Contents

• Introduction
• Steps to Create a Reactive REST API with Spring WebFlux
  • 1. Setting Up Spring Boot with Spring WebFlux
    • Maven Configuration:
    • Gradle Configuration:
  • 2. Creating a Reactive Controller
    • Example: Simple Reactive REST API
    • Output:
  • 3. Using **Flux** for Multiple Items
    • Example: Returning a List of Items
    • Output:
  • 4. Handling Asynchronous Data (Non-blocking)
    • Example: Fetching Data Asynchronously from a Remote Service
    • Example: Calling the External Service in a Controller
  • 5. Error Handling in Reactive APIs
    • Example: Handling Errors with onErrorResume
    • Output:
  • 6. Customizing the API with Path Variables and Query Parameters
    • Example: Path Variables and Query Parameters
    • Output:
• Conclusion

Introduction

Spring WebFlux is a reactive programming framework for building non-blocking, asynchronous applications in Java. It was introduced in Spring 5 and provides an alternative to traditional Spring MVC, enabling highly scalable applications that handle a large number of concurrent requests without blocking the server's threads.

In this guide, we will walk through the process of creating a reactive REST API using Spring WebFlux in Spring Boot. We will leverage Mono and Flux to represent asynchronous streams of data, providing a powerful way to handle requests and responses in a non-blocking manner.

Steps to Create a Reactive REST API with Spring WebFlux

1. Setting Up Spring Boot with Spring WebFlux

To start building a reactive REST API with Spring WebFlux, you need to include the necessary dependencies in your Spring Boot project. If you’re using Maven, add the spring-boot-starter-webflux dependency to your pom.xml file:

Maven Configuration:

This dependency includes Spring WebFlux and Project Reactor for building reactive applications. If you're using Gradle, you can add the following dependency:

Gradle Configuration:

2. Creating a Reactive Controller

In a traditional Spring MVC application, a controller method might return a simple String, a ModelAndView, or some other object. In Spring WebFlux, the controller method typically returns a **Mono** or **Flux**, which are the reactive types for handling asynchronous data.

• **Mono**: Represents a single asynchronous item (0..1).
• **Flux**: Represents a stream of asynchronous items (0..N).

Here’s how to create a reactive controller that handles HTTP GET requests asynchronously:

Example: Simple Reactive REST API

• **Mono.just("Hello, World!")**: This returns a single value asynchronously inside a Mono. This is a basic example, where we simply return a greeting message in response to a GET request.
• **@RestController**: Indicates that this class is a REST controller where all methods return data directly (usually as JSON or XML).
• **@RequestMapping("/api")**: Specifies the base path for all the routes defined in the controller.

Output:

When you access /api/hello, it will return:

3. Using **Flux** for Multiple Items

In reactive programming, if you expect a sequence of items (rather than just a single item), you should use Flux. For example, if you wanted to return a list of items as a stream, you could do the following:

Example: Returning a List of Items

• **Flux.just(1, 2, 3, 4, 5)**: This returns a stream of integers asynchronously, where each item will be emitted one by one.

Output:

When you access /api/numbers, it will return the sequence:

4. Handling Asynchronous Data (Non-blocking)

One of the major benefits of reactive programming is that it allows you to handle large volumes of requests without blocking the server's threads. This is particularly useful when dealing with I/O-bound operations, such as making REST API calls, reading from a database, or accessing files.

You can leverage Mono and Flux for asynchronous data fetching.

Example: Fetching Data Asynchronously from a Remote Service

Imagine you want to fetch data from an external API asynchronously. You can use WebClient (which is also non-blocking) to call the remote service:

• **WebClient** is a non-blocking HTTP client that allows you to make asynchronous HTTP requests.
• **Mono<String>**: This returns the fetched data asynchronously.

Example: Calling the External Service in a Controller

When you access /api/post?id=1, it fetches data asynchronously from the external service.

5. Error Handling in Reactive APIs

In Spring WebFlux, you can use reactive operators like **onErrorReturn**, **onErrorResume**, and **doOnError** to handle errors in a non-blocking way.

Example: Handling Errors with onErrorResume

• **Mono.error()**: Creates a Mono that immediately emits an error.
• **onErrorResume()**: Handles the error and provides a fallback value (in this case, a string message).

Output:

If you access /api/error, it will return:

6. Customizing the API with Path Variables and Query Parameters

You can also customize your reactive REST API to handle dynamic path variables and query parameters:

Example: Path Variables and Query Parameters

In this example:

• **@PathVariable** captures the dynamic part of the URL (name).
• **@RequestParam** captures the query parameter (greeting).

Output:

When you access /api/greet/John?greeting=Hello, it will return:

Conclusion

Creating a reactive REST API with Spring WebFlux is straightforward, and it allows you to build non-blocking, asynchronous services that scale efficiently. By using **Mono** and **Flux**, you can handle single and multiple asynchronous data streams with ease. With features like WebClient, error handling, and customized routing, Spring WebFlux is an ideal framework for building high-performance, reactive APIs. Whether you're fetching data from external services, dealing with large volumes of requests, or handling errors in a non-blocking manner, Spring WebFlux provides the necessary tools for efficient, reactive programming.