Spring Boot 3 brings significant changes that every Java developer should understand. After successfully migrating several enterprise applications, I want to share the key insights and lessons learned.

Why Upgrade?

Spring Boot 3 requires Java 17 as the minimum version, but I recommend going straight to Java 21 for its LTS status and powerful features like virtual threads, pattern matching, and record patterns.

Key Changes in Spring Boot 3

1. Jakarta EE 9+ Migration

The most significant change is the shift from javax. to jakarta. namespace. This affects:
  • Servlet API (javax.servlet → jakarta.servlet)
  • JPA annotations (javax.persistence → jakarta.persistence)
  • Validation annotations (javax.validation → jakarta.validation)
// Before (Spring Boot 2.x)
import javax.persistence.Entity;
import javax.validation.constraints.NotNull;

// After (Spring Boot 3.x)
import jakarta.persistence.Entity;
import jakarta.validation.constraints.NotNull;

2. Native Image Support

Spring Boot 3 has first-class support for GraalVM native images: 
@SpringBootApplication
public class MyApplication {
    public static void main(String[] args) {
        SpringApplication.run(MyApplication.class, args);
    }
}

Build native image:

./mvnw -Pnative native:compile

3. Observability with Micrometer

The new observability features make distributed tracing seamless: 
@RestController
public class OrderController {

    private final ObservationRegistry registry;

    @GetMapping("/orders/{id}")
    public Order getOrder(@PathVariable Long id) {
        return Observation.createNotStarted("order.fetch", registry)
            .observe(() -> orderService.findById(id));
    }
}

Migration Steps

  1. Update Java Version: Ensure Java 17+ (preferably 21)
  2. Update Dependencies: Modify pom.xml or build.gradle
  3. Fix Namespace Changes: javax. to jakarta.
  4. Review Security Config: WebSecurityConfigurerAdapter is removed
  5. Test Thoroughly: Especially integration tests

Performance Improvements

In our production environment, we observed:

  • 30% faster startup time

  • 25% reduction in memory footprint

  • Improved throughput with virtual threads

Conclusion

While the migration requires effort, Spring Boot 3 with Java 21 provides significant benefits in performance, security, and developer experience. Start planning your migration today!