When trying to compile an JavaFX application in the environment:
java -version
openjdk version \"1.8.0_212\"
OpenJDK Runtime Environment (build 1.8.0_212-8u2
What I did was build it from source. The instructions here on the OpenJDK wiki worked for me. Once I built it, I had to put the jars in the right place, like this other stackoverflow post talked about.
cd build/sdk/
sudo cp -R rt/lib /usr/lib/jvm/java-8-openjdk-amd64/jre/lib/
sudo cp ./lib/ant-javafx.jar /usr/lib/jvm/java-8-openjdk-amd64/lib/
sudo cp ./lib/javafx-mx.jar /usr/lib/jvm/java-8-openjdk-amd64/lib/
After that I was able to compile my project!