Spring Boot vs Micronaut: JVM Startup Compared
Spring Boot uses runtime reflection and a vast ecosystem; Micronaut resolves dependency injection at compile time for fast startup and low memory.
Spring Boot relies on runtime reflection and classpath scanning, which is flexible but adds startup cost. Micronaut performs DI and AOP at compile time, eliminating much reflection for faster boot, lower memory, and good GraalVM native support. The trade is Spring Boot ecosystem depth versus Micronaut startup and footprint efficiency.
| Spring Boot | Micronaut | |
|---|---|---|
| DI / AOP | Runtime reflection | Compile-time |
| Startup time | Slower | Fast |
| Memory footprint | Higher | Low |
| Ecosystem | Vast | Growing |
| Best for | Broad enterprise apps | Fast-start microservices |
In CI
Both build with Maven or Gradle and test with JUnit. Micronaut shifts work to compile time, so builds do more but startup in tests is fast; cache the dependency cache for both. Native-image builds (either framework) are slow - consider doing them only on release. Choose by startup/footprint needs versus ecosystem breadth.
Speed it up
Cache the Maven/Gradle caches between runs. Both build on CI runners; faster managed runners shorten the build and any native-image steps.
The verdict
Want the largest ecosystem and conventions: Spring Boot. Want compile-time DI, fast startup, and low memory for microservices: Micronaut. Spring Boot for breadth, Micronaut for lean, fast-starting services.