Spring Boot 4 Modularization Explained: What you need to know

Spring Boot 4 modularizes its auto-configuration by splitting the previously monolithic auto-configure jar into smaller, focused modules. This breaks existing apps that relied on the old giant jar — for example, the H2 console no longer works automatically after upgrading because its auto-configuration now lives in a separate `spring-boot-h2-console` dependency. The fix is to add the appropriate starter (e.g., `spring-boot-starter-h2-console`) instead of the raw H2 dependency. For teams needing a quick migration path, a `spring-boot-autoconfigure-classic` dependency restores the old behavior, allowing gradual migration to the new modular starters.