Java version errors

Errors like "Unsupported class file major version" or "requires Java 21".

The Java version does not match: Versions sets the right Java for the Minecraft version, and plugins need a Java at least as new as they were built for.

Before you start: The error message.

Step by step

1. Open Versions and reinstall your current software and version: this resets the Java image to the right one.
2. Class file major version 65 = Java 21, 61 = Java 17, 52 = Java 8. If a plugin needs a newer Java than your Minecraft version uses, get an older build of the plugin.
3. Old Minecraft (1.16 and below) needs Java 8 or 11; very new plugins will not run there.
4. Restart and check the console.
5. If you changed the Docker image or startup command by hand, set it back.
6. Check the plugin page for which Java it needs; most modern plugins state it in their requirements.

Check it worked: No Java errors on startup.

Why it happens: Each Minecraft version needs a minimum Java (1.20.5+ needs Java 21), and newer plugins need newer Java.

More guides on Crashes, startup failures, and errors:

• Find and read crash reports and logs
• Server stuck in a restart loop or will not start after an update
• Failed to bind to port or unable to access jarfile
• Crash or error caused by a plugin
• Crash caused by a mod
• Server freezes or watchdog kills it