I haven't tried this, but I think it should work.

1.) Make a directory. In that directory put your JAR file, your start
script and anything else the app needs to run.
2.) From that directory, run `cf push <app-name> -b java_buildpack -c
'$PWD/start-script.sh'`. This will upload your script, the JAR file and
everything else in the current directory. It will also tell CF that you
specifically want to use the Java build pack (which will install Java) and
that you want to use your script to start your app.

What could be tricky about this is your start script. It's going to need
to reference JAVA_HOME as `/home/vcap/app/.java-buildpack/open_jdk_jre`,
and `java` as `$JAVA_HOME/bin/java` since `java` is not going to be on the
$PATH.

You're also going to need to handle some of the things that the JBP would
normally do like set -Xmx and other JVM memory settings to keep the JVM
from exceeding the containers MEMORY_LIMIT. Note, *all* memory needs to
fit under the limit, not just the JVM's heap. In other words, setting -Xmx
== MEMORY_LIMIT is 100% wrong.

Beyond that, you'd need to make sure the app is listening on $PORT or if
it's not taking web requests, disable that health check (`cf push
--no-route` & `cf set-health-check none`).

Dan