Spring Boot 4 Upgrade Pitfalls Guide: Java 21, Jakarta, Jackson 3, and Copilot Automation

Spring Boot 4 introduces major ecosystem changes and many teams are planning to upgrade their applications to Java 21 and Spring Boot 4.0.x.

However, real-world upgrades often reveal multiple compatibility issues such as:

• Jakarta package migration breaking compilation
• Jackson 3 JSON behavior changes
• Spring Cloud Gateway configuration updates
• Test framework migration issues
• Micrometer / Observability runtime errors

This article summarizes a production-ready upgrade strategy and demonstrates how to use GitHub Copilot Agent to automate much of the migration work.

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Table of Contents

1. Why Upgrade to Spring Boot 4
2. Recommended Upgrade Strategy
3. Using GitHub Copilot for Automated Migration
4. Java 21 Upgrade
5. Spring Boot 4 Upgrade
6. Jakarta Migration
7. Jackson 3 Changes
8. Spring Cloud Gateway Changes
9. Common Upgrade Pitfalls
10. Upgrade Verification Checklist

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Why Upgrade to Spring Boot 4

Spring Boot 4 introduces several important platform improvements:

• Official support for Java 21
• Full migration to Jakarta APIs
• Adoption of Jackson 3 for JSON processing
• Improved Micrometer Observability
• Modern Spring Security configuration

Upgrading provides long-term benefits including:

• Better performance
• Improved security
• Better cloud-native compatibility

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Recommended Upgrade Strategy

Large systems should upgrade in incremental phases.

1. Upgrade Java runtime
2. Upgrade Spring Boot version
3. Perform Jakarta package migration
4. Align dependency versions
5. Fix configuration changes
6. Update and fix tests
7. Run full build verification

Each phase should include:

• Compilation verification
• Unit tests
• Application startup validation

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Using GitHub Copilot for Automated Migration

For large repositories, manual upgrades can be time-consuming.

GitHub Copilot Agent can analyze the project structure and generate a migration Pull Request automatically.

Recommended Copilot Prompt

Goal:
Upgrade this project to Java 21 and Spring Boot 4.0.x.

Constraints:
- Preserve existing behaviour
- Do NOT remove business logic
- Ensure project builds and tests pass

Tasks:
1 Upgrade Java version
2 Upgrade Spring Boot parent
3 Replace javax with jakarta
4 Align dependencies
5 Update configuration
6 Fix tests
7 Verify application startup
8 Provide migration summary

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Java 21 Upgrade

Update the Maven configuration:

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

Verification steps:

• The project compiles successfully
• Unit tests pass
• The application starts correctly

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Spring Boot 4 Upgrade

Update the Maven parent:

<parent>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-parent</artifactId>
<version>4.0.3</version>
</parent>

Upgrade principles:

• Remove dependency versions managed by Spring Boot
• Use official Spring Boot starters
• Avoid overriding Boot-managed dependencies

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Jakarta Migration

Spring Boot 4 fully adopts the Jakarta ecosystem.

Old Package	New Package
javax.servlet	jakarta.servlet
javax.validation	jakarta.validation
javax.persistence	jakarta.persistence

Most IDEs can automatically refactor these imports.

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Jackson 3 Changes

Spring Boot 4 now uses Jackson 3.

Important changes include:

• Group ID moved from com.fasterxml.jackson to tools.jackson
• @JsonComponent renamed to @JacksonComponent
• JSON configuration properties moved under spring.jackson.json

If migration to Jackson 3 is not immediately possible, Spring Boot provides a temporary compatibility module.

<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-jackson2</artifactId>
</dependency>

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Spring Cloud Gateway Configuration Changes

Spring Cloud Gateway introduced structural changes in the configuration.

New starters include:

• spring-cloud-starter-gateway-server-webflux
• spring-cloud-starter-gateway-server-webmvc

Example configuration structure:

spring:
  cloud:
    gateway:
      server:
        webflux:
          routes:

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Common Upgrade Pitfalls

1 Jackson Serialization Errors

Jackson 3 introduces subtle changes in JSON parsing behavior, which may break existing serialization logic.

2 Missing WebTestClient Annotation

Add the required test dependency:

<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-webflux-test</artifactId>
<scope>test</scope>
</dependency>

3 Micrometer Assertion Errors

Reactive applications may trigger Observability assertion errors if the Reactor context is overwritten.

Always merge the Reactor context instead of replacing it.

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Upgrade Verification Checklist

Before merging the upgrade PR, verify the following:

• Application starts successfully
• Health endpoints respond correctly
• Authentication and authorization work
• OAuth token flow functions correctly
• External API calls succeed
• Logs include trace or transaction IDs
• Metrics are emitted through Micrometer
• Integration tests pass

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Conclusion

The most challenging parts of upgrading to Spring Boot 4 typically involve:

• Jakarta package migration
• Jackson 3 compatibility
• Spring Cloud configuration changes
• Test framework updates

Using GitHub Copilot Agent can significantly reduce upgrade effort and even generate migration pull requests automatically.

By following a phased migration strategy, teams can safely upgrade their applications to Java 21 and Spring Boot 4.

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