it appears how, exactly?
We don't support Java 9 yet, you'll want to use Java 8 for version 17.3.1. I think 17.4 on a Mac will be ok with Java 9.
Turns out that in order to run my old version of Photoshop, I have to use the 1.6 JVM - which means I can't use the latest version (17.4) of SQL Developer.
So, what I ended up doing is creating a bootable USB with MacOS and loading the JDK onto the USB. I can now use SQL Developer. Good enough for now.