Spring Boot 4.0.0 Released – Deep Dive into the Next Generation of Spring Applications

Spring Boot 4.0.0 Released – Deep Dive into the Next Generation of Spring Applications

Spring Boot 4.0.0 has officially arrived, and this is not just another incremental upgrade. It marks the beginning of a new generation of Spring Boot, built on top of Spring Framework 7, and designed to future-proof Java applications for the coming years.


With a focus on modularity, performance, API evolution, modern Java support, and developer productivity, Spring Boot 4.0.0 introduces architectural changes that every Java developer should understand before upgrading.

In this blog, we take a deep dive into the major features, explain why they matter, and explore practical examples.

1. Spring Boot 4.0.0 at a Glance

Spring Boot 4.0.0 introduces:

  • A fully modularized codebase
  • Portfolio-wide null safety improvements using JSpecify
  • First-class support for Java 25 (while retaining Java 17 compatibility)
  • Native API Versioning support
  • Improved HTTP Service Clients for REST-based applications

Since this is a major release, upgrading from Spring Boot 3.x requires careful planning and following the official migration guide.

2. Complete Modularization of Spring Boot

What Changed?

Spring Boot 4.0.0 introduces a fully modular architecture. Earlier versions relied on large, monolithic starter dependencies. Now, Spring Boot is broken into smaller, purpose-focused modules.

Why This Matters

  • Smaller application footprints
  • Faster startup times
  • Better dependency control
  • Clear separation of concerns
  • Easier native and cloud-native optimizations

Example

Earlier:

<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-web</artifactId>
</dependency>

Now, internally this starter pulls modular components such as:

  • Web MVC core
  • JSON serialization
  • Validation
  • HTTP handling
Impact for developers:
You still use familiar starters, but your application becomes leaner and more maintainable under the hood.

3. Portfolio-Wide Null Safety with JSpecify

What is JSpecify?

JSpecify is a standard for expressing nullness in Java APIs. Spring Boot 4 adopts it across the ecosystem.

Why This Is Huge

  • APIs explicitly declare nullability
  • IDEs catch issues at compile time
  • Safer refactoring
  • Better contract clarity between components

Example

import org.jspecify.annotations.Nullable;

public String findUsername(@Nullable String userId) {
    if (userId == null) {
        return "anonymous";
    }
    return userService.find(userId);
}
Impact:
Spring Boot applications become more robust, especially in large enterprise systems.

4. First-Class Support for Java 25 (Java 17 Still Supported)

Java Compatibility

  • Java 25 (latest)
  • Java 17 (baseline LTS)

Why This Matters

  • Use modern Java features
  • Deploy on stable enterprise environments
  • Gradually modernize existing codebases

Example: Virtual Threads (Java 21+)

@Bean
public Executor taskExecutor() {
    return Executors.newVirtualThreadPerTaskExecutor();
}
  • Massive concurrency
  • Lower memory usage
  • Better scalability for I/O-heavy applications

5. Built-In API Versioning Support

The Problem

API evolution is hard. Changing endpoints often breaks existing clients.

The Solution in Spring Boot 4

Spring Boot 4 introduces first-class API versioning support, making versioned REST APIs clean and consistent.

Example: Header-Based Versioning

@RestController
@RequestMapping("/api/users")
public class UserController {

    @GetMapping(headers = "API-Version=1")
    public UserV1 getUserV1() {
        return new UserV1("John");
    }

    @GetMapping(headers = "API-Version=2")
    public UserV2 getUserV2() {
        return new UserV2("John", "Doe");
    }
}

Supported strategies:

  • Header-based
  • URL-based
  • Media-type based

6. HTTP Service Clients for REST Applications

What Are HTTP Service Clients?

Spring Boot 4 improves HTTP Service Clients, making REST calls:

  • Declarative
  • Type-safe
  • Less boilerplate

Example

@HttpExchange("/users")
public interface UserClient {

    @GetExchange("/{id}")
    User getUser(@PathVariable Long id);
}

Configuration:

@Bean
UserClient userClient(HttpServiceProxyFactory factory) {
    return factory.createClient(UserClient.class);
}
  • Cleaner code
  • Easier testing
  • Better readability
  • Ideal for microservices

7. Migration Considerations

  • Dependency alignment updates
  • Package and module changes
  • Stricter nullability checks
  • Updated configuration defaults

Recommendations

  • Read the official migration guide
  • Upgrade Java version carefully
  • Test API behavior thoroughly
  • Validate third-party library compatibility

8. Final Thoughts

Spring Boot 4.0.0 is not just an upgrade — it’s a strategic reset that aligns Spring with:

  • Modern Java
  • Cloud-native architecture
  • Safer APIs
  • Long-term sustainability

If you are building enterprise applications, microservices, or high-performance backends, Spring Boot 4.0.0 sets the foundation for the next decade.

Conclusion

  • Cleaner architecture
  • Safer code
  • Better performance
  • Future-ready Java support

This is the Spring Boot evolution developers have been waiting for. Do visit official notes : Spring Boot 4.0.0

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