Tomcat and Jetty are not application servers according to the Jakarta EE definition. They're just servlet containers.
The reason Spring includes those libraries is partly historical - Spring is old, and dates from the applications server days. Newer frameworks like Micronaut and Quarkus use more focused and performant libraries like Netty, Vert.x, and Undertow instead.
The reason Spring includes those libraries is partly historical - Spring is old, and dates from the applications server days. Newer frameworks like Micronaut and Quarkus use more focused and performant libraries like Netty, Vert.x, and Undertow instead.