Why Spring Boot Is Moving From RestTemplate to WebClient

RestTemplate Background and Design

Early Spring Boot projects used RestTemplate as the default HTTP client because it wrapped low‑level clients ( HttpURLConnection, Apache HttpClient), offered a unified API, automatic JSON/XML conversion, and simplified exception handling.

The core design is a synchronous blocking model:

Current thread initiates the HTTP request.

Thread is suspended while waiting for the response.

Thread resumes after the response arrives.

This model is simple and reliable for low‑concurrency applications.

Limitations of Blocking I/O in Modern Microservice Environments

When services communicate frequently (API gateways, inter‑service calls) and traffic becomes highly concurrent, each request occupying a thread leads to:

Rapid thread‑pool exhaustion.

Reduced CPU utilization.

Frequent context switches.

Increased memory consumption.

QPS cannot increase while machine resources are already saturated.

These constraints motivated Spring to adopt a reactive, non‑blocking programming model.

RestTemplate Features and Usage

Supports GET, POST, PUT, DELETE.

Automatic request/response serialization.

Interceptor mechanism.

Low configuration overhead.

Bean Definition

package com.icoderoad.config;

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.boot.web.client.RestTemplateBuilder;
import org.springframework.web.client.RestTemplate;

@Configuration
public class AppConfig {
    @Bean
    public RestTemplate restTemplate(RestTemplateBuilder builder) {
        // Global settings such as timeout and interceptors can be applied here
        return builder.build();
    }
}

Service Example

package com.icoderoad.service;

import org.springframework.stereotype.Service;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.client.RestTemplate;

@Service
public class UserService {
    @Autowired
    private RestTemplate restTemplate;

    public User getUserById(Long userId) {
        String url = "https://api.example.com/users/" + userId;
        // Synchronous blocking call
        return restTemplate.getForObject(url, User.class);
    }
}

Execution flow: the calling thread sends the request, blocks until the response is received, then returns the User object. Concurrency is limited because each request ties up a thread.

WebClient: Reactive HTTP Client

WebClient

belongs to Spring 5’s WebFlux module and follows the Reactive Streams model (Mono/Flux). Its characteristics are non‑blocking, asynchronous, stream‑oriented, and high‑throughput.

Why WebClient Fits Modern Systems

No thread blocking during I/O.

Few threads can handle many concurrent requests.

Improved resource utilization.

Supports back‑pressure.

Suitable for large‑scale microservice architectures.

Bean Definition

package com.icoderoad.config;

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.web.reactive.function.client.WebClient;

@Configuration
public class AppConfig {
    @Bean
    public WebClient webClient() {
        return WebClient.builder()
                .baseUrl("https://api.example.com")
                .build();
    }
}

Service Example

package com.icoderoad.service;

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

@Service
public class UserService {
    @Autowired
    private WebClient webClient;

    public Mono<User> getUserById(Long userId) {
        return webClient.get()
                .uri("/users/{id}", userId)
                .retrieve()
                .bodyToMono(User.class);
    }
}

Execution flow: the request is issued, the calling thread returns a Mono immediately without blocking, and the response is processed asynchronously when it arrives.

Side‑by‑Side Comparison

Model: RestTemplate – synchronous blocking; WebClient – asynchronous non‑blocking.

Programming style: RestTemplate – imperative; WebClient – reactive.

Concurrency capability: RestTemplate – low to medium; WebClient – high.

Resource utilization: RestTemplate – one thread per request; WebClient – few threads handle many requests.

Recommendation: RestTemplate – continue for legacy maintenance; WebClient – preferred for new projects.

Performance Perspective

RestTemplate

Each request occupies a dedicated thread.

Thread‑pool pressure grows with concurrency.

Risk of thread exhaustion.

WebClient

Event‑driven, non‑blocking architecture.

Operates with a small number of threads.

Achieves high throughput with low resource consumption.

Well‑suited for high‑concurrency scenarios.

Choosing the Appropriate Client

When RestTemplate Is Appropriate

Small internal systems.

Low concurrency requirements.

No need for reactive programming.

Maintaining legacy codebases.

When WebClient Is Appropriate

Microservice architectures.

High‑concurrency workloads.

Real‑time data streams.

Large file downloads.

API‑gateway layers.

Migration Guidance

For new projects, adopt WebClient directly. For existing projects, introduce WebClient alongside RestTemplate, replace calls incrementally, and keep code style consistent. Spring has not removed RestTemplate, but the strategic focus is on WebClient.