Install Java 21
Make sure the server launcher uses the Java version you have checked.
Windows
- Download a 64-bit Java 21 JDK or runtime from an official distributor, such as Eclipse Temurin.
- Select the correct architecture and install it. Enable PATH integration if offered.
- Open a new PowerShell window and check:
java -version
Get-Command javaThe version should report 21. Get-Command shows which executable is selected. If several Java versions are installed, use the full java.exe path in the start script, quoting paths containing spaces.
Linux
Install Java 21 from your distribution’s repository or the official Temurin package instructions. Package names and repository setup depend on the distribution.
java -version
command -v javaCheck as the account that runs the server. A service or container can have a different PATH from your interactive shell.
Hosting panels
Select a Java 21 runtime/image in the panel and verify its version in startup output. Ask the provider if it is unavailable. Changing Java on your personal computer does not change the hosting container.
If the version is still wrong
Reopen the terminal, check PATH or the panel runtime, and repeat the check. UnsupportedClassVersionError usually indicates Java is too old for the server or a plugin. See startup troubleshooting.