Spring Boot 3 to GraalVM Native Images: The Complete Migration Guide

For over a decade, the narrative around Java in the cloud has been consistent: it's robust, scalable, and powerful, but it's heavy. The "cold start" problem has historically kept Java out of the serverless conversation, where Go and Node.js have thrived.

With Spring Boot 3, that narrative changes. By treating GraalVM Native Images as a first-class citizen, the Spring team has unlocked a new tier of performance for enterprise Java applications. We are no longer talking about incremental gains; we are talking about reducing startup times from seconds to milliseconds.

The Paradigm Shift: JIT vs. AOT

Traditionally, Java uses Just-In-Time (JIT) compilation. The JVM loads bytecode, interprets it, and compiles hot paths into native machine code at runtime. This enables aggressive runtime optimizations but incurs startup cost and memory overhead.

GraalVM Native Image uses Ahead-Of-Time (AOT) compilation. It compiles your entire application into a standalone native executable at build time—no JVM required. The result is a single binary containing your application, dependencies, and a minimal runtime.

The Closed World Assumption

The critical concept: Closed World Assumption. The AOT compiler must know about every class, method, and field at build time. If it's not reachable during static analysis, it's removed from the final binary.

This aggressive dead-code elimination creates small binaries but breaks dynamic features like reflection and proxies if not explicitly configured. Traditional JVM apps that dynamically load classes at runtime require metadata hints to work in the native world.

Migration Steps

Migrating an existing Spring Boot application isn't just about changing a build flag. It requires a methodical approach to handling dynamic behavior.

1. Prerequisites and Dependencies

Ensure you are running Java 17 or higher. Spring Boot 3 requires it.

For Gradle, apply the GraalVM Native Build Tools plugin:

plugins {
    id 'org.springframework.boot' version '3.2.0'
    id 'io.spring.dependency-management' version '1.1.4'
    id 'org.graalvm.buildtools.native' version '0.9.28'
}

For Maven, ensure the native-maven-plugin is active within a profile:

<profiles>
    <profile>
        <id>native</id>
        <build>
            <plugins>
                <plugin>
                    <groupId>org.graalvm.buildtools</groupId>
                    <artifactId>native-maven-plugin</artifactId>
                </plugin>
            </plugins>
        </build>
    </profile>
</profiles>

2. Handling Dynamic Code with RuntimeHints

Spring Boot 3 does a lot of heavy lifting to automatically register hints for its own ecosystem (Spring MVC, Data, Security). However, if you use third-party libraries that rely on reflection or serialization, you may encounter ClassNotFoundException or NoSuchMethodException at runtime.

You can register these manually using the RuntimeHintsRegistrar interface:

public class MyLibraryHints implements RuntimeHintsRegistrar {

    @Override
    public void registerHints(RuntimeHints hints, ClassLoader classLoader) {
        // Register reflection for a specific class
        hints.reflection().registerType(MyCustomDto.class,
            MemberCategory.INVOKE_PUBLIC_CONSTRUCTORS,
            MemberCategory.INVOKE_PUBLIC_METHODS);

        // Register a resource pattern
        hints.resources().registerPattern("*.json");
    }
}

Then, import this registrar in your configuration:

@Configuration
@ImportRuntimeHints(MyLibraryHints.class)
public class AppConfig {
}

3. The Build Process

Building a native image is computationally expensive. It can take several minutes and consumes a significant amount of RAM.

# Gradle
./gradlew nativeCompile

# Maven
./mvnw -Pnative native:compile

Tech Note: We recommend running native builds in a CI/CD pipeline rather than on your local development machine to avoid locking up resources.

4. Testing in Native Mode

Your standard JUnit tests will pass on the JVM but fail in native mode if you haven't registered the correct hints. Run tests against the native image:

# Gradle
./gradlew nativeTest

# Maven
./mvnw -Pnative test

This compiles your test suite into a native binary and executes it. It's slower than JVM tests but catches native-specific issues before production.

Performance Benchmarks

We benchmarked a standard Spring Boot 3 microservice (REST API + JPA + PostgreSQL) running on a 2 vCPU / 4GB RAM instance.

These numbers are transformative for scale-to-zero architectures like AWS Lambda or Google Cloud Run, where you pay for execution time and cold starts directly impact user experience.

Trade-offs and Limitations

The benefits are substantial, but the trade-offs require careful consideration.

• Build Time: Native builds take minutes, not seconds. Strategy: Develop and test on the JVM. Build native images only in CI/CD for integration tests and production.
• Peak Throughput: JIT can outperform AOT for long-running processes because it optimizes based on runtime behavior. Native Images prioritize startup and footprint, not sustained throughput.
• Observability: Bytecode instrumentation agents (NewRelic, Datadog) don't work. Use native-compatible tools like OpenTelemetry or vendor-specific native agents.
• Platform Lock-in: Native binaries are platform-specific (Linux ARM64, Linux x86-64, macOS ARM64). You must build for your deployment target.

Conclusion

Spring Boot 3's GraalVM support represents a watershed moment for enterprise Java. Teams can now leverage the Spring ecosystem while achieving the performance characteristics demanded by serverless and Kubernetes-native architectures.

When to migrate:

• Microservices running in serverless environments (AWS Lambda, Google Cloud Run)
• Applications requiring scale-to-zero capabilities
• CLI tools and batch jobs with short lifecycles

When to stay on the JVM:

• Long-running monoliths with stable traffic patterns
• Applications requiring maximum peak throughput
• Legacy codebases with heavy reflection usage

If you're building cloud-native Java systems and need engineers who understand these trade-offs, we specialize in placing senior Java developers with deep Spring Boot and cloud infrastructure experience.

Frequently Asked Questions

Can I use my existing third-party libraries?

Most popular libraries now support GraalVM or have metadata available in the GraalVM Reachability Metadata Repository. However, legacy libraries heavily reliant on complex reflection might require significant manual configuration.

How do I debug a native image?

Debugging a native binary is difficult and requires tools like GDB. We strongly recommend ensuring your application passes all tests on the JVM first. If a bug only appears in native mode, use the nativeTest task to run JUnit tests within the native context.

Is Native Image suitable for monolithic applications?

Technically yes, but the build times may become prohibitive (15+ minutes). Native Images shine brightest in microservices and serverless functions where startup speed and density are critical.

Does AOT compilation support all Java features?

No. Features like dynamic class loading, finalizers, and SecurityManager are not supported. Most modern application development patterns avoid these, but legacy codebases may need refactoring.

References

• Spring Boot Docs: GraalVM Native Image Support
• GraalVM: Native Image Reference
• Reachability Metadata: GitHub Repository