Please see:
This will (potentially) require a manual user action during upgrade:
- if you have both JDK and JRE installed you can manually install the JDK
with pacman -Sy jdk-openjdk and this removes the JRE related packages. - if you have both JRE and JRE-headless you will need to choose one of
them and install it manually since they would conflict each other now. - If you only have one of the JDK/JRE/JRE-headless pacman should resolve
dependencies normally and no action is needed.
At the moment this is only valid for the upcoming JDK 21 release.
https://archlinux.org/news/incoming-changes-in-jdk-jre-21-packages-may-require-manual-intervention/