Ability to use a custom Java binary

[Tajmoti]

Due to a problem with Forge when using Java 8 >= 8u312 with 1.16+ modpacks, I am unable to play this modpack. The problem is better described here.

To resolve the issue, I have tried downgrading Java, but quickly realized that the Java versions are baked into the flatpak and cannot be changed easily. I was not able to get the flatpak version of PolyMC to use a custom Java version from my machine.

Additionally, I have tried toying with flatpak extensions (namely this one), but that lead nowhere.

Is there a way to use a custom Java binary with the flatpak version of PolyMC?

[d-513]

Flatpak is sandboxed, so sadly not, unless I make an extension with that version myself

[guihkx]

You can definitely do that by importing a custom Java build into ~/.var/app/org.polymc.PolyMC.

Here's how you do that using the command line (easier to explain):

# navigate into PolyMC's sandboxed home folder
cd ~/.var/app/org.polymc.PolyMC/
# create a folder to store the JRE 8 (build 312) we'll download below
mkdir jre8.0.312
# download the jre (without keeping the archive on disk) and extract it to the folder above
curl -sL https://cdn.azul.com/zulu/bin/zulu8.58.0.13-ca-jre8.0.312-linux_x64.tar.gz | tar xvz --strip-components 1 -C jre8.0.312

Now, configure your 1.16 instance in PolyMC like so (also click the "Test" button afterward just to make sure it works):

If you did everything correctly, you should have it working: