wildfly (tried with 8.2 and 9.0.0.CR1) sends "100-Continue" twice to the client. It is observable with commons-httpclient v3.1, which logs at each POST- and PUT-request the info-message "Discarding unexpected response: HTTP/1.1 100 Continue", which is annoying in a high-load environment.
A little research in the sources revealed, that io.undertow.server.handlers.HttpContinueReadHandler is contained twice in the request-handler-chain:
1. inserted by org.wildfly.extension.undertow.Host (org.wildfly:wildfly-undertow) building the root-handler
2. inserted by io.undertow.servlet.core.DeploymentManagerImpl (io.undertow:undertow-servlet) as a deployment-level handler
I'm considering this an undertow-bug, because in my view the protocol-level handler HttpContinueReadHandler shouldn't be used on deployment-level.