Transition to WebClient

About

Spring introduced WebClient as a non-blocking, reactive alternative to RestTemplate. With increasing demand for scalable, resilient, and high-performance services, especially in microservices architecture, WebClient became the preferred choice in reactive and even traditional applications.

Spring has not deprecated RestTemplate yet, but it is considered feature-frozen.

Why Transition ?

As systems scale and enterprise architectures shift toward reactive, event-driven, or cloud-native patterns, RestTemplate starts to become a bottleneck. It’s built on the traditional blocking I/O model, which limits concurrency and consumes more threads under load.

On the other hand, WebClient is part of Spring WebFlux and is designed for non-blocking, reactive communication, which aligns better with modern microservices and service mesh environments.

1. Resource Efficiency

• RestTemplate uses a per-request thread model, which is fine for small workloads but scales poorly under heavy load.

• WebClient runs on a reactive, event-loop model, allowing thousands of concurrent requests with a small number of threads.

• Enterprises aiming to run efficiently on Kubernetes, with limited memory and CPU, will benefit from this shift.

2. Designed for Modern Architectures

• Newer systems are often microservice-based, relying on high-volume inter-service calls.

• WebClient supports backpressure, streaming, and non-blocking retries, which are critical in distributed systems.

• It's suitable for service meshes, API gateways, and reactive databases, where latency and throughput matter.

3. Native Support for Reactive and Asynchronous Programming

• WebClient works natively with Project Reactor, returning Mono or Flux, and integrates seamlessly with WebFlux, RSocket, and reactive data stores.

• This enables end-to-end non-blocking flows from database to API response, which is not possible with RestTemplate.

4. Full Support for Streaming Use Cases

• Use cases like server-sent events, chunked uploads/downloads, or real-time dashboards require streaming support.

• RestTemplate doesn’t support this well, while WebClient provides direct support for streaming responses, backpressure, and partial consumption of results.

5. Better Extensibility and Customization

• WebClient uses a fluent builder and filter-based API, which is more modular and adaptable.

• Customizations such as:

  • Logging

  • Interceptors

  • Headers or auth injection

  • Retry strategies

  • Global error handling are easier to implement in WebClient.

6. Future-Proofing our Codebase

• RestTemplate is in maintenance mode, meaning no new features, only bug/security fixes.

• WebClient is actively developed and will receive enhancements, integration with newer protocols (e.g., HTTP/2), and tooling support.

• Modern observability platforms and tracing libraries (like Sleuth, OpenTelemetry) offer first-class support for WebClient.

7. Better Suitability for Resilience Patterns

• Patterns like:

  • Retry with backoff

  • Timeout handling

  • Circuit breakers

  • Fallback strategies are more naturally implemented with WebClient and tools like Resilience4j.

8. Unified Client for All Scenarios

• While RestTemplate is limited to HTTP and REST, WebClient works with:

  • REST

  • Streaming APIs

  • WebSocket

  • Server-Sent Events

  • Multipart uploads

• This makes it a single client abstraction for all communication needs, reducing fragmentation across teams.

When to Transition ?

Transitioning to WebClient is not always about replacing everything at once. It’s about identifying scenarios where the benefits of non-blocking, reactive HTTP communication outweigh the migration costs. Below are real-world triggers that indicate it's time to shift.

1. Performance Bottlenecks Under Load

If our application starts experiencing:

• Thread pool exhaustion

• Increased latency during spikes

• High memory and CPU consumption under concurrent API traffic ...it’s a strong signal that blocking I/O (RestTemplate) is hitting its limit. Transitioning to WebClient enables efficient scaling with fewer resources.

2. High Concurrency Requirements

If our service needs to:

• Call many downstream services concurrently

• Make parallel API calls (e.g., aggregators, facades)

• Perform fire-and-forget notifications or long-polling …WebClient handles this with far fewer threads by leveraging non-blocking async execution.

3. Moving Toward Reactive Stack

If our application is:

• Using or planning to use Spring WebFlux

• Incorporating reactive data sources (e.g., R2DBC, reactive Mongo)

• Adopting reactive message queues, Kafka, or WebSockets ...we should switch to WebClient to maintain a fully reactive data flow, avoiding thread-blocking points like RestTemplate.

4. Need for Streaming and Advanced Protocol Support

If we require

• Server-sent events (SSE)

• Chunked transfer or streaming large files

• Integration with GraphQL over HTTP, gRPC-Web, or HTTP/2 ...WebClient is the only viable client in the Spring ecosystem that supports these natively.

5. Centralized Error Handling and Observability

If our organization is maturing toward:

• Standardized observability using Sleuth, OpenTelemetry, Zipkin

• Implementing retry logic, timeouts, and circuit breakers

• Using log enrichment or request tracing …WebClient allows for better integration with Resilience4j, Micrometer, and custom filters to build a robust HTTP client layer.

6. Application is Gateway, Proxy, or Aggregator

When our Spring Boot app is acting as a:

• API Gateway

• Backend for frontend (BFF)

• Service aggregator … we are essentially performing high-throughput, multi-destination communication. WebClient is designed to maximize concurrency, minimize thread usage, and deliver low latency in such scenarios.

7. Migrating to Cloud-Native or Serverless Architecture

If We are deploying to:

• Kubernetes, where CPU and memory are constrained

• Serverless functions, which need minimal cold start time and fast I/O

• Autoscaled containerized environments where lightweight thread usage is critical Then WebClient helps our services remain scalable and responsive without bloating resource usage.

8. Starting a New Module or Greenfield Service

When building:

• A new microservice

• An internal tool

• A client SDK or adapter It’s better to start with WebClient from day one, since RestTemplate is in maintenance mode and lacks forward-looking capabilities.

9. Integration with External Reactive Systems

If external services (e.g., third-party APIs, streaming platforms) expose:

• Streaming responses

• Reactive Web APIs

• Expect non-blocking backpressure support We will need WebClient to leverage full compatibility and throughput benefits.

10. Incremental Migration of Legacy Codebase

We don’t have to rip and replace everything. Introduce WebClient:

• For high-load or streaming endpoints

• Where retry/timeouts/circuit-breaking are must-haves

• In utility/service layers without touching the controller logic This allows gradual adoption with measurable benefits.

How to Transition ?

Transitioning from RestTemplate to WebClient should be strategic and incremental. It involves evaluating existing usage patterns, defining boundaries, preparing infrastructure, and gradually rolling out changes without disrupting the application.

This guide breaks down the process into phases with techniques and real-world practices.

1. Assess Existing Usage

Start with an inventory of current RestTemplate usages:

• Where are the HTTP calls happening?

• Which services are most frequently called?

• Are any calls parallel, streaming, or long-running?

• Are retry/timeouts or circuit breakers applied?

Tip: Use static code analysis tools or manual inspection to categorize usage.

2. Identify Candidates for Migration

Look for areas that:

• Face performance or scaling limitations

• Involve parallel calls, batch processing, or high-frequency endpoints

• Are already part of a reactive pipeline

• Need fine-grained timeout, retry, or circuit-breaking logic

Don’t migrate everything at once — start small and focus on high-impact scenarios.

3. Introduce WebClient in Parallel

There is no need to delete RestTemplate immediately. Spring allows both to coexist:

• Inject WebClient.Builder as a bean

• Use it selectively in service layers

• Build utility wrappers to standardize usage

This lets us experiment with WebClient without affecting stable paths.

4. Build an Adapter Layer

To abstract the usage:

• Create a reusable HTTP client utility or adapter service

• Internally use WebClient, but expose a uniform API to the rest of our app

• This makes swapping the HTTP client easier without ripple effects

Example Adapter Interface:

public interface HttpClientAdapter {
    <T> Mono<T> get(String url, Class<T> responseType);
    <T, R> Mono<R> post(String url, T request, Class<R> responseType);
}

5. Update Configuration

Add a WebClient bean globally:

@Configuration
public class WebClientConfig {
    @Bean
    public WebClient webClient(WebClient.Builder builder) {
        return builder
            .baseUrl("https://api.example.com")
            .defaultHeader(HttpHeaders.CONTENT_TYPE, MediaType.APPLICATION_JSON_VALUE)
            .build();
    }
}

Use WebClient.Builder for customizations per service, like timeouts, interceptors, authentication, or logging.

6. Refactor Calls Gradually

For each identified method or service:

• Replace RestTemplate call with WebClient

• Handle response using Mono<T> or Flux<T>

• Adapt result using block() if calling from imperative code temporarily

• Use .onErrorResume() or .retry() for resilience

Before (RestTemplate):

ResponseEntity<User> response = restTemplate.getForEntity(url, User.class);

After (WebClient):

User user = webClient.get()
    .uri(url)
    .retrieve()
    .bodyToMono(User.class)
    .block(); // Only if needed in imperative flow

7. Handle Exception & Timeouts

• Customize ExchangeFilterFunction to handle global errors

• Use onStatus(...) to manage HTTP status codes

• Set timeouts using reactor.netty.http.client.HttpClient

• Integrate with Resilience4j or Spring Retry for robust behavior

8. Update Tests

• Mock WebClient using WebClient.Builder with ExchangeFunction

• Use tools like WireMock or MockWebServer to simulate downstream APIs

• Avoid overusing .block() in tests; use .subscribe() or StepVerifier if we are already on reactive stack

9. Train Our Team

• Conduct internal knowledge-sharing or training sessions

• Share wrapper patterns and common pitfalls

• Document when to use block() and when not to

10. Monitor and Iterate

• Track error rates and latency after introducing WebClient

• Measure thread usage, response time, GC impact

• Use APM tools (e.g., New Relic, Grafana, Zipkin) to trace impact