Spring Boot 4.0 removes support for the Undertow embedded server because it is not compatible with the required Servlet 6.1 baseline and lacks sufficient maintenance, prompting developers to switch to Tomcat or Jetty and follow detailed migration steps to avoid startup failures and configuration loss.
Spring Boot 4.0, built on Spring Framework 7, requires Servlet 6.1, leading to the removal of Undertow support; the article explains the technical reasons, the impact on existing projects, and provides step‑by‑step migration paths to Tomcat or Jetty, including configuration changes and risk considerations.
Spring Boot 4.0 removes Undertow because it cannot meet the Servlet 6.1 baseline, leading to build failures for projects that depend on spring-boot-starter-undertow, and the article analyzes the technical reasons, compatibility matrix, and lessons for future web‑container selection.
Spring Boot 4.0 RC1 removes Undertow support because it cannot meet the Servlet 6.1 baseline, forcing developers to migrate web containers, and the article explains the technical reasons, new Servlet 6.1 features, Jakarta EE 11 changes, compatibility matrix, and migration guidance.
