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.
Web containers, such as Tomcat, keep running by continuously listening on network ports with socket servers and thread pools, handling I/O requests; this article explains the role of the main function, servlet lifecycle, Netty’s NIO framework, and why WebFlux‑based gateways outperform traditional Tomcat setups.
