How do you create a REST client using WebClient in Spring Boot?

Table of Contents

Introduction

In modern Spring Boot applications, WebClient is the preferred tool for creating REST clients that need to make asynchronous or non-blocking HTTP requests. Unlike the traditional RestTemplate, which is synchronous, WebClient is part of the Spring WebFlux module and supports both synchronous and asynchronous operations, making it ideal for scalable applications that require high concurrency.

This guide will walk you through how to set up and use WebClient to create a REST client in a Spring Boot application, explaining both basic and advanced usage.

1. Setting Up WebClient in Spring Boot

Step 1: Add WebFlux Dependency

To use WebClient, you need to include the spring-boot-starter-webflux dependency in your pom.xml file. This starter provides all necessary components for using WebClient in your application.

This will bring in the necessary libraries to work with WebClient and make HTTP requests.

2. Create a WebClient Bean

To make WebClient available throughout your Spring Boot application, define it as a @Bean in a configuration class. This is typically done in the @Configuration class of your Spring Boot application.

Example: Configuring WebClient Bean

In this setup:

  • WebClient.Builder is provided as a bean so it can be injected anywhere in the application.

3. Using WebClient for Making API Calls

Now that you've configured WebClient, you can start making asynchronous HTTP requests to external APIs.

Step 1: Inject WebClient into a Service

You can inject the WebClient.Builder into your service classes to create WebClient instances.

Example: Basic GET Request with WebClient

Explanation:

  • **baseUrl(apiUrl)**: Sets the base URL for the requests.
  • **get()**: Specifies the HTTP method (GET).
  • **retrieve()**: Starts the request and prepares the response for handling.
  • **bodyToMono(String.class)**: Maps the response body to a Mono of type String.

Step 2: Subscribe to the Mono to Get the Result

Since WebClient is non-blocking, you need to subscribe to the Mono returned by the API call to actually perform the HTTP request and process the response.

In this example:

  • **subscribe()** triggers the request and processes the response once it's received.
  • **doOnTerminate()** is used to print a message after the request completes, regardless of success or failure.

4. Handling Responses and Errors

WebClient allows you to handle both successful responses and errors in a convenient way.

Example: Handling Errors with WebClient

Explanation:

  • **onStatus(HttpStatus::is4xxClientError, response -> Mono.error(...))**: Handles client errors (4xx status codes).
  • **onStatus(HttpStatus::is5xxServerError, response -> Mono.error(...))**: Handles server errors (5xx status codes).
  • **doOnError()**: Logs or handles errors during the request lifecycle.

5. Sending POST Requests with WebClient

In addition to GET requests, WebClient supports sending POST requests to submit data to external APIs.

Example: Sending a POST Request with WebClient

Explanation:

  • **post()**: Specifies the HTTP method (POST).
  • **body(BodyInserters.fromValue(requestBody))**: Sends the provided object in the body of the POST request.
  • **bodyToMono(String.class)**: Converts the response body into a Mono of type String.

6. Configuring Timeouts and Retries

When working with external APIs, it’s important to set up timeout and retry logic to handle slow or failed requests.

Example: Configuring Timeouts for WebClient

Explanation:

  • **responseTimeout(Duration.ofSeconds(5))**: Sets the timeout for receiving a response from the server.
  • **CONNECT_TIMEOUT_MILLIS**: Sets the timeout for establishing a connection.

Conclusion

WebClient is a powerful tool for creating REST clients in Spring Boot applications, enabling asynchronous and non-blocking HTTP requests to external APIs. By setting up the WebClient correctly, you can easily handle different HTTP methods (GET, POST), process responses, manage errors, and configure timeouts for your API interactions. For more advanced use cases, WebClient’s fluent API allows fine-grained control over the request and response cycle, making it an excellent choice for modern, high-concurrency applications.

Share
Similar Questions