RestTemplate vs WebClient in Spring Boot

In Spring applications, both RestTemplate and WebClient are used for making HTTP requests to external services, but they have different design philosophies and capabilities. Below is a comparison between the two, highlighting their features, differences, and when to use each.

Overview

RestTemplate

• Blocking: RestTemplate is a synchronous (blocking) client for making RESTful HTTP requests.
• Legacy: It has been a part of Spring since version 3.x and is now considered somewhat outdated as Spring encourages the use of WebClient for new projects.
• Simplicity: It provides a simpler way to make HTTP requests and handle responses, which is useful for straightforward applications.

WebClient

• Non-blocking: WebClient is an asynchronous (non-blocking) client introduced in Spring 5 as part of the Spring WebFlux project. It supports reactive programming principles.
• Modern: It is intended for use in modern applications that require high scalability and efficiency, especially when dealing with a large number of concurrent requests.
• Functional-style API: WebClient provides a more flexible and powerful API, supporting request and response handling with functional programming paradigms.

Key Differences

Feature	RestTemplate	WebClient
Nature	Synchronous (blocking)	Asynchronous (non-blocking)
Introduced in	Spring 3.x	Spring 5.0
Reactive Support	No	Yes (supports Project Reactor)
Error Handling	Simple error handling with exceptions	Advanced error handling mechanisms
Performance	Potentially less performant in high-load scenarios due to blocking behavior	Better suited for high concurrent loads
Ease of Use	Simple and straightforward to use	More complex but more flexible
Builder Pattern	Limited to providing basic options	Richer builder pattern with fluent API
Streaming Support	Limited support for streaming	Excellent support for streaming data

When to Use RestTemplate

• Simplicity: If you are building a simple, small-scale application where synchronous calls are sufficient.
• Legacy Applications: If you are maintaining an older application already using RestTemplate.
• Existing Code: If the existing project already uses RestTemplate, and you want to maintain consistency without introducing new patterns.

Example Using RestTemplate

import org.springframework.stereotype.Service;
import org.springframework.web.client.RestTemplate;

@Service
public class MyService {

    private final RestTemplate restTemplate;

    public MyService(RestTemplate restTemplate) {
        this.restTemplate = restTemplate;
    }

    public String getExternalData(String url) {
        return restTemplate.getForObject(url, String.class);
    }
}

When to Use WebClient

• Reactive Applications: If you are building a reactive application using Spring WebFlux, where non-blocking calls are essential.
• High Concurrency: If your application requires handling a large number of concurrent requests efficiently.
• Streaming Data: If you need to work with streaming data or Server-Sent Events (SSE).
• Asynchronous Processing: If you want to take advantage of reactive programming paradigms for more complex asynchronous workflows.

Example Using WebClient

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

@Service
public class MyService {

    private final WebClient webClient;

    public MyService(WebClient.Builder webClientBuilder) {
        this.webClient = webClientBuilder.baseUrl("http://api.example.com").build();
    }

    public Mono<String> getExternalData(String endpoint) {
        return webClient.get()
                .uri(endpoint)
                .retrieve()
                .bodyToMono(String.class);
    }
}

Additional Features of WebClient

• Error Handling: WebClient provides more advanced error handling capabilities, allowing you to handle different response statuses.
• Request Composition: It supports functional-style request composition, enabling you to chain calls and handle response transformations more elegantly.
• Timeouts: You can configure timeouts and other request settings easily.

Conclusion

While both RestTemplate and WebClient serve a similar purpose in making HTTP requests, your choice should depend on the requirements of your application:

• Use RestTemplate for simpler, synchronous scenarios or when maintaining legacy code.
• Use WebClient for modern, reactive applications requiring asynchronous processing and better scalability.

As of Spring 5, WebClient is the recommended approach for new applications, particularly those that leverage reactive programming. It provides greater flexibility, performance, and support for modern web application needs.