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What to Expect from Spring Boot 4.x: Preparing for the Next Generation

Cowrie
Cowrie
Dev @ Bswen

Purpose

When I tried to deploy a Spring Boot application to a serverless platform last month, I got this:

Terminal window
[INFO] Starting application...
[WARN] JVM warm-up time: 8.5 seconds
[ERROR] Function timed out after 10 seconds

My Spring Boot 3.x application couldn’t start fast enough for serverless environments. The cold start was killing me.

This is one of the main problems Spring Boot 4.x aims to solve. In this post, I’ll explore what we can expect from the next major Spring Boot release and how to prepare for it.

The Problems We Face Today

Before diving into Spring Boot 4.x, let me explain the challenges I encounter with Spring Boot 3.x:

Problem 1: Cold Start Latency

Cold start comparison
Traditional JVM:        8-15 seconds
Native Image (GraalVM): 0.05-0.5 seconds

In serverless environments like AWS Lambda or Google Cloud Functions, every millisecond counts. My Spring Boot apps start too slowly because of:

  • Classpath scanning at startup
  • Bean initialization overhead
  • Reflection-based configuration

Problem 2: Native Image Complexity

I tried to convert my app to a native image with GraalVM:

Native image build attempt
./mvnw -Pnative native:compile


[ERROR] Failed to reach 'org.mybatis.spring.SqlSessionFactoryBean'
[ERROR] No metadata found for reflection
[ERROR] Build failed after 4 minutes

The configuration was painful. I spent hours adding reflection hints and reachability metadata.

Problem 3: Container Image Size

Docker image layers
REPOSITORY          TAG       SIZE
myapp:latest        jvm       450MB
myapp:latest        native    85MB

But building the native image required so much configuration that I gave up and used the JVM version.

Problem 4: Observability Fragmentation

I had to integrate multiple tools for observability:

pom.xml - Dependencies I needed
<!-- Metrics -->
<dependency>
    <groupId>io.micrometer</groupId>
    <artifactId>micrometer-core</artifactId>
</dependency>


<!-- Tracing -->
<dependency>
    <groupId>io.micrometer</groupId>
    <artifactId>micrometer-tracing-bridge-brave</artifactId>
</dependency>


<!-- Exporters -->
<dependency>
    <groupId>io.micrometer</groupId>
    <artifactId>micrometer-registry-prometheus</artifactId>
</dependency>

Each tool had different configuration approaches. It felt fragmented.

What Spring Boot 4.x Might Bring

Based on current trends and Spring Framework 7 roadmap, here’s what I expect:

1. Enhanced Native Image Support

Spring Boot 4.x will likely make native images a first-class citizen:

Expected improvement
Spring Boot 3.x:  Manual metadata, complex setup
Spring Boot 4.x:  Auto-configured hints, simpler build process

What this means for you:

pom.xml - Simplified native build
<plugin>
    <groupId>org.graalvm.buildtools</groupId>
    <artifactId>native-maven-plugin</artifactId>
    <!-- Spring Boot 4.x handles metadata automatically -->
</plugin>
One-command native build
./mvnw native:compile -Pnative
# No manual reflection hints needed

2. Improved Startup Performance

The expected startup improvements:

Expected startup times
Spring Boot 3.x (JVM):      2-8 seconds
Spring Boot 3.x (Native):   0.1-0.5 seconds
Spring Boot 4.x (AOT):      0.5-2 seconds  <- New middle ground
Spring Boot 4.x (Native):   0.05-0.2 seconds

Spring Boot 4.x introduces better AOT (Ahead-of-Time) compilation that gives you faster startup without full native image complexity.

3. Built-in Observability

Spring Boot 4.x will likely integrate OpenTelemetry by default:

application.yml - Expected configuration
management:
  observations:
    enabled: true
  tracing:
    enabled: true
    sampling:
      probability: 1.0
  otlp:
    metrics:
      export:
        enabled: true
    tracing:
      endpoint: "http://localhost:4318/v1/traces"

No more piecing together Micrometer, Brave, and various exporters.

4. Container-First Design

Expected container improvements:

Container layering strategy
Spring Boot 3.x:  Manual layer extraction
Spring Boot 4.x:  Layered JARs by default, optimized for Docker
Dockerfile - Expected pattern
FROM eclipse-temurin:21-jre
WORKDIR /app
COPY target/myapp.jar app.jar
# Spring Boot 4.x automatically extracts layers efficiently
ENTRYPOINT ["java", "-jar", "app.jar"]

5. Java 21+ Requirement

Spring Boot 4.x will require at least Java 21:

Java version requirements
Spring Boot 2.x:  Java 8+
Spring Boot 3.x:  Java 17+
Spring Boot 4.x:  Java 21+  <- Virtual threads, pattern matching

This enables:

  • Virtual threads for better concurrency
  • Pattern matching for switch expressions
  • Record patterns for destructuring
  • Sequenced collections

How to Prepare Now

I’m already preparing my applications for Spring Boot 4.x. Here’s my checklist:

Step 1: Upgrade to Java 21

pom.xml
<properties>
    <java.version>21</java.version>
</properties>

Test your application with Java 21 before Spring Boot 4.x arrives.

Step 2: Clean Up Deprecated APIs

Run your tests and fix deprecation warnings:

Find deprecated usage
./mvnw compile -X 2>&1 | grep -i "deprecated"

Step 3: Try Native Images Now

Get familiar with native image builds:

pom.xml
<profile>
    <id>native</id>
    <build>
        <plugins>
            <plugin>
                <groupId>org.graalvm.buildtools</groupId>
                <artifactId>native-maven-plugin</artifactId>
                <version>0.10.2</version>
            </plugin>
        </plugins>
    </build>
</profile>
Build and test
./mvnw -Pnative native:compile
./target/myapp

Even if you encounter errors, you’ll learn what metadata your application needs.

Step 4: Add Observability Now

Start using Micrometer and OpenTelemetry today:

pom.xml
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-actuator</artifactId>
</dependency>
<dependency>
    <groupId>io.micrometer</groupId>
    <artifactId>micrometer-registry-prometheus</artifactId>
</dependency>
<dependency>
    <groupId>io.micrometer</groupId>
    <artifactId>micrometer-tracing-bridge-otel</artifactId>
</dependency>
application.yml
management:
  endpoints:
    web:
      exposure:
        include: health,info,metrics,prometheus
  observations:
    enabled: true

This way, when Spring Boot 4.x arrives, you’re already familiar with the concepts.

Step 5: Review Your Dependencies

Check for incompatible dependencies:

Check dependency compatibility
./mvnw dependency:tree | grep -E "(javax|jakarta)"

Make sure you’re using Jakarta EE 10+ namespaces, not the old javax packages.

Common Migration Concerns

I anticipate these challenges when upgrading to Spring Boot 4.x:

Concern 1: Breaking Changes

Every major version has breaking changes. Spring Boot 3.x required migration from javax to jakarta packages. Spring Boot 4.x might have similar shifts.

My advice: Stay on Spring Boot 3.x for production until 4.x has been stable for at least 3 months.

Concern 2: Third-Party Library Compatibility

Compatibility timeline
Spring Boot 4.x release:     Your libraries might not work
3 months after release:      Major libraries updated
6 months after release:      Most libraries compatible

My advice: Check your critical dependencies before upgrading.

Concern 3: Build Tool Updates

You might need to update:

Expected requirements
Maven:      3.9.0+
Gradle:     8.5+
Java:       21+

My advice: Update your build tools early.

What I’m Excited About

The features I’m most looking forward to:

Virtual Threads Integration

Java 21’s virtual threads will make concurrent programming simpler:

Expected pattern
@GetMapping("/users")
List<User> getUsers() {
    // Virtual threads handle blocking I/O efficiently
    return userRepository.findAll();
}

No more reactive programming complexity for simple use cases.

AOT Compilation Improvements

Spring Boot 4.x will likely offer AOT compilation without full native image builds:

Compilation options
JVM:        Traditional, slow startup
AOT:        Compiled, medium startup, easy debugging
Native:     Maximum speed, limited reflection

This gives you a middle ground between JVM flexibility and native performance.

Unified Observability

One configuration for metrics, traces, and logs:

application.yml - Expected unified config
management:
  observations:
    enabled: true
  otlp:
    endpoint: "http://localhost:4318"

Summary

Spring Boot 4.x will address the problems I face with cold starts, native image complexity, and observability fragmentation. Here’s what to remember:

  • Java 21+ is required - upgrade now if you haven’t
  • Native images will be easier - less manual configuration
  • Observability is built-in - start learning Micrometer and OpenTelemetry today
  • Cold starts will improve - especially important for serverless
  • Migration takes planning - don’t rush, wait for ecosystem support

I recommend preparing now by upgrading to Java 21, adding observability, and experimenting with native images. When Spring Boot 4.x arrives, you’ll be ready.

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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