which is great when you realize that not all software is updated at the same time.

how managing multiple java runtime versions is supposed to work is still beyond me... it's a different tool at every company, and the instructions never seem to work

It's less complicated than you might think. A Java Development Kit (JDK) is a filesystem directory, and includes everything necessary to run a Java program. Most of the mysterious installers and version managers are managing a collection of these JDK directories in some fixed location on disk. You can download a JDK directory (tarball), and use the `java` binary within it directly.

There is also a convention of using the `JAVA_HOME` environment variable to allow tools to locate the correct JDK directory. For example, in a unix shell, add `$JAVA_HOME/bin` to your `PATH`.

Java runtimes is just: export JAVA_HOME=/path; ./app.sh

But the java runtime needs to be at /path then, and it needs to stay there as long as ./app.sh needs it. And when app2.sh needs a different version you need that to be at /path2

You need the runtime though