Unsupported Class Version Error

The UnsupportedClassVersionError appears when the server, a plugin, or a mod is compiled for a newer Java version than the one your server runs.

Example message:

java.lang.UnsupportedClassVersionError: ... has been compiled by a more recent
version of the Java Runtime (class file version 65.0), this version of the
Java Runtime only recognizes class file versions up to 61.0

This means the file needs Java 21 (class 65) while the server runs Java 17 (class 61).

How to Fix It#

1. Check the Server's Java Version#

   Run this in the Console:

   java -version

   Note the version shown (e.g., 17, 21).

2. Match Java to the Plugin/Mod Requirement#

   The best fix is to raise the server's Java version to match the plugin/mod.

   1. Open the Startup tab.
   2. Find the Docker Image or Java Version option.
   3. Select the required Java image (e.g., Java 21).
   4. Restart the server.

3. Or Downgrade the Plugin/Mod#

   If you cannot raise Java, download a plugin/mod version that matches your server's Java (usually listed on the download page).

Java Version Table#

Match the class file version number in the error with the table above to know which Java is required.

Need Help?#