Java class / Mach-O fat file signature
Magic number at the start of the file
The bytes CA FE BA BE (the readable word cafebabe) are a famous collision: they begin both a compiled Java .class file and a Mach-O universal (fat) binary on macOS.
What this magic number means
A magic number is a short, fixed run of bytes at a known position that tells a program what a file really is, no matter what the filename says. If a file starts with the bytes CA FE BA BE, it is a Java class / Mach-O fat file.
They are distinguished by what follows. In a Java class the next four bytes are the minor and major version (for example 00 00 00 34 for Java 8). In a Mach-O fat binary the next four bytes are a plausible architecture count, and each slice inside is a normal Mach-O starting feedface or feedfacf.
Formats that use the CA FE BA BE signature
No single file extension owns this signature exclusively; it belongs to the Java class / Mach-O fat format itself.
It is also seen in: CLASS, Mach-O universal binary.
How to check a file's signature
You can read the first bytes of any file yourself. The magic bytes are shown in hex, the same way this page lists them.
Linux & macOS
- xxd -l 16 example.bin
- hexdump -C -n 16 example.bin
- file example.bin
Windows (PowerShell)
- Format-Hex -Path example.bin -Count 16
Python
- open("example.bin","rb").read(4).hex()
In your browser
- Drop the file into the WhatFileType identifier, which reads the signature without uploading it.
Frequently asked questions
What is the Java class / Mach-O fat file signature?
Java class / Mach-O fat files start with the hex bytes CA FE BA BE. This magic number identifies the format regardless of the file's name or extension.
How do I check a file's magic number?
Open the file in a hex editor, or run a command such as xxd -l 16 example.bin on Linux or macOS, or Format-Hex -Path example.bin -Count 16 in Windows PowerShell, and read the first bytes.
Can a file fake the Java class / Mach-O fat signature?
Renaming a file does not change its bytes, so the extension can lie but the signature usually cannot. A genuine Java class / Mach-O fat file has these exact bytes; a file with the wrong bytes is not really Java class / Mach-O fat, whatever its name says.