How do you use WebClient to consume RESTful services?

Table of Contents

• Introduction
  • Setting Up WebClient in Your Spring Application
    • 1. Add WebFlux Dependency
    • 2. Create a WebClient Bean (Optional)
  • Consuming RESTful Services with WebClient
    • 1. Sending a GET Request
      • Example: GET Request to Retrieve Data
    • 2. Sending a POST Request
      • Example: POST Request to Create a New Post
    • 3. Sending a PUT Request
      • Example: PUT Request to Update a Post
    • 4. Sending a DELETE Request
      • Example: DELETE Request to Delete a Post
  • Handling Responses
    • 1. Handling Status Codes
      • Example: Handling 4xx and 5xx Errors
    • 2. Handling JSON Responses
      • Example: Mapping JSON to Java Object
• Conclusion

Introduction

Spring WebClient is a non-blocking, reactive HTTP client used for making HTTP requests in Spring applications. Unlike the traditional RestTemplate, WebClient provides a more modern approach to consuming RESTful APIs, especially in reactive applications built with Spring WebFlux. This tutorial will explain how to use WebClient to interact with RESTful services, including performing GET, POST, PUT, and DELETE requests, and handling JSON responses.

Setting Up WebClient in Your Spring Application

1. Add WebFlux Dependency

To use WebClient, you need to include the spring-boot-starter-webflux dependency in your pom.xml or build.gradle file. This starter includes everything needed for Spring WebFlux, including WebClient.

For Maven:

For Gradle:

2. Create a WebClient Bean (Optional)

While you can create a WebClient instance directly in your service classes, it's a good practice to configure it as a bean in your Spring context for reusability.

Now, you can inject WebClient.Builder into any service or controller class to create a WebClient instance.

Consuming RESTful Services with WebClient

1. Sending a GET Request

To fetch data from a RESTful API using WebClient, you can use the get() method to create a GET request. You can chain additional methods like uri() and retrieve() to customize the request and handle the response.

Example: GET Request to Retrieve Data

In this example, we send a GET request to retrieve a post by its ID. The bodyToMono(Post.class) method converts the response body to a Mono<Post>, which is part of the reactive programming model in Spring WebFlux.

2. Sending a POST Request

To send data to the server, you can use the post() method. This is often used for creating new resources or submitting data.

Example: POST Request to Create a New Post

Here, bodyValue(newPost) sends a Post object as the body of the request. The server responds with the newly created post, and bodyToMono(Post.class) maps the response back to a Post object.

3. Sending a PUT Request

The put() method is used to update an existing resource on the server.

Example: PUT Request to Update a Post

This example sends a PUT request to update the post with the given postId. The updatedPost object is sent as the request body, and the response is mapped to the Post object.

4. Sending a DELETE Request

To delete a resource on the server, use the delete() method.

Example: DELETE Request to Delete a Post

In this case, the deletePost() method sends a DELETE request to remove the post with the specified postId. Since DELETE requests often don’t return data, we use Mono<Void> to represent the absence of a response body.

Handling Responses

1. Handling Status Codes

WebClient allows you to handle different HTTP status codes using methods like onStatus(). You can define custom behavior for different types of errors, such as client-side errors (4xx) or server-side errors (5xx).

Example: Handling 4xx and 5xx Errors

In this example, if the HTTP status code is a 4xx or 5xx, a custom exception is thrown.

2. Handling JSON Responses

WebClient automatically converts JSON responses to Java objects if you specify the class type with methods like bodyToMono() or bodyToFlux(). You can also use exchangeToMono() for more advanced response handling.

Example: Mapping JSON to Java Object

Here, the JSON response from the API is converted into a User object.

Conclusion

Using WebClient in Spring WebFlux to consume RESTful services is straightforward and efficient for reactive applications. It provides a non-blocking, asynchronous way to interact with APIs and is highly flexible for handling different HTTP methods (GET, POST, PUT, DELETE). By using WebClient, you can take full advantage of reactive programming in Spring WebFlux, making your application more scalable and responsive.