What embedded servers does Spring Boot support, and how do you change the default?

Spring Boot ships with embedded Tomcat by default (via spring-boot-starter-web), and also supports Jetty and Undertow. To switch, exclude the default Tomcat starter dependency and add the starter for the server you want instead (e.g. spring-boot-starter-undertow) — the embedded-server model means the application itself is a runnable JAR with the server built in, rather than a WAR deployed into an externally-installed server.