It is easy enough to read the class file signature and get these values without a 3rd party API. All you need to do is read the first 8 bytes.

ClassFile {
    u4 magic;
    u2 minor_version;
    u2 major_version;

e.g.: Read a file which is version 61.0 (Java 17)

hexdump -C Identification.class 
00000000  ca fe ba be 00 00 00 3d  00 20 0a 00 02 00 03 07  |.......=. ......|
00000010  00 04 0c 00 05 00 06 01  00 10 6a 61 76 61 2f 6c  |..........java/l|
00000020  61 6e 67 2f 4f 62 6a 65  63 74 01 00 06 3c 69 6e  |ang/Object...<in|
00000030  69 74 3e 01 00 03 28 29  56 09 00 08 00 09 07 00  |it>...()V.......|
...

the opening bytes are:

ca fe ba be 00 00 00 3d

…where ca fe ba be are the magic bytes, 00 00 is the minor version and 00 3d is the major version.

major version number of the class file format being used

  • Java SE 18 = 62 (0x3E hex)
  • Java SE 17 = 61 (0x3D hex)
  • Java SE 16 = 60 (0x3C hex)
  • Java SE 15 = 59 (0x3B hex)
  • Java SE 14 = 58 (0x3A hex)
  • Java SE 13 = 57 (0x39 hex)
  • Java SE 12 = 56 (0x38 hex)
  • Java SE 11 = 55 (0x37 hex)
  • Java SE 10 = 54 (0x36 hex)
  • Java SE 9 = 53 (0x35 hex)
  • Java SE 8 = 52 (0x34 hex)
  • Java SE 7 = 51 (0x33 hex)
  • Java SE 6.0 = 50 (0x32 hex)
  • Java SE 5.0 = 49 (0x31 hex)
  • JDK 1.4 = 48 (0x30 hex)
  • JDK 1.3 = 47 (0x2F hex)
  • JDK 1.2 = 46 (0x2E hex)
  • JDK 1.1 = 45 (0x2D hex)

References