Sitting at home in a not so decent state made me finally fiddle with java-package to deal with Oracle Java 9 builds. For now I've added only some half-assed support for JDK 9 amd64 builds. That's what you download as "jdk-9.0.1_linux-x64_bin.tar.gz" from the Oracle Java pages. It's a works for me thing, but maybe someone finds it useful, the source is here.

git clone https://git.sven.stormbind.net/java-package.git
cd java-package
sed -i -e 's#lib_dir="/usr/share/java-package"#lib_dir="./lib"#' make-jpkg

and you can just start using it in this directory without creating and installing the java-package Debian package.

Side note: If you try out Java within a chroot mount /proc into it. Wasted half an hour to find that out this morning.