Try casting the ServletOutputStream to io.undertow.io.BufferWritableOutputStream (or io.undertow.servlet.spec.ServletOutputStreamImpl) and then use the transferFrom(FileChannel source) method.
Thanks for the reply. I tried this and it appears to work. Easy drop-in replacement for the old Tomcat/APR/sendfile. I don't know how the performance compares but I would guess similar assuming they use sendfile under the hood. Thanks!