Class Data Sharing and JVM Startup Tuning

GraalVM native images give near-instant startup, but most teams still ship a regular JVM build for the vast majority of deployments. The JVM is easier to debug, supports full reflection, and works with every agent and library out of the box.

The good news: a JVM-mode Spring Boot 4 app does not have to be slow to start. Three levers move the needle the most:

• Class Data Sharing (CDS) — skip re-parsing class metadata on every boot.
• Lazy initialization — create beans only when first used.
• Bean instantiation tuning — avoid expensive work in constructors and at refresh time.

This lesson walks through all three, in JVM mode, no native image required.

When the JVM starts, it loads, parses, and verifies hundreds or thousands of classes. Class Data Sharing (CDS) writes the parsed in-memory class representation into a read-only archive file once, then memory-maps that archive on every subsequent start.

The payoff:

• Less class loading and verification work per boot.
• The archive can be shared across multiple JVM processes (same physical memory pages).

Two flavors matter for Spring Boot 4:

• Dynamic CDS (AppCDS) — archive your application classes, not just the JDK ones.
• Project Leyden / Ahead-of-Time cache — an evolution layered on CDS in newer JDKs.