It's time to upgrade to Spring Boot 4.1 (and Java 25)

Still on Spring Boot 3.x, or even 2.7 in production? Spring Boot 4 is more than a framework upgrade: it’s a chance to reduce dependency weight, improve startup behavior, modernize your Java runtime, and selectively evaluate GraalVM for cold-start-sensitive services. This guide lays out a practical upgrade path across Spring Boot 4, Java 25, and native images, with a focus on measurable performance gains and migration risk.

Upgrade Track at a Glance (Spring, Java, GraalVM)

Startup time, runtime efficiency, garbage collection, and migration path

Track	What improves	Migration work	Best Fit For
Spring Boot 4	Modular autoconfiguration, smaller dependency footprint, enhanced virtual-thread support, and dependency modernization.	Upgrade to latest Spring Boot 3.5.x first; review deprecated APIs, starters, and third-party Spring modules.	Current support, performance tuning, and framework modernization.
Java 25	Faster startup, improved runtime performance, container CPU and memory limits, tuned G1/ZGC behavior, and enhanced concurrent GC work.	Low if the application avoids deprecated JDK internals; validate build plugins, JVM flags, containers, and observability agents.	Default runtime uplift for JVM-based services.
GraalVM Native Image	Very fast cold starts, lower resident set size, and improved fit for serverless or rapid autoscaling patterns.	Native image build; testing for reflection, proxies, JNI, classpath scanning; longer builds, and edge-case runtime behavior.	Short-lived services, bursty autoscaling, and latency-sensitive cold-start paths.

Spring Boot 4.1 Performance and Migration

Spring Boot 4.1 is the right choice for OSS support, security, and dependency modernization. When you upgrade to Spring Boot 4, dependency management, starter structure, framework APIs, and deprecations all need review/updates.

Virtual-thread integration

Spring Boot 4’s virtual-thread support is excellent for I/O-bound work over the network or filesystem. Each request can be backed by a lightweight JVM-managed virtual thread, reducing the need for application-managed request thread pools or reactive programming. I’ve started using virtual threads in all my applications and appreciate how they enable lightweight, imperative concurrency.

Modular Dependencies

Spring Boot 4 modularizes auto-configuration, replacing the spring-boot-autoconfigure artifact with smaller, technology-focused modules and matching starters.

When upgrading, review dependencies, adding explicit starters for just the technologies you use. Doing so will lower your applications autoconfiguration overhead on startup, and reduce the amount of classes bundled into your JAR.

Applications with custom starters or shared libraries should plan for Boot 4-specific artifacts, while spring-boot-starter-classic and spring-boot-starter-test-classic can provide a temporary bridge for incremental upgrades.

Migration path

Upgrade to the latest Spring Boot 3.5.x, then verify deprecations and dependency compatibility before moving to Spring Boot 4.
If your application doesn’t use starter dependencies, expect more dependency adjustment: Spring Boot 4 uses smaller, more modular dependencies.
Consider starter-classic flavor dependencies as a transition step: these transitory dependencies allow you to upgrade to Boot 4 before fully modularizing dependencies.
Review third-party Spring modules for compatibility independently. Depending on the Spring Boot version you’re migrating from, some dependencies have changed coordinates or may have been deprecated/replaced entirely. Users of Spring Boot 2.7 or older will encounter more deprecated dependencies.

Startup tuning checklist

Your Spring Boot app may be able to drastically improve its startup time – which is easier than ever in Spring Boot 4:

Run startup with –debug to inspect autoconfiguration condition evaluation and identify unnecessary work.
Remove unused starters and transitive dependencies that trigger optional-but-unused autoconfiguration: this can greatly lower startup time! fewer classes to scan + fewer beans to construct = faster app startup.
Exclude unused autoconfiguration classes with @SpringBootApplication(exclude = {…}) if the autoconfiguration must be imported but is not used.
Constrain entity and component discovery with @EntityScan and @ComponentScan rather than allowing broad classpath scanning.
Mark non-critical beans with @Lazy so they initialize only when actually needed.
Prefer async log appenders for Log4j or Logback when synchronous logging appears in startup or request hot paths.

Java 25 Upgrade

Upgrading to Java 25 is usually a safe upgrade. However, if you use deprecated JDK internals, unsupported flags, or agents/frameworks that assume older JVM behavior, additional migration work is required.

Startup and warm-up

A straightforward upgrade to Java 25 is likely to improve startup and runtime behavior with minimal to no application changes. Always verify performance before and after a Java upgrade using existing metrics/benchmarks.

Container and profiling accuracy

With Java 25, the JVM is fully container aware. the -XX:+UseContainerSupport flag is enabled by default, and the JVM automatically detects container resource limits set by a your container platform. If you’re running your Java apps on a platform like Kubernetes, I recommend configuring CPU and Memory requests/limits.

Garbage collection (GC)

GC continues to improve as Java evolves. If you’re using the default G1 (which is suitable for most Spring Boot apps) you may have no changes in this area.

Most Spring Boot services do great with G1, balancing latency and throughput. Start with G1 and evaluate over time.
Evaluate ZGC or Shenandoah only where pause-time reduction and low-latency behavior are first-order requirements.
Measure GC pause time, allocation rate, CPU overhead, heap behavior, and tail latency under the same container limits used in production.

See Garbage Collection Tuning for an in-depth look at GC tuning.

Useful Java 25 platform features

Scoped values can provide lightweight, immutable context passing that works well with virtual threads.
Flexible constructor bodies, module import declarations, compact source files, instance main methods, and pattern matching improvements reduce boilerplate.
Remember all that clunky crypto wrapper code you’ve written? Crypto API improvements around PEM handling and key derivatio reduce the amount of utility code in security-aware services.

See the Java 25 release notes for full JDK/JVM enhancements

Evaluating GraalVM

GraalVM dramatically improves your app’s cold start and memory footprint. Most Spring Boot 4 native images start in milliseconds. GraalVM also changes the build, packaging, and runtime model.

If startup time is a critical concern, consider GraalVM.
Best used for serverless functions, short-lived services, rapid autoscaling, and services that lose traffic during one-to-three-minute startup windows.
Be cautious for long-running services where a warmed JVM may outperform the native image over time.
Budget extra testing for reflection, proxies, JNI, classpath scanning, serialization, security providers, observability agents, and runtime edge cases.

POC Measurement Plan

Metric	How to capture	Decision signal
Cold start and warm-up	Launch to readiness, classloading, autoconfiguration, and first-traffic p95/p99 latency.	Confirms startup and early latency improvements without readiness regression.
GC and container fit	Pause time, allocation rate, heap, CPU, memory, thread limits, and Kubernetes constraints.	Decides whether G1 remains enough or ZGC/Shenandoah merits testing.
Memory/RSS	Resident set size, heap, metaspace, native memory, and container pressure.	GraalVM only wins if memory reduction justifies native complexity.
Compatibility	Smoke, regression, integration, logging, security, and observability tests.	Migration proceeds only with equivalent functional behavior.

Discovery questions for your apps

Use these discovery questions to help plan your upgrade path.

Application stack

Which Spring Boot versions and major Spring modules are in use, such as Spring Kafka, Security, Data, Batch, and WebFlux/MVC?
Does the app use Maven, Gradle, custom build logic, bytecode enhancement, reflection-heavy libraries, JNI, or Java agents?
Are current GC behavior, profiling data, Micrometer metrics, OpenTelemetry traces, and startup dashboards trusted enough for before/after performance comparison?

Operational fit

Is the main business problem cold start, slow warm-up, GC pauses, memory pressure, autoscaling lag, or support/security posture?
Which services are safe candidates for a simple POC, and which services are too dependency-heavy to use as the first migration proof point?

Suggested Roadmap

A pragmatic upgrade path can start with Java 25, then adopt Spring Boot 4 (upgrade to Spring Boot 3.5.x first). GraalVM may be considered as optional profile for specific services where startup time and memory usage are critical concerns. Keep each step measurable, and limits the number of variables changed at once.