The items in the ClassFile structure are as follows:
The magic item supplies the magic number identifying the class file format; it has the value 0xCAFEBABE.
The values of the minor_version and major_version items are the minor and major version numbers of the compiler that produced this class file. An implementation of the Java Virtual Machine normally supports class files having a given major version number and minor version numbers 0 through some particular minor_version. .......
so that spec I linked to before says "In Sun's Java Developer's Kit (JDK) 1.0.2 release, documented by this book, the value of major_version is 45. The value of minor_version is 3. Only Sun may define the meaning of new class file version numbers."
So, I guess if you can find the magic list of numbers to JDK versios -- then you're good to go!