Why Spring Boot 4’s Modular Architecture Makes Your Apps Faster and Lighter

In 2014 Spring Boot 1.0 launched with the "out‑of‑the‑box" philosophy, and its core auto‑configuration package spring-boot-autoconfigure was only 182 KB. By Spring Boot 3.5 the same package grew to about 2 MB, bringing more features but also more complexity and size.

To keep Spring Boot lightweight and easy to use, the Spring team completely re‑designed its architecture for the upcoming Spring Boot 4, introducing a modularization architecture.

The modular approach breaks the original monolithic auto‑configuration package into multiple independent modules, each responsible for a specific technology: spring-boot-webmvc – traditional Servlet web applications spring-boot-webflux – reactive web applications spring-boot-data-jdbc – JDBC data access spring-boot-flyway – database migration management spring-boot-webclient – standalone WebClient support

Each module has clear boundaries, single responsibilities, and explicit dependencies, which improves overall maintainability.

Problems with the previous monolithic package included:

Bloated auto‑configuration : all auto‑configuration classes were packaged regardless of usage.

IDE noise : many irrelevant classes and configuration suggestions appeared.

Startup scan overhead : larger classpaths slowed startup.

Benefits of the new modularization are:

Higher maintainability : clear module boundaries let developers focus on specific technologies and IDEs provide more accurate code hints.

Faster startup and lower memory usage : only required modules are included, reducing class‑path scanning.

More precise configuration : for example, to use only WebClient, add the spring-boot-webclient module without pulling in server auto‑configuration.

Support for flexible use cases : monitoring modules like Micrometer can be used independently without the full Actuator dependency chain.

Testing support is also modularized. New test‑specific modules include:

spring-boot-data-jdbc-test

spring-boot-starter-webmvc-test

spring-boot-starter-security-test

spring-boot-starter-flyway-test

Each functional module has a corresponding test starter, ensuring test dependencies stay aligned with production dependencies while remaining lightweight.

For projects migrating from Spring Boot 3 to Spring Boot 4, most applications only need to:

Update starter dependencies.

Add the appropriate test starter.

Adjust package paths and custom configurations (new packages follow org.springframework.boot.<module>).

To ease the transition, Spring Boot 4 provides “Classic Starters” that automatically bring in all modules’ auto‑configuration without adopting the new package structure. Example Maven dependency:

<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-classic</artifactId>
</dependency>

Developers can first migrate to the Classic mode and then gradually switch to the individual modules for a leaner application.

Overall, Spring Boot 4’s modularization makes the framework clearer, lighter, and more efficient, offering a better development experience for building enterprise‑grade Java applications.