Java only needs to be installed on the fw Server app host machine; it is used by the built-in web server and powers the Web Client and Mobile Web Client (and fw Play if licensed).
Download OpenJDK e.g. here: https://adoptopenjdk.net/
Use the "Hotspot" version.
The installers are called for example:
On Windows: OpenJDK8U-jdk_x64_windows_hotspot_8u222b10.msi
On Mac: OpenJDK8U-jdk_x64_mac_hotspot_8u222b10.pkg
On Linux: OpenJDK8U-jdk_x64_linux_hotspot_8u222b10.tar.gz
Update July 2021:
By now the latest "LTS" Hotspot version is 11!
We are using this on our side, and we recommend you do the same on yours.
Then first UNINSTALL ALL previous Java versions - especially the old deprecated v1.6 and v1.7 - on the fw Server host machine; you might need to search for uninstall instructions.
It can help to use the Windows Task Manager or Activity Monitor on Mac to ensure that no "Java" process is still running.
Only then Install OpenJDK.
After launching the fw Server application again, check e.g. that the Web Client is properly running.
Also here, using the Windows Task Manager or Activity Monitor on Mac can help to ensure that now the "Java" process is running.
INFO:
fw - farmerswife
JRE - Java Runtime Environment
JDK - Java Development Kit