Spring RestTemplate vs WebClient

You’re asking about two different HTTP clients within the Spring ecosystem: RestTemplate and WebClient. They represent different paradigms: RestTemplate is blocking and imperative, while WebClient is non-blocking and reactive.

Spring RestTemplate (Blocking & Imperative)

RestTemplate is Spring’s original HTTP client. It’s built on top of the traditional Servlet API and uses blocking I/O.

Pros of RestTemplate

1. Simplicity and Familiarity:

   • Easy to Use: Its API is straightforward and maps directly to HTTP methods (e.g., getForObject(), postForEntity()).
   • Familiar Paradigm: Developers accustomed to imperative programming find it very intuitive. It blocks until a response is received, which aligns with traditional synchronous code flow.

2. Maturity:

   • It has been around for a long time, is very stable, and has a vast amount of documentation and community support.

3. Debugging:

   • Synchronous execution makes debugging easier, as the call stack follows a linear path.

Cons of RestTemplate

1. Blocking I/O (Main Drawback):

   • When RestTemplate makes an HTTP call, the thread executing that call becomes blocked and waits for the response from the remote server.
   • Scalability Issues: In a high-concurrency environment, this thread-per-request model can lead to a large number of blocked threads, consuming significant memory and CPU resources without doing actual work. This limits the number of concurrent requests your application can efficiently handle.

2. Deprecated:

   • As of Spring Framework 5, RestTemplate is officially in maintenance mode and deprecated in favor of WebClient. While it’s not being removed, new features will not be added, and its use is discouraged for new development.

3. Less Flexible:

   • Its API is less fluent and flexible compared to WebClient for building complex requests or handling streaming data.

Example (RestTemplate)

import org.springframework.web.client.RestTemplate;
import org.springframework.http.ResponseEntity;

public class RestTemplateExample {
    public static void main(String[] args) {
        RestTemplate restTemplate = new RestTemplate();
        String url = "https://jsonplaceholder.typicode.com/posts/1";

        try {
            // This call blocks until the response is received
            ResponseEntity<String> response = restTemplate.getForEntity(url, String.class);
            System.out.println("Response Status: " + response.getStatusCode());
            System.out.println("Response Body: " + response.getBody());
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

Spring WebClient (Non-Blocking & Reactive)

WebClient is Spring’s modern, non-blocking, and reactive HTTP client introduced in Spring 5 as part of the Spring WebFlux framework. It’s built on Project Reactor and is designed to leverage non-blocking I/O.

Pros of WebClient

1. Non-Blocking I/O (Main Advantage):

   • WebClient does not block the executing thread while waiting for an HTTP response. Instead, it leverages a small number of event loop threads to handle many concurrent connections.
   • High Scalability and Efficiency: This makes it highly efficient for I/O-bound operations and enables applications to handle a much larger number of concurrent requests with fewer threads and less memory, especially in microservices or API gateway scenarios.

2. Reactive Programming Model:

   • Integrates seamlessly with Spring WebFlux applications, returning Mono (for 0 or 1 item) or Flux (for 0 to N items). This allows for a completely reactive, end-to-end programming model.
   • Even in a traditional Spring MVC application, WebClient can be used to perform non-blocking calls (though if you immediately .block() the result, you lose most of the non-blocking benefits).

3. Fluent and Functional API:

   • Offers a more expressive, chainable API for building requests, handling responses, and composing asynchronous operations.

4. Streaming Support:

   • Excellent support for streaming requests and responses, Server-Sent Events (SSE), and WebSockets.

5. Future-Proof:

   • It is the recommended HTTP client for all new Spring applications.

Cons of WebClient

1. Steep Learning Curve (Reactive Paradigm):

   • Requires a shift in mindset to reactive programming concepts (e.g., Mono, Flux, operators, publishers, subscribers).
   • Debugging can be more challenging due to the asynchronous and non-linear execution flow.

2. Complexity for Simple Cases:

   • For very simple, blocking-only applications where concurrency is not an issue, the reactive paradigm might feel like overkill and introduce unnecessary complexity.

3. Requires Reactive Downstream:

   • To fully benefit from WebClient’s non-blocking nature, your entire stack (including databases, message queues, and other services) should ideally also be non-blocking. If you make a non-blocking call using WebClient and then immediately hit a blocking database driver, you’re still introducing blocking.

Example (WebClient)

import org.springframework.web.reactive.function.client.WebClient;
import reactor.core.publisher.Mono;

public class WebClientExample {
    public static void main(String[] args) {
        WebClient webClient = WebClient.create("https://jsonplaceholder.typicode.com");

        // This call is non-blocking. It returns a Mono, which is a publisher of 0 or 1 item.
        Mono<String> responseMono = webClient.get()
            .uri("/posts/1")
            .retrieve()
            .bodyToMono(String.class);

        // To see the result in a blocking main method, we block explicitly.
        // In a reactive Spring WebFlux app, you would typically subscribe
        // and return the Mono/Flux directly, letting the framework handle subscription.
        responseMono.subscribe(
            responseBody -> System.out.println("Response Body (non-blocking): " + responseBody),
            error -> System.err.println("Error: " + error.getMessage()),
            () -> System.out.println("Request completed.") // On completion
        );

        // Important: In a real application (e.g., Spring Boot), the thread would stay alive.
        // For a simple main method, we need to block the main thread to allow async operations to complete.
        // DON'T DO THIS IN A REAL REACTIVE APP! It defeats the purpose.
        responseMono.block(); // For demonstration purposes in a main method.
    }
}

When to Use Which

The choice between RestTemplate and WebClient boils down to your application’s architecture, performance requirements, and team’s familiarity with reactive programming.

Use RestTemplate When:

• Existing Legacy Projects: If you’re working on an older Spring MVC application that already uses RestTemplate, and there’s no compelling need to switch, stick with it to avoid unnecessary refactoring.
• Simple, Low-Concurrency Blocking Applications: For very basic applications where performance and scalability under high load are not primary concerns, RestTemplate is straightforward.
• Team Familiarity: If your development team is new to reactive programming and doesn’t have the time or resources to learn Mono/Flux.
• As a stop-gap: While migrating to WebClient in an existing application, you might use RestTemplate for parts not yet converted.

However, be aware of its deprecated status. For any new development, even in blocking applications, WebClient is generally preferred.

Use WebClient When:

• New Projects (Recommended Default): For any new Spring application, especially Spring Boot 2.x and later, WebClient is the recommended default HTTP client.
• Spring WebFlux Applications: If your application is built with Spring WebFlux, WebClient is the natural and necessary choice for interacting with external services in a non-blocking manner.
• High Concurrency & I/O-Bound Workloads:
  • Microservices/API Gateways: When building services that need to handle a large number of concurrent requests with low latency, especially when aggregating data from multiple downstream services.
  • Applications with many external calls: If your service frequently calls other APIs or microservices.
• Asynchronous Operations within Spring MVC: Even in a blocking Spring MVC application, if you want to perform HTTP calls in a non-blocking way (e.g., in a background task using CompletableFuture or Callable), WebClient is a good choice. Crucially, avoid immediately calling .block() on the Mono/Flux in your controller, as that negates the non-blocking benefit.
• Streaming Data: For applications that need to consume or produce streams of data (e.g., Server-Sent Events, large file uploads/downloads).
• Resource Efficiency: When you need to maximize the number of concurrent connections a single instance can handle with minimal hardware resources.
• Reactive Ecosystem: When interacting with other reactive services or databases.

Conclusion

For any new Spring development, WebClient should be your default choice. Its non-blocking nature makes it far more scalable and efficient for modern, I/O-intensive applications, even if your application itself is not fully reactive (as long as you avoid immediate .block() calls).

RestTemplate is largely for maintaining legacy code or very simple, low-concurrency scenarios where the overhead of reactive programming is not justified and its deprecation is accepted. The industry trend and Spring’s direction are firmly towards reactive programming and WebClient.