I develop on MacOS with Eclipse IDE. I would like to add Java 11 to the runtimes available for Eclipse Projects without making it the system default. I use Adopt OpenJDK Java 8 as my system default JDK/Runtime.
Whats the best way to accomplish this to I run the .pkg installer for Java 11 and then re-install Java 8 or should I try and find a zip of the JDK contents which I can extract somewhere ?
Java's native format is a self-contained ZIP file. You download it, unzip it and it's ready to go.
The installers do 2 things, neither of which are essential.
First, they catalog the product in the system's software inventory. Which is nice, since you can query the inventory database to see what's installed. And in the case of Red Hat RPMs at least, also audit for possible damage to the installation.
Secondly, they help the JDK/JRE live more comfortably in its new home, mostly by adding softlinks so that the Java components look more like native OS product installations.
In the case of the RPM installer, multiple installed JDKs and JVM's are supported by a convention. The installer places the unzipped files in a directory under /usr/java. The directory name corresponds to the Java release ID, so to select a given JDK or JRE you simply explicitly request the Java under the directory you want, or, if you're running something like Tomcat, Maven, or Ant that looks for JAVA_HOME, export the /usr/java/whatever-jbm-release to JAVA_HOME. And incidentally, the RPM installer also makes an alias /usr/java/latest that points to the latest installed JVM.
I've not installed Java under MacOS, but you can reasonably expect similar behavior there.
Even Microsoft Windows can have multiple Java VM's installed, just doesn't have a decent set of OS conventions to manage them. Comes from not being Unix-like.
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Building a Better World in your Backyard by Paul Wheaton and Shawn Klassen-Koop