NUHMAN.com

In a Spring Boot application, calling an external REST API is a common task. Depending on whether your application is synchronous or reactive, Spring provides two main tools for this: RestTemplate and WebClient. Here’s a comprehensive guide to both, with examples.

🥇 Option 1: Using RestTemplate (Synchronous, Legacy-Friendly)

✅ When to Use:

• For simple, blocking HTTP requests
• In traditional Spring MVC applications

🔧 Step 1: Add Dependency

Include the Spring Web dependency in your pom.xml:

xml

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

🧱 Step 2: Define a RestTemplate Bean

java

@Configuration
public class AppConfig {

    @Bean
    public RestTemplate restTemplate() {
        return new RestTemplate();
    }
}

💡 Step 3: Call the API

java

@Service
public class ApiService {

    @Autowired
    private RestTemplate restTemplate;

    public String getDataFromApi() {
        String url = "https://api.example.com/data";
        ResponseEntity<String> response = restTemplate.getForEntity(url, String.class);
        return response.getBody();
    }
}

• getForEntity() performs a GET request
• getBody() extracts the actual response

🚀 Option 2: Using WebClient (Asynchronous & Reactive)

✅ When to Use:

• In modern apps with non-blocking, asynchronous communication
• If you're using Spring WebFlux

🔧 Step 1: Add WebFlux Dependency

xml

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

🧱 Step 2: Define a WebClient Bean

java

@Configuration
public class AppConfig {

    @Bean
    public WebClient.Builder webClientBuilder() {
        return WebClient.builder();
    }
}

💡 Step 3: Call the API

java

@Service
public class ApiService {

    @Autowired
    private WebClient.Builder webClientBuilder;

    public Mono<String> getDataFromApi() {
        String url = "https://api.example.com/data";
        return webClientBuilder.build()
                               .get()
                               .uri(url)
                               .retrieve()
                               .bodyToMono(String.class);
    }
}

• .retrieve() initiates the call
• .bodyToMono() returns a Mono<String> (a reactive container)

🕹️ Use It in a Controller

java

@RestController
@RequestMapping("/api")
public class ApiController {

    @Autowired
    private ApiService apiService;

    @GetMapping("/data")
    public Mono<String> getData() {
        return apiService.getDataFromApi();
    }
}

🧠 Summary: RestTemplate vs WebClient

FeatureRestTemplateWebClientTypeSynchronous / BlockingAsynchronous / Non-blockingSuitabilityLegacy / Simple use-casesModern / Reactive appsProject TypeSpring MVCSpring WebFlux / modern MVCPerformanceLess scalableHighly scalableRecommended forOlder or simpler appsNew applications

✅ Recommendation:

Use RestTemplate for simple synchronous tasks or legacy projects.

Use WebClient for scalable, modern, and non-blocking applications.