Java class file major version by release: 52 is Java 8, 61 is 17, 65 is 21

Every .class file starts with a major version number that records the Java release that wrote it: 52 for Java 8, 55 for Java 11, 61 for Java 17 and 65 for Java 21. A JVM runs a class file only if its major version is not newer than the highest one that JVM knows, so code compiled by Java 21 cannot run on Java 17.

Version table

From Java 2 on the number is the release number plus 44. Java 5 is 49 and Java 8 is 52.

For releases 10, 12 to 16 and others the same rule applies: release plus 44.

What works with what

The rule is one-directional: backward compatibility is the platform's goal, but nothing runs on a JVM older than the one that wrote the class. The failure is java.lang.UnsupportedClassVersionError: ... has been compiled by a more recent version of the Java Runtime; see UnsupportedClassVersionError.

Check it on your machine

The block compiles one class for three releases and prints the major version that javap finds. It also shows what javac says for releases it no longer supports. JAVA_TOOL_OPTIONS is unset so no "Picked up" line is printed.

unset JAVA_TOOL_OPTIONS
cat > Hello.java <<'JAVA'
public class Hello {
    public static void main(String[] args) {
        System.out.println("hi");
    }
}
JAVA
for r in 11 17 21; do
  rm -f Hello.class
  javac --release $r Hello.java
  printf "release $r: "
  javap -v Hello.class | grep "major version"
done
echo "--- unsupported release"
javac --release 7 Hello.java 2>&1 | head -1

release 11:   major version: 55
release 17:   major version: 61
release 21:   major version: 65
--- unsupported release
error: release version 7 not supported

The output follows the table: 55, 61 and 65. Release 7 is refused by this JDK, because each JDK keeps only a window of older targets (the number depends on the JDK version you use).

Choosing a target

• Libraries: compile for the oldest release your users still run, with --release. Compiling on a new JDK without it produces class files only that JDK or newer can load.
• Applications: compile for the runtime you deploy. A mismatch between the build JDK and the runtime JRE is the usual cause of the version error.
• Maven and Gradle: set maven.compiler.release or the Gradle options.release property so the same rule applies in the build tool. See javac --release.

Why --release and not --target

--target only writes the class file version. --release N also limits the code to the public API of release N, so a call to a method added in a later release fails at compile time instead of failing at run time with NoSuchMethodError on the older JVM.

Compiled classes are found through the classpath, and a JAR can mix class versions, so the check above is per class, not per JAR.

FAQ