What is CA FE BA BE 00 00 00 3D in Java Class Bytecode

If you read a Java .class file using a hex editor you will see that the first 4 hex codes are CA FE BA BE followed by something like 00 00 00 3D

Example:

public class MyClass {
 
 //Some Java code...

}

Let's compile the class file,

Using xxd command to display the .class file as HEX

xxd MyClass.class

00000000: cafe babe 0000 003d 000d 0a00 0200 0307  .......=........
00000010: 0004 0c00 0500 0601 0010 6a61 7661 2f6c  ..........java/l
00000020: 616e 672f 4f62 6a65 6374 0100 063c 696e  ang/Object...<in
00000030: 6974 3e01 0003 2829 5607 0008 0100 0653  it>...()V......S
00000040: 616d 706c 6501 0004 436f 6465 0100 0f4c  ample...Code...L
00000050: 696e 654e 756d 6265 7254 6162 6c65 0100  ineNumberTable..
00000060: 0a53 6f75 7263 6546 696c 6501 000b 5361  .SourceFile...Sa
00000070: 6d70 6c65 2e6a 6176 6100 2100 0700 0200  mple.java.!.....
00000080: 0000 0000 0100 0100 0500 0600 0100 0900  ................
00000090: 0000 1d00 0100 0100 0000 052a b700 01b1  ...........*....
000000a0: 0000 0001 000a 0000 0006 0001 0000 0001  ................
000000b0: 0001 000b 0000 0002 000c                 ..........

CAFE BABE - This is called the Magic Number - A java class file is identified by these first 4 byte headers

00 00 00 3D - The next first 2 bytes (00 00) signify the Minor Version of Java, and the next two (00 3D) when converted to decimal you get 61 - which implies the Java Major version - which is Java 17, so this class was built using Java-17 version

Java Version = Decimal (Hex)
Java SE 20 = 64 (40)
Java SE 19 = 63 (3F)
Java SE 18 = 62 (3E)
Java SE 17 = 61 (3D)
Java SE 16 = 60 (3C)
Java SE 15 = 59 (3B)
Java SE 14 = 58 (3A)
Java SE 13 = 57 (39)
Java SE 12 = 56 (38)
Java SE 11 = 55 (37)
Java SE 10 = 54 (36)
Java SE 9  = 53 (35)
Java SE 8  = 52 (34)
Java SE 7  = 51 (33)
Java SE 6  = 50 (32)
Java SE 5  = 49 (31)
JDK 1.4    = 48 (30)
JDK 1.3    = 47 (2F)
JDK 1.2    = 46 (2E)
JDK 1.1    = 45 (2D)

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