How to fix UnsupportedClassVersionError

The message

java.lang.UnsupportedClassVersionError: com/example/App 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

The first number is what the code needs. The second is what your server has. Translate them:

In the example above the application needs Java 21 but the server runs Java 17.

1. See which Java is used

java -version
update-alternatives --list java       # Debian and Ubuntu
ps -o args= -p $(pgrep -f myapp.jar | head -1) | cut -d' ' -f1

Many servers have several Java versions installed. The one in your shell is not always the one the service uses. The last command shows the binary the running process was started with.

2. Install the right Java

Follow Install Java 21 on Ubuntu, or for another version, sudo apt install openjdk-17-jre-headless. Pick by LTS version.

3. Point the service at it

Spring Boot or any JAR service. In the systemd unit, give the full path to Java:

ExecStart=/usr/lib/jvm/java-21-openjdk-amd64/bin/java -jar /opt/myapp/app.jar

then sudo systemctl daemon-reload && sudo systemctl restart myapp.

Tomcat. Set JAVA_HOME. On Debian and Ubuntu packages put it in /etc/default/tomcat10:

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

For a tarball install, export JAVA_HOME=... in bin/setenv.sh. Restart Tomcat. Check the first lines of the log, which print the Java version in use.

4. Or compile for the older Java

If you must stay on an older Java, build with the matching release target. Maven: <maven.compiler.release>17</maven.compiler.release>. Gradle: java { toolchain { languageVersion = JavaLanguageVersion.of(17) } }. This works only if the code and its libraries do not use newer APIs. Spring Boot 3 needs Java 17 or newer, so downgrading below that is not possible without changing the framework version.

Last updated October 29, 2026.