Spring Boot 4: The Dawn of Intelligent Java
Introduction
The wait is over. Spring Boot 4.0 has officially landed, marking the biggest shift in the ecosystem since the introduction of reactive programming. Built strictly on Java 25, this release focuses on three pillars: Speed (Leyden), Simplicity (Virtual Threads), and Intelligence (Spring AI).
Key Features
1. Java 25 Baseline
Spring Boot 4 drops support for older Java versions. By requiring Java 25, the framework now fully embraces:
- Pattern Matching for everything: Resulting in significantly cleaner code.
- Pinned Virtual Threads: The days of synchronized blocking virtual threads are long gone.
- Value Objects: Reducing memory footprint for data-heavy applications.
2. Project Leyden Integration
The most hyped feature is the native integration with Project Leyden. Unlike GraalVM native images which require a complex build process and compromise on dynamic features, Leyden allows for "Static Images" that start in milliseconds but retain the flexibility of the JVM.
- Startup Time: Reduced by ~400ms for standard REST apps.
- Warmup: Practically zero.
3. Virtual Threads are Default
In Spring Boot 3.2, Virtual Threads were an opt-in. In version 4, they are the default execution model for Tomcat and Jetty.
- Say goodbye to CompletableFuture hell.
- Write blocking code that scales like reactive code. The need for WebFlux has diminished for 95% of use cases.
4. Spring AI Core
AI is no longer an extension; it's baked into the core.
- @AiClient Annotation: Inject pre-configured LLM clients (OpenAI, Gemini, Claude) as easily as a database connection.
- Vector Store Autoconfiguration: Spring Data now auto-configures vector databases (Pinecone, Chroma, PGVector) based on your classpath.
Migration Guide
Upgrading from 3.x to 4.0 is smoother than the 2.x to 3.x jump, but there are gotchas:
- Jakarta EE 12: Ensure your dependencies are compatible with the latest Jakarta namespaces.
- WebFlux Deprecation warnings: While not removed, parts of the reactive stack are marked for future optionality in favor of Virtual Threads.
Conclusion
Spring Boot 4 isn't just an update; it's a statement that Java is the premier language for the AI era. It combines the massive ecosystem we love with the performance required for modern workloads.
