This configuration uses the PortableApps Portable Java
(called jPortable for some reason).
I don't have a use for a download manager at the moment, but could you give a few details on how to get this to work?
Hmmm, I don't understand this one. JDownloader already includes Java so you can run it standalone. You don't have to do anything to make it run. In the JDownloader app folder, you can find a folder named jre which has an older JRE version 1.8.0_40. Current is 1.8.0_144 but you can just replace the included runtime files with updated ones with no issues usually.
I'm guessing you're asking on how to make it work with PA Portable Java. Just delete the one that's included with JDownloader in "JDownloaderPortable\App\JDownloader\jre". Then it will look for the the usual portable \CommonFiles\Java relative to where JDownloaderPortable is located. You can then check in Help > About JDownloader > Java: and clicking on the version string to see which JRE its actually using.
I might be dumb and the question is actually on how to use JDownloader itself as a download manager... Well I use it quite a bit. It's very good for downloading from lots of different file hosters or even video streaming sites like youtube, twitch, etc.
Anyway, here's some tips for users to disable some annoying stuff like banners and nags. You can find the options to turn these off in advanced settings. Maybe it can be included in the readme of jdownloader portable or something.
You just need to filter using these words to find the right settings to toggle:
premium disabled warning flash
Alternatively, edit the cfg file "org.jdownloader.settings.GraphicalUserInterfaceSettings.json"
"premiumalertetacolumnenabled" : false,
"bannerenabled" : false,
"donatebuttonstate" : "CUSTOM_HIDDEN",
"specialdealoboomdialogvisibleonstartup" : false,
"premiumalerttaskcolumnenabled" : false,
"premiumalertspeedcolumnenabled" : false,
"premiumdisabledwarningflashenabled" : false,
"specialdealsenabled" : false,