How do you implement a reactive web client in Spring WebFlux?

Table of Contents

Introduction

In Spring WebFlux, **WebClient** is the primary class used to make reactive, non-blocking HTTP requests. It is a part of the Spring WebFlux module and provides a fully reactive approach to interact with web services. Unlike the traditional **RestTemplate**, which is synchronous and blocking, **WebClient** supports non-blocking I/O and is designed to be used in reactive applications.

This guide demonstrates how to implement a reactive web client using Spring WebFlux with **WebClient** for making asynchronous HTTP requests in a reactive environment.

1. Setting Up Spring WebFlux and WebClient

Before you start implementing a reactive web client, make sure that Spring WebFlux is included as a dependency in your Spring Boot project.

Maven Dependency for Spring WebFlux:

Gradle Dependency for Spring WebFlux:

Spring WebFlux will automatically configure the **WebClient** bean for you when you add this dependency. However, if you want to manually configure it, you can define the **WebClient.Builder** in a configuration class.

Example of WebClient Configuration:

2. Using **WebClient** to Make HTTP Requests

Once the setup is complete, you can use the **WebClient** class to make HTTP requests. Below are some basic examples to demonstrate how to create a **WebClient** instance and use it to perform various types of HTTP operations (GET, POST, etc.).

Example: Making a GET Request

In this example:

  • We create a **WebClient** instance with the base URL **https://jsonplaceholder.typicode.com**.
  • The **get()** method is used to send a GET request to **/todos/1**.
  • The **retrieve()** method initiates the request.
  • **bodyToMono()** specifies that we want to retrieve the response body as a **Mono<String>** (since it's a single value response).

Example: Making a POST Request

For POST requests, you can use the **post()** method on the **WebClient** instance.

Here:

  • We use **post()** to send a POST request to the endpoint **/todos**.
  • **bodyValue()** is used to set the request body (a **Todo** object in this case).
  • The response body is again retrieved using **bodyToMono(String.class)**, but you can deserialize the response to any Java object you prefer.

3. Handling Responses and Errors

WebClient also provides a way to handle different kinds of HTTP responses and errors in a reactive manner.

Handling Successful Responses:

You can use **onStatus()** to check for specific status codes in the response and handle them accordingly.

In this case, if the server responds with a 4xx or 5xx error, the **onStatus()** handler is invoked, and the error is propagated as a Mono.error.

Handling Empty Responses:

If the response body might be empty, you can use **switchIfEmpty()** to provide a default value.

In this example:

  • If the **/todos/1000** endpoint returns an empty body, the **switchIfEmpty()** operator ensures that a default value is returned.

4. Making Concurrent Requests

You can also make multiple concurrent requests using **WebClient**. Project Reactor makes this easy with operators like **flatMap** or **zip** for combining multiple requests.

Example of Concurrent Requests with flatMap:

Here, both requests are made concurrently, and the results are combined using **Mono.zip()**.

5. Customizing WebClient

You can also customize **WebClient** by adding common configurations like headers, timeouts, and authentication for all requests.

Example of Custom Headers and Timeout:

This custom **WebClient** will include the Authorization header in all requests and set a response timeout.

Conclusion

The **WebClient** in Spring WebFlux is a powerful and flexible tool for building reactive web clients that handle non-blocking HTTP requests in a reactive application. By using **WebClient**, you can:

  • Make GET, POST, and other types of HTTP requests asynchronously.
  • Handle responses and errors reactively.
  • Execute multiple requests concurrently with operators like **flatMap** and **Mono.zip()**.
  • Customize request headers, timeouts, and other settings to meet your application's needs.

By leveraging **WebClient**, you can integrate with remote services in a way that fully embraces the principles of reactive programming, ensuring scalability and responsiveness in your reactive applications.

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