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What's New in Spring Boot 4.0.0: Key Features Explained

Cowrie
Cowrie
Dev @ Bswen

Purpose

I’ve been exploring Spring Boot 4.0.0 to understand what’s changed in this major release and whether it’s worth upgrading. This post captures what I learned about the key new features, why they matter, and how they might affect real-world applications.

Spring Boot 4.0.0 represents a significant milestone - it requires JDK 24, builds on Spring Framework 7.0.1, and introduces some major capabilities. I wanted to cut through the release notes and understand what actually matters for developers building applications today.

Context

Before diving into features, let me acknowledge the prerequisites. Spring Boot 4.0.0 isn’t a drop-in upgrade for everyone:

  • JDK 24 baseline: You’ll need Java 24 to run Spring Boot 4.0.0 applications
  • Spring Framework 7.0.1: The framework dependency has jumped to version 7.x
  • Breaking changes: As with any major version bump, some things work differently

This is important context because the JDK 24 requirement means you can’t just flip a switch on production systems. You need to plan for runtime changes, CI/CD pipeline updates, and potential library compatibility issues.

Virtual Threads: The Headline Feature

Virtual threads are the big story in Spring Boot 4.0.0. If you’ve been following Java development since Java 21, you know virtual threads (Project Loom) promise to simplify concurrent programming by making threads lightweight and cheap.

Spring Boot 4.0.0 adds first-class support for virtual threads. Here’s why this matters:

The Problem with Traditional Threads

  • Platform threads are expensive (each consumes ~1MB stack memory)
  • Thread pools limit scalability
  • Context switching overhead increases with thread count
  • Complex async programming models (reactive, CompletableFuture chains)

How Virtual Threads Help

  • Virtual threads are lightweight (managed by the JVM, not the OS)
  • You can create millions of them without exhausting resources
  • Blocking operations don’t block platform threads
  • Write simple blocking code that scales

Enabling Virtual Threads

spring:
  threads:
    virtual:
      enabled: true

That’s the basic configuration. For more control over the thread executor:

@Bean
public AsyncTaskExecutor applicationTaskExecutor(SimpleAsyncTaskExecutorBuilder builder) {
    return builder
        .virtualThreads(true)
        .threadNamePrefix("vthread-")
        .build();
}

When to Use Virtual Threads

I found that virtual threads shine for I/O-bound workloads:

  • REST APIs calling external services
  • Database-heavy applications
  • Microservices with network communication
  • Applications handling many concurrent requests

They’re less beneficial for CPU-bound tasks where you’re limited by processor cores anyway.

Caveats I Discovered

  • Not all libraries support virtual threads yet (some use fixed thread pools or synchronized blocks)
  • Thread-local usage needs careful consideration (you’ll have way more threads)
  • Monitoring tools may need updates to handle virtual thread metrics

Enhanced Observability

Observability gets a significant boost in Spring Boot 4.0.0. The Micrometer Observation API is now better integrated, which means you get better tracing and metrics out of the box.

What’s Improved

  1. Built-in Observation Context: Spring Boot 4.0.0 automatically creates observations for common operations
  2. Simplified Custom Observations: Creating your own observations is more straightforward
  3. Better Distributed Tracing: Automatic propagation of trace context across service boundaries
  4. Enhanced Actuator Endpoints: More metrics and health indicators available

Why This Matters

In microservices architectures, understanding what’s happening across service boundaries is critical. The improved observability means you spend less time instrumenting code and more time analyzing real data.

I found the automatic observation particularly useful for:

  • Database query performance tracking
  • HTTP client/server latency measurements
  • Cache hit/miss monitoring
  • Custom business metric collection

The key insight is that Spring Boot 4.0.0 shifts observability from “something you add later” to “something that works by default.”

Docker Compose & Testcontainers Integration

This feature caught my attention because it addresses a real pain point: managing service dependencies during development and testing.

Docker Compose Support

Spring Boot 4.0.0 introduces the spring-boot-docker-compose dependency. Add it to your project:

implementation 'org.springframework.boot:spring-boot-docker-compose'

Place a docker-compose.yml file in your project root, and Spring Boot will:

  • Automatically start defined services when your application starts
  • Configure connection properties (datasource URLs, etc.)
  • Shut down services when your application stops

This means no more manually starting PostgreSQL, Redis, or other services before running your app. The environment matches production more closely because services run in containers just like they would in deployment.

Testcontainers Integration

For integration testing, the new spring-boot-testcontainers dependency simplifies working with Testcontainers:

testImplementation 'org.springframework.boot:spring-boot-testcontainers'
testImplementation 'org.testcontainers:postgresql'

Why This Matters

I’ve seen too many integration tests that fail in CI but pass locally because environment differences. Testcontainers integration means:

  • Tests run against actual containerized services (not mocks)
  • Consistent test environments across machines
  • Easy testing against multiple database versions
  • No shared state between test runs

The combination of Docker Compose for development and Testcontainers for testing creates a consistent environment story that reduces “works on my machine” problems.

Security Improvements

Spring Security configuration patterns continue to evolve in Spring Boot 4.0.0. The changes aren’t revolutionary, but they refine the developer experience.

Updated Configuration Patterns

The recommended SecurityFilterChain configuration continues to emphasize the lambda-based DSL:

@Bean
public SecurityFilterChain securityFilterChain(HttpSecurity http) throws Exception {
    http.authorizeHttpRequests(authz -> authz
        .requestMatchers("/public/**").permitAll()
        .anyRequest().authenticated()
    );
    return http.build();
}

What’s Better

  1. Enhanced Auto-Configuration: Security setup requires less explicit configuration
  2. Improved Defaults: Authorization rules follow security best practices by default
  3. Better Error Messages: Security configuration failures provide clearer guidance

Why This Matters

Security configuration is notoriously difficult to get right. The improvements in Spring Boot 4.0.0 reduce the surface area for mistakes. However, if you have complex custom security logic (multi-tenant setups, custom authentication providers, etc.), you’ll still need to test thoroughly during migration.

Property Migration Tool

This is one of those practical features that makes major version upgrades less painful. Spring Boot 4.0.0 includes a properties migrator tool.

How It Works

Add the dependency:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-properties-migrator</artifactId>
    <scope>runtime</scope>
</dependency>

When you start your application, the migrator:

  • Scans your application properties/yaml files
  • Compares them against known deprecated/renamed properties
  • Prints diagnostics about what needs to change
  • Temporarily maps old properties to new ones at runtime

Important: Remove this dependency after migration is complete. It’s not meant to stay in production builds.

Why This Matters

I’ve spent hours debugging production issues caused by renamed properties that nobody updated. The migrator tool automates what used to be manual documentation searching. It’s particularly useful if you have:

  • Large property files spread across multiple profiles
  • Properties defined in environment variables
  • Custom Spring Boot starters with their own configuration

Performance Enhancements

Spring Boot 4.0.0 includes several performance improvements, though the impact varies based on your workload.

What I Found

  1. Startup Time: Improvements in auto-configuration processing reduce startup time
  2. Memory Footprint: Virtual threads dramatically reduce memory usage for concurrent workloads
  3. AOT Compilation: Enhanced ahead-of-time compilation support for native images
  4. Native Image: Better GraalVM native image compilation

Quantifying the Impact

The performance gains depend on your use case:

  • I/O-bound applications benefit most from virtual threads
  • Startup improvements are more noticeable in large applications with many auto-configurations
  • Native image compilation requires careful testing (not all libraries support it)

The key takeaway: Spring Boot 4.0.0 is faster, but you need to measure your specific application to see meaningful improvements.

Developer Experience Improvements

Beyond the headline features, Spring Boot 4.0.0 includes several quality-of-life enhancements.

Better Diagnostics

Error messages are more actionable. Instead of generic “bean creation failed” errors, Spring Boot 4.0.0 provides:

  • Specific suggestions for missing configuration
  • Clearer explanations of auto-configuration failures
  • Links to documentation for common problems

Enhanced IDE Support

The Spring Boot 4.0.0 metadata improvements mean:

  • Better auto-completion in application.properties
  • More accurate syntax validation
  • Improved navigation between configuration and code

Simplified Testing

Testcontainers integration and improved test auto-configuration make it easier to write tests that actually verify behavior rather than implementation details.

Breaking Changes & Migration Considerations

Major version bumps always involve breaking changes. Here’s what I found most significant:

Property Changes

Several properties have been renamed or removed. Use the properties migrator tool to identify what needs updating in your application.

Dependency Updates

  • Spring Framework 7.0.1 may introduce API changes in custom code
  • Some third-party libraries may not yet support Spring Boot 4.0.0
  • Jakarta EE namespace changes (if any) require package import updates

Auto-Configuration Changes

Some auto-configurations have been removed or their defaults changed. Pay particular attention to:

  • Security configuration
  • DataSource configuration
  • Actuator endpoints

Migration Checklist

Based on what I learned, here’s a practical migration approach:

  1. Verify JDK 24 is available in all environments (dev, test, prod)
  2. Update Spring Boot parent version to 4.0.0
  3. Add properties migrator dependency
  4. Run application locally and review diagnostics
  5. Update deprecated properties
  6. Remove properties migrator
  7. Test thoroughly with virtual threads enabled
  8. Update CI/CD pipelines for JDK 24
  9. Validate all third-party dependencies
  10. Performance test in staging environment

Should You Upgrade to Spring Boot 4.0.0?

Based on my exploration, here’s when the upgrade makes sense:

Upgrade If:

  • You’re starting a new project (clean slate is ideal)
  • You need virtual threads for scalability (I/O-bound workloads)
  • Improved observability would help your operations
  • You want Docker Compose/Testcontainers integration
  • You have resources for thorough testing

Wait If:

  • You’re in the middle of critical feature development
  • You don’t need the new features urgently
  • You have complex Spring Security customizations
  • Critical dependencies don’t yet support 4.0.0
  • Your team hasn’t adopted JDK 24 yet

The decision isn’t just about features - it’s about timing. Virtual threads are compelling, but they require JDK 24, which may conflict with other constraints in your environment.

Summary

Spring Boot 4.0.0 introduces meaningful improvements, with virtual threads as the headline feature. The enhanced observability, Docker Compose integration, and developer experience refinements make it a compelling upgrade for new projects and applications that can benefit from improved concurrency.

The JDK 24 requirement is the biggest barrier to adoption. If you can meet that prerequisite, the upgrade path is manageable with the properties migrator tool. The performance improvements from virtual threads are real, but they require testing to quantify in your specific context.

For production systems, I’d recommend a phased approach: start with non-production workloads, validate virtual threads behavior, test all third-party dependencies, and migrate gradually when you have capacity for thorough testing.

Final Words + More Resources

My intention with this article was to help others share my knowledge and experience. If you want to contact me, you can contact by email: Email me

Here are also the most important links from this article along with some further resources that will help you in this scope:

Oh, and if you found these resources useful, don’t forget to support me by starring the repo on GitHub!

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