Not sure if anyone of you has tried opening a java .class file using some hex editor. If you haven’t yet, try it once!
The first thing that you will notice on doing so, is it’s very first 4 byte header “CAFE BABE” 😉
Every java class files are identified by 4 byte header (in hex) CA FE BA BE (first 4 entries). This signature is just an easy way of verifying that a block of data really does claim to be an instance of the Java class format. Every Java binary class, even one that isn’t present on the file system, needs to start with these four bytes.
The history of this magic number was explained by James Gosling:-
“We used to go to lunch at a place called St Michael’s Alley. According to local legend, in the deep dark past, the Grateful Dead used to perform there before they made it big. It was a pretty funky place that was definitely a Grateful Dead Kinda Place. When Jerry died, they even put up a little Buddhist-esque shrine. When we used to go there, we referred to the place as Cafe Dead. Somewhere along the line it was noticed that this was a HEX number. I was re-vamping some file format code and needed a couple of magic numbers: one for the persistent object file, and one for classes. I used CAFEDEAD for the object file format, and in grepping for 4 character hex words that fit after “CAFE” (it seemed to be a good theme) I hit on BABE and decided to use it. At that time, it didn’t seem terribly important or destined to go anywhere but the trash-can of history. So CAFEBABE became the class file format, and CAFEDEAD was the persistent object format. But the persistent object facility went away, and along with it went the use of CAFEDEAD – it was eventually replaced by RMI.”