Spring Boot has revolutionized Java development, especially when it comes to building RESTful services and clients. While RestTemplate has been the go-to choice for such operations, Spring introduced WebClient with its reactive stack, offering a more powerful and flexible way to make HTTP requests. In this post, we'll create a simple REST API and then consume it using WebClient, demonstrating how to pass parameters to our API.

Creating a Simple REST API

Firstly, let's set up a basic Spring Boot application with a REST controller. We'll have a simple endpoint that accepts path parameters.

Step 1: Setting Up the Spring Boot Application

Create a Spring Boot project using Spring Initializr with the following Maven dependency:

 <dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-web</artifactId> </dependency>

Step 2: Creating the REST Controller

Create a UserController with an endpoint that retrieves a user's details based on the user ID provided as a path variable.

UserController.java:

import org.springframework.http.ResponseEntity; import org.springframework.web.bind.annotation.*; @RestController @RequestMapping("/api/users") public class UserController { @GetMapping("/{id}") public ResponseEntity<User> getUserById(@PathVariable Long id) { // For demonstration, let's mock a user lookup. User user = new User(id, "John Doe", "johndoe@example.com"); return ResponseEntity.ok(user); } // Nested class for User private static class User { private Long id; private String name; private String email; // Constructor, getters, and setters } }

Consuming the API with WebClient

With our REST API ready, let's consume it using WebClient. We’ll make a GET request to retrieve a user by their ID, passing the ID as a parameter.

Step 1: Adding WebClient Dependency

To use WebClient, you need to add the spring-boot-starter-webflux dependency:

 <dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-webflux</artifactId> </dependency>

Step 2: Configuring WebClient

Create a configuration class to provide a WebClient bean.

WebClientConfig.java:

import org.springframework.context.annotation.Bean; import org.springframework.context.annotation.Configuration; import org.springframework.web.reactive.function.client.WebClient; @Configuration public class WebClientConfig { @Bean public WebClient webClient() { return WebClient.builder() .baseUrl("http://localhost:8080/api/users") .build(); } }

Step 3: Consuming the API

Create a service class that uses WebClient to call the API.

UserServiceClient.java:

import org.springframework.beans.factory.annotation.Autowired; import org.springframework.stereotype.Service; import org.springframework.web.reactive.function.client.WebClient; import reactor.core.publisher.Mono; @Service public class UserServiceClient { private final WebClient webClient; @Autowired public UserServiceClient(WebClient webClient) { this.webClient = webClient; } public Mono<User> getUserById(Long id) { return webClient.get() .uri("/{id}", id) .retrieve() .bodyToMono(User.class); } // Nested class for User to match the API's response private static class User { private Long id; private String name; private String email; // Constructor, getters, and setters } }

In getUserById, we're using a path variable to pass the user ID to our GET request.

Step 4: Using the Client Service

Now, let's make use of our UserServiceClient to make a call to the REST API. We could do this in a CommandLineRunner bean for demonstration purposes.

DemoApplicationRunner.java:

import org.springframework.boot.CommandLineRunner; import org.springframework.stereotype.Component; @Component public class DemoApplicationRunner implements CommandLineRunner { private final UserServiceClient userServiceClient; public DemoApplicationRunner(UserServiceClient userServiceClient) { this.userServiceClient = userServiceClient; } @Override public void run(String... args) { userServiceClient.getUserById(1L) .subscribe(user -> System.out.println("Retrieved User: " + user)); } }

The subscribe method is part of the reactive paradigm and here it's simply used to print the user details to the console. Remember, in a fully reactive environment, you'd return the Mono<User> from your web endpoint instead of subscribing to it.

Conclusion

WebClient is a powerful part of Spring's reactive stack that allows for robust and efficient HTTP communication. By following the above steps, you can quickly set up a REST API in Spring Boot and use WebClient to consume it, making use of path variables to pass parameters.

Source Code Examples

Search This Blog

Spring Boot WebClient GET Request with Parameters

Creating a Simple REST API

Step 1: Setting Up the Spring Boot Application

Step 2: Creating the REST Controller

UserController.java:

Consuming the API with WebClient

Step 1: Adding WebClient Dependency

Step 2: Configuring WebClient

WebClientConfig.java:

Step 3: Consuming the API

UserServiceClient.java:

Step 4: Using the Client Service

DemoApplicationRunner.java:

Conclusion

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Related Spring Boot Source Code Examples

Spring Core Annotations with Examples

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