Class (file format)

From Free net encyclopedia

The Java source files (.java files) get compiled into .class files. Since Java is a platform-independent language, it compiles its sources into bytecode, which it stores in a .class file. If the source code has more than one class, each class is compiled into a separate .class file. These .class files can be loaded by any Java Virtual Machine (JVM).

Since JVMs are available for many platforms, the .class file compiled in one platform will execute in a JVM of another platform. This makes Java platform-independent.

As of 2006, the modification of the class file format is being considered under Java Specification Request (JSR) 202.

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Trivia

Class files are identified by the following 4 byte header (in hexadecimal): CA FE BA BE.

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."

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