What are the differences between Spring MVC and Spring WebFlux?

Table of Contents

• Introduction
  • 1. Programming Model
    • Spring MVC (Blocking)
    • Spring WebFlux (Non-blocking)
  • 2. Architecture and Frameworks
    • Spring MVC
    • Spring WebFlux
  • 3. Performance and Scalability
    • Spring MVC
    • Spring WebFlux
  • 4. Use Cases
    • Spring MVC
    • Spring WebFlux
  • 5. Response Handling
    • Spring MVC
    • Spring WebFlux
  • 6. Framework and Server Support
    • Spring MVC
    • Spring WebFlux
  • 7. Thread Management
    • Spring MVC
    • Spring WebFlux
• Conclusion
  • When to Choose Each

Introduction

Both Spring MVC and Spring WebFlux are frameworks in the Spring ecosystem used to build web applications and REST APIs. However, they differ significantly in terms of their architecture, programming models, and use cases. While Spring MVC is based on the traditional Servlet API and follows a blocking model, Spring WebFlux is built for reactive programming and supports a non-blocking and asynchronous model.

In this guide, we’ll explore the key differences between Spring MVC and Spring WebFlux, and help you decide when to use each framework based on your application’s requirements.

1. Programming Model

Spring MVC (Blocking)

Spring MVC follows the Servlet-based model where each request is handled by a dedicated thread. This is a blocking model, meaning that each HTTP request is processed synchronously: when a request arrives, it occupies a thread until the response is sent back to the client. This approach can become inefficient under high load, especially for I/O-bound operations like database queries, network calls, or file access, as each request consumes server resources (threads) during the entire processing time.

Spring WebFlux (Non-blocking)

Spring WebFlux, on the other hand, is designed around the reactive programming paradigm. It supports non-blocking I/O, meaning that a single thread can handle multiple requests concurrently by releasing the thread during waiting periods (such as waiting for data from a database or an external API). Instead of blocking the thread while waiting for I/O operations to complete, WebFlux uses reactive streams to asynchronously process data. This model is ideal for building highly scalable, low-latency, and high-throughput applications.

2. Architecture and Frameworks

Spring MVC

• Servlet API-based: Spring MVC relies on the traditional Servlet API, meaning each incoming HTTP request is assigned a thread from the server's thread pool.
• Blocking I/O: The framework is inherently synchronous, meaning the processing of each HTTP request will block a thread until it’s completed.
• Designed for traditional, synchronous web applications: Spring MVC is well-suited for applications that have traditional workflows (like web forms, user interfaces) where each action (HTTP request) can be handled sequentially.

Spring WebFlux

• Reactive Streams-based: Spring WebFlux is based on Project Reactor, which implements the Reactive Streams API for handling asynchronous data flows.
• Non-blocking I/O: It uses reactor-core to support non-blocking backpressure handling and event-driven programming, which allows better resource utilization and performance.
• Designed for reactive and asynchronous applications: WebFlux is ideal for building real-time applications, streaming data, and handling concurrent requests where each action (HTTP request) might involve waiting for I/O operations like reading from databases, calling external services, or processing large streams of data.

3. Performance and Scalability

Spring MVC

• Thread-per-request model: In Spring MVC, each request gets a dedicated thread, which is a limiting factor for scalability under high load. When the application needs to handle large amounts of traffic, the thread pool may get exhausted, resulting in slow response times or errors.
• Better for synchronous, small-scale applications: Spring MVC is optimal for small to medium-scale applications with lower concurrency demands. However, for applications that handle a high volume of I/O-bound requests or need to scale massively, Spring MVC may not be the best choice.

Spring WebFlux

• Scalable, non-blocking model: Spring WebFlux is designed for scalability. By using a small number of threads and handling requests asynchronously, it can process many more requests concurrently, making it ideal for high-traffic applications, APIs, and systems where resources are constrained.
• Better performance under high load: Because WebFlux can handle thousands of concurrent requests with a small number of threads, it performs better in situations where there are many I/O-bound tasks (e.g., waiting for database queries, HTTP calls, etc.).

4. Use Cases

Spring MVC

• Synchronous, traditional web applications: Spring MVC is best suited for applications that require a request-response lifecycle, like web applications that do not involve complex data streams or require real-time data.
• Applications with low concurrency requirements: If your app doesn’t need to handle a large number of concurrent users or I/O-bound operations, Spring MVC will work fine, especially for applications like e-commerce websites, admin portals, and simple CRUD-based applications.

Spring WebFlux

• Reactive applications: Spring WebFlux is ideal for building real-time applications, microservices, and APIs where responses are streamed, data changes frequently, and the system needs to handle many concurrent connections.
• High-concurrency environments: WebFlux is perfect for applications that need to process high volumes of traffic asynchronously, such as social media platforms, chat applications, streaming services, or IoT applications.
• Non-blocking services: It is particularly well-suited for systems that require integration with reactive data sources (e.g., MongoDB, Cassandra, or R2DBC) or APIs that make long-lasting HTTP calls to third-party services.

5. Response Handling

Spring MVC

• Request-response model: Responses in Spring MVC are handled in a synchronous manner. Each controller method returns a model and a view or JSON/XML data (for REST APIs). The entire response lifecycle is handled synchronously by a thread.
• Single-threaded per request: Each request and its corresponding response block a thread from the server pool until the request is fully processed and the response is returned.

Spring WebFlux

• Asynchronous, event-driven model: In WebFlux, the response is represented by a reactive type such as Mono or Flux. These types return asynchronous data that is processed and consumed as it arrives, without blocking threads.
• Streams and backpressure handling: Responses can be streamed or chunked as they are computed, and WebFlux supports backpressure to control how data is handled in case the consumer is slower than the producer.

6. Framework and Server Support

Spring MVC

• Servlet container: Spring MVC is designed to run on traditional servlet containers like Tomcat, Jetty, or Undertow. These containers are based on the servlet model, which requires threads to be allocated for each HTTP request.

Spring WebFlux

• Supports both servlet and non-servlet containers: Spring WebFlux supports both traditional servlet containers (Tomcat, Jetty) and non-servlet containers (e.g., Netty, Undertow). Non-servlet containers provide better performance for handling asynchronous and non-blocking operations.
• Can run on reactive runtimes: WebFlux can be run on reactive runtimes like Netty, which is designed for high-performance, non-blocking, event-driven I/O. It can also run on Undertow or Jetty in a reactive mode.

7. Thread Management

Spring MVC

• Thread per request: Each request is handled by a separate thread from a thread pool, which makes Spring MVC more resource-intensive when dealing with high concurrency.

Spring WebFlux

• Event-loop model (reactive threads): WebFlux uses an event-loop model with a small number of threads, where each thread can handle multiple requests asynchronously without blocking. This is more efficient and can scale much better under heavy load.

Conclusion

Feature	Spring MVC	Spring WebFlux
Programming Model	Blocking (synchronous)	Non-blocking (asynchronous)
I/O Handling	Blocking I/O, thread-per-request	Non-blocking I/O, event-driven
Performance and Scalability	Limited scalability, good for small apps	High scalability, handles high load
Use Cases	Traditional, synchronous web apps	Reactive, real-time, and scalable apps
Framework Support	Servlet containers (e.g., Tomcat)	Servlet and non-servlet containers (e.g., Netty)
Response Handling	Synchronous request-response cycle	Asynchronous, reactive responses

When to Choose Each

• Choose Spring MVC if your application is traditional, requires synchronous request-response cycles, and doesn’t need to handle high-concurrency, I/O-bound operations, or real-time streaming.
• Choose Spring WebFlux if your application requires asynchronous processing, high scalability, real-time data streaming, or has heavy I/O-bound tasks that can benefit from non-blocking execution.

By understanding these differences, you can make an informed decision on which framework best suits your needs, whether you're building a traditional web application or a modern, reactive API.