Spring Boot 3 (on Spring Framework 6) was the first release in years to break source compatibility on purpose — and understanding why is the fastest way to show you've actually shipped on the modern stack rather than copied a tutorial. The headline change is the javax.* → jakarta.* namespace migration: Jakarta EE 9 renamed every enterprise package (servlet, persistence, validation, JMS…), so Boot 3 can't run on the old imports at all. The other baseline shift is Java 17 minimum — Boot 3 is built and compiled against it, so records, sealed types, and pattern matching are now table stakes.
On top of that floor sit the features that define the "modern stack":
Boot 3 / Framework 6 modern stack
├── GraalVM native image (Spring AOT: build-time reflection/proxy hints)
├── Spring Modulith (enforce module boundaries in a monolith)
├── ProblemDetail (RFC 7807/9457 error bodies, built in)
├── @HttpExchange clients (declarative HTTP, often replaces Feign)
├── RestClient (6.1+, fluent synchronous client)
├── Micrometer Tracing (replaces Spring Cloud Sleuth)
└── spring.config.import (config trees, Vault, Consul)Two themes tie these together. First, observability and clients moved into the core. What used to be Spring Cloud add-ons (Sleuth tracing, Feign clients) now have first-class equivalents — Micrometer Tracing and @HttpExchange — so you depend on fewer separate libraries with their own release cadence. Second, the framework leans toward ahead-of-time and closed-world thinking. Spring AOT processes your context at build time, generating the reflection and proxy hints GraalVM needs, which is what makes sub-100ms startup and tiny memory footprints possible.
You should also know what left: WebSecurityConfigurerAdapter is gone (Security 6 wants component-based SecurityFilterChain beans), WebMvcConfigurerAdapter was long replaced by the WebMvcConfigurer interface, and RestTemplate is in maintenance mode (not deprecated) with RestClient as the forward path.
The questions below dig into each: the migration mechanics, AOT's limits, Modulith, ProblemDetail, the new clients, the observability rework, config import, and exactly what was removed.