Home / Questions / 272
Accepted
Ruslan López
Asked: 2020-12-03 09:07:43 +0800 CST 2020-12-03 09:07:43 +0800 CST

How do I find the magic number 0xCAFEBABE?

  • 772

I've heard that Java classes have something called a magic number in their headers that identifies them, and it says something like 0xCAFEBABE.

Could someone explain to me how I can check such information?

java

2 Answers

  1. Best Answer

    Well, regarding your question, when you open a .class file with a hex editor, you can see "cafe babe" if it contains this magic number.

    In fact here I found an explanation of James Gosling himself

    We used to go to a place called St Michael's Alley for lunch. According to a local legend, in the deep and dark past, the Grateful Dead used to play there, before they became famous. It was a fairly modern place that lived up to the name of the band (a place worthy to die). When Jerry the gang leader died they put up a small Buddhist altar. When we went there we used to call the place "Cafe Dead". Someone noticed that it was a hexadecimal number. I was reworking the class file format code and needed some magic numbers: one for the persistent object file and one for the classes. I used CAFEDEAD for the persistent object file, and looking for a pattern in the first 4 digits "CAFE", and a variant in the others, BABE came to mind and I used it. At the time it didn't seem too important or destined to go anywhere like the trash can of history. So CAFEBABE became the class file format and CAFEDEAD the persistent object file format, but over time the persistent object went away along with the use of CAFEDEAD which was replaced by RMI."

    enter image description here

    The first 4 bytes are "the magic number",

    • 0xCAFEBABE,

    to identify a valid class file.

    The next 2 bytes minor version number identify the version of the .class being used.

    The next 2 bytes major version number

    • J2SE 8 = 52 (0x34 hex),
    • J2SE 7 = 51 (0x33 hex),
    • J2SE 6.0 = 50 (0x32 hex),
    • J2SE 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).
    • 15

  2. The magic number is present at the beginning of all .class files as a result of the source code compilation process. With a simple text editor like Notepad on Windows it is possible to observe this, or with any hexadecimal text editor, like Notepad++, also opening the files with a common editor present in GNU/Linux operating systems like gedit, nano or vi.

    • 3