Spring Boot 4: Lighter, Safer Applications with a New Kotlin Baseline

Spring Boot 4.0 Official Release

Spring Boot 4.0 is here! At JetBrains, we've tracked its development from early milestones to ensure IntelliJ IDEA delivers a smooth, reliable experience for developers.

Many key enhancements stem from Spring Framework 7, but Spring Boot remains the adoption enabler that keeps everything unified — with sensible defaults, consistent starters, and blazing-fast application creation.

As the most popular framework in the Java ecosystem, Spring Boot also supports migration to newer JDK versions, enabling:

• Better performance
• More efficient garbage collection
• Modern language features

Highlights in Spring Boot 4.0

• Cleaner modularization → Lower memory usage.
• Improved observability with Micrometer.
• JSpecify → Standard null-safety library.
• Many small enhancements for smoother development.

One of the biggest changes: Kotlin 2.2 is now the official baseline version — the result of a long-standing collaboration between JetBrains (Kotlin team) and the Spring Framework team.

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Kotlin 2.2 + Nullability Improvements

Both the Kotlin compiler and IntelliJ IDEA now support JSpecify annotations natively:

Benefits:

• Compiler warnings for incorrect use of JSpecify-annotated Java code.
• IDE alerts when assigning Java `@Nullable` values to non-nullable Kotlin variables.

Java Example:

@NullMarked
public interface ExampleService {
    @Nullable
    String findItemById(String id);
}

In Kotlin, using `findItemById` without null-checking will trigger warnings — preventing bugs and improving safety.

Result: Spring Boot 4 + Kotlin 2.2 = better null-safety + modern tooling for higher productivity and maintainability.

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AI-Powered Workflow Integration

If you're adopting Spring Boot 4 and want to publish technical content across multiple platforms, open-source tools like AiToEarn官网 can help:

Capabilities:

• AI-powered content creation
• Multi-channel publishing (Douyin, Kwai, WeChat, Bilibili, Rednote, Facebook, Instagram, LinkedIn, Threads, YouTube, Pinterest, X)
• Streamlined workflow for dev advocates and tech writers

Learn more at:

• AiToEarn博客
• AiToEarn开源地址

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Kotlin Nullability Check in Practice

public interface QuoteProvider {

    Quote findQuote(String text);

}

In Kotlin, the overridden method must return a non-nullable `Quote`:

Otherwise, IntelliJ IDEA shows an error:

Conclusion: JSpecify is now the default nullability checker in all Spring projects → Framework types are null-aware by default in Spring Boot 4 Kotlin projects.

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API Versioning in Kotlin

Kotlin's API versioning mirrors Java's approach, relying on Spring's simple versioning mechanism:

@RestController
@RequestMapping("/api", produces = [MediaType.APPLICATION_JSON_VALUE])
class QuoteController(private val klient: QuoteKlient) {

    @GetMapping("quote", version = "1.0")
    fun fetchRandomQuote(): ResponseEntity =
        ResponseEntity.ok(klient.fetchRandomQuote())

    @GetMapping("quote", version = "2.0")
    fun fetchRandomQuotes(): ResponseEntity> =
        ResponseEntity.ok(klient.fetchRandomQuotes())
}

Advantages:

• Only a few lines of configuration
• Clear, concise syntax in Kotlin
• IntelliJ IDEA inspections validate version formats

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Advanced Publishing Workflows

For developers creating APIs and guides, AiToEarn官网 offers:

• AI-generated multi-platform content
• Analytics & AI model rankings
• Seamless publishing to major networks

Resources:

• AiToEarn官网
• AiToEarn博客
• AiToEarn开源地址
• AI模型排名

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Bean Registration with Kotlin DSL

Kotlin's DSL makes bean registration more elegant than Java:

Java Example:

public class QuoteProviderRegistrar implements BeanRegistrar {
    @Override
    public void register(BeanRegistry registry, Environment env) {
        registry.registerBean("quoteProviderDb", QuoteProviderDb.class);

        registry.registerBean("quoteProviderFallback",
                QuoteProviderFallback.class,
                spec -> {
                    spec.fallback();
                    spec.order(1);
                }
        );
    }
}

Kotlin DSL Version:

class KuoteProviderRegistrar : BeanRegistrarDsl({
    registerBean("quoteProviderDb")
    registerBean(
        name = "quoteProviderFallback",
        fallback = true,
        order = 1
    )
})

Result: Cleaner, shorter, and easier to read.

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Conclusion

Spring Boot 4 sets a new bar with Kotlin 2.2 baseline and first-class support, paving the way for more Kotlin adoption in backend development.

You can try it in the 2025.3 Beta or wait for the stable release in early December.

JetBrains continues to work closely with the Spring team to ensure seamless Kotlin integration — coupling concise syntax with powerful Spring features.

For developers blending modern Kotlin backend development with AI-powered publishing, AiToEarn官网 offers an open-source platform for content creation, cross-platform publishing, analytics, and monetization — echoing the integration and productivity goals of the Kotlin–Spring collaboration.