I saw this list of major version numbers for Java in another post:
- Java 1.2 uses major version 46
- Java 1.3 uses major version 47
- Java 1.4 uses major version 48
- Java 5 uses major version 49
- Java 6 uses major version 50
- Java 7 uses major version 51
- Java 8 uses major version 52
- Java 9 uses major version 53
These come from the class version. If you try to load something compiled for java 6 in a java 5 runtime you’ll get the error, incompatible class version, got 50, expected 49. Or something like that.
See here in byte offset 7 for more info.
Additional info can also be found here.
I found a list of Java class file versions on the Wikipedia page that describes the class file format:
Under byte offset 6 & 7, the versions are listed with which Java VM they correspond to.