Installed the OpenJDK 8 JRE:

pacman -S  jre8-openjdk-headless

The java-rxtx package puts a dependency on the non-headless Java runtime. I force installed it with:

pacman -Sdd  java-rxtx

Then I created the symlink from the JRE manually:

ln -s /usr/share/java/rxtx/RXTXcomm.jar /usr/lib/jvm/java-8-openjdk/jre/lib/ext/RXTXcomm.jar

Then, as the package noted, added user solar to the uucp and lock system groups.