Spring / Spring7 Intermediate to Advanced Interview questions
How is a GraalVM native image build different from a traditional JAR deployment in Spring Boot 4?
A traditional JAR deployment ships portable bytecode that runs on any matching JVM: the JVM interprets and JIT-compiles code as it runs, resolves reflection and dynamic proxies at runtime, and pays a warm-up cost (several seconds of startup, growing memory as the JIT optimizes hot paths) in exchange for broad portability and full dynamic-language features.
| Aspect | JAR on JVM | GraalVM native image |
| Startup time | Seconds | Milliseconds |
| Memory footprint | Higher, grows with JIT | Low, fixed ahead of time |
| Portability | Any machine with a matching JVM | One binary per OS/architecture |
| Reflection/dynamic proxies | Fully dynamic at runtime | Must be known ahead of time (via AOT hints) |
| Build time | Fast | Much slower (whole-program analysis) |
A native image instead compiles the application ahead of time into a self-contained, platform-specific executable: Spring's own AOT processing step precomputes the application context so the GraalVM compiler knows exactly which beans, proxies, and reflective accesses are needed, bakes only those reachable code paths into the binary, and skips the JVM's runtime class-loading machinery entirely - trading a much longer, heavier build and reduced runtime flexibility for near-instant startup and a small, predictable memory footprint.
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