Imagine Your application requires data from an external REST API to function. Describe how you would use RestTemplate or WebClient to consume the REST API in your Spring Boot application.

Question

Accepted Answer

RestTemplate and WebClient are the two main options for consuming an external REST API from Spring Boot -- RestTemplate is the classic, blocking client, and WebClient is the modern, non-blocking, reactive client.

Key Points:
• RestTemplate is defined as a @Bean and called with methods like getForObject() or exchange(), blocking the calling thread until the response arrives.
• WebClient is built with WebClient.builder() and used with a fluent chain: get(), uri(), retrieve().
• WebClient returns Mono (single value) or Flux (stream of values) instead of a raw object, fitting reactive pipelines.
• RestTemplate is now in maintenance mode; new code, especially anything performance-sensitive, should prefer WebClient or the newer RestClient.
• Both need timeout and error-handling configuration to avoid hanging on a slow or failing downstream service.

Example:
Fetching a user profile from an external service with RestTemplate returns a UserDto directly, while the same call with WebClient returns a Mono that you subscribe to or map further downstream.

Code Example:
```
// RestTemplate
UserDto user = restTemplate.getForObject("/api/users/{id}", UserDto.class, id);

// WebClient
Mono user = webClient.get()
        .uri("/api/users/{id}", id)
        .retrieve()
        .bodyToMono(UserDto.class);
```

Interview Tip:
A concise interview answer is:

"For a blocking call I'd define a RestTemplate bean and use getForObject() or exchange(). For a non-blocking, reactive flow I'd use WebClient, chaining get().uri().retrieve() and mapping the result with bodyToMono() or bodyToFlux(). Since RestTemplate is in maintenance mode, I'd default to WebClient for new code."