Java '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 52.0': how do I fix it?

Cause: The class (or a library jar) was compiled for a newer Java version than the JVM that is running it. Each Java release has a class file major version (for example 52 for Java 8, 65 for Java 21), and a JVM can only load versions up to its own. The error is a subclass of ClassFormatError and is raised by the JVM at load time. Usual triggers: the build used a newer JDK than production, an IDE or CI uses a different JDK from the server, a dependency was upgraded to a release that requires a newer Java, or JAVA_HOME / the PATH java is not the one you think. Fix (best first): 1. Check which runtime you are using: java -version and echo the JAVA_HOME; check which one the service, container image or CI job really starts. 2. Easiest and preferred: run on a JDK at least as new as the version used for compilation (upgrade the runtime or container base image). 3. If you must support an older runtime, compile for it. Use javac --release N, which sets language level, class file version and the API surface to release N, unlike -source/-target which do not limit the API. In Maven set maven.compiler.release (recommended with maven-compiler-plugin 3.13.0 and newer; older source/target properties do not stop use of newer APIs). 4. If the offending class is inside a third-party jar, either use a version of that library that supports your Java release, or move to a newer runtime. Reading the library's release notes tells you its minimum Java. 5. Make the toolchain explicit (Maven toolchains or Gradle toolchains) so CI and developers build with the same JDK. Verify: The application starts on the target runtime. javap -v on a sample class shows the expected major version. Sources: https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/UnsupportedClassVersionError.html https://docs.oracle.com/en/java/javase/21/docs/specs/man/javac.html https://maven.apache.org/plugins/maven-compiler-plugin/examples/set-compiler-source-and-target.html