Undertow is an open-source lightweight, flexible and performant Java server, they say. I can confirm that it’s
- lightweight: just have a look at those few lines of code to start a server and 1MB core JAR
- flexible: always feel free to provide your own implementations or use Undertow helpers to delegate usual server glue code to a more specific implementation you provide

I didn’t check or compare performance. It is the default server implementation of Wildfly Application Server and sponsored by JBoss.

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