I thought Java is supposed to be backwards compatible? Oh well. I guess I'm installing Java 8. The strange thing is I was able to run the server with Java 9 for a few days. I had purged OpenJDK completely from the system after installing Java 9 the day I installed the Minecraft server.
sudo apt-get purge openjdk-8-jdk
and
sudo apt-get purge openjdk-8-jre
UPDATE: Well, installing Java 8 worked. I usually install Java from the Oracle website but I was too lazy to go through that whole process (there's probably an easier way to install it that I don't know about other than OpenJDK). I just installed openjdk-8-jdk and it works fine! Thank you @quadraxis!