Symptoms: Unsupported Java version, UnsupportedClassVersionError, java.lang.UnsupportedClassVersionError

Possible Causes

  • The wrong Java version is installed
  • MC 1.17+ requires Java 17+
  • MC 1.20.5+ requires Java 21+

Step-by-Step Fix

Step 1

Check the Java version currently installed

bash
java -version

Step 2

Install the correct Java version (Java 21 for MC 1.20.5+)

bash
sudo apt install openjdk-21-jre-headless

Step 3

Point the server at an alternative Java path

bash
/usr/lib/jvm/java-21-openjdk-amd64/bin/java -jar server.jar

Step 4

Update the JAVA_HOME environment variable

bash
export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64