What is the complete modularisation of Spring Boot 4 and why does it matter?

One of the most architecturally significant changes in Spring Boot 4 is the complete modularisation of the codebase into 70+ focused JAR modules. Previously, the two monolithic JARs -- spring-boot-autoconfigure and spring-boot-test-autoconfigure -- contained auto-configuration for every supported technology regardless of what your project actually used.

Why it matters:

• Smaller application footprint -- you only include modules for technologies you actually use
• Faster builds and startup times -- less classpath scanning and fewer classes to load
• Better GraalVM native images -- smaller reachability surface means smaller native binaries
• Cleaner IDE auto-complete -- no longer suggests classes from libraries you haven't included
• Clearer dependency management -- easier to reason about what is on the classpath and why

<!-- Spring Boot 3 (monolithic starter) -->
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
</dependency>

<!-- Spring Boot 4 (modular starters) -->
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-webmvc</artifactId>
</dependency>
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-jackson</artifactId>
</dependency>

<!-- For gradual migration: classic starters bridge the gap -->
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-classic</artifactId>  <!-- bundles old behaviour -->
</dependency>

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