This is a major upgrade across three major versions (3.x → 6.x) of the Spring Framework, introducing significant breaking changes. A direct upgrade is not feasible and will require a substantial migration effort.

Key Breaking Changes:

• Java Version Requirement: Spring Framework 6.x requires a Java 17 baseline at minimum. This is a major jump from the Java 6/7/8 requirements of Spring 3.x and 4.x.

• Jakarta EE Namespace Migration: Spring 6 moves from Java EE to Jakarta EE 9+. This requires changing all package imports from javax.* to jakarta.* for APIs like Servlet, JPA, and Bean Validation. This change impacts the entire codebase and its dependencies.

• Removed APIs and Integrations: Many classes and methods deprecated in versions 4.x and 5.x have been removed. Notably, Spring 6 removes support for several outdated integrations, including Apache Commons FileUpload (CommonsMultipartResolver) and Apache Tiles.

• Configuration and Behavior Changes:

  • Trailing Slash Matching: In Spring MVC, requests with a trailing slash (e.g., /endpoint/) no longer match handlers for /endpoint by default.
  • XML to Java Config: While not a hard break, applications using legacy XML configuration from the Spring 3 era should be migrated to modern Java-based @Configuration.

• Dependency Ecosystem: The upgrade requires compatible versions of the entire ecosystem, including web servers (e.g., Tomcat 10.1+, Jetty 11+) and persistence providers (e.g., Hibernate ORM 6.1+).

Recommendation:
This is a major migration project, not a simple dependency update. A direct version bump will fail. It is strongly recommended to treat this as a new development effort, potentially by creating a new application using the latest Spring Boot (which manages Spring Framework 6) and migrating code, rather than attempting an in-place upgrade.

Source: Spring Framework 6.0 Release Notes, Upgrading to Spring Framework 5.x, Migrating from Spring Framework 3.2 to 4.0.1

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