.CLASS file signature
A Java compiled bytecode file begins with the bytes CA FE BA BE at the start of the file. The signature is the same whatever the file is named, which is why a renamed or extensionless JAVABYTECODE file is still recognisable.
Signature
| Offset | Hex | ASCII |
|---|---|---|
| 0 | CA FE BA BEFollowed by minor and major version; major 0x41 (65) is Java 21. |
MIME type: application/java-vm · usual extensions: .class
How to check a file's signature
Without installing anything: drop the file on our detector. It reads the bytes in your browser and shows the hex dump alongside the verdict; nothing is uploaded.
macOS and Linux (Terminal):
xxd -l 32 file.class
file file.classWindows (PowerShell):
Format-Hex -Path file.class -Count 32If the bytes do not match
A file named .class whose first bytes are something else is not a JAVABYTECODE file. The common explanations are a wrong or deliberately misleading extension, a download that saved an HTML error page instead of the file, or a container such as ZIP wrapping the real content. The detector reports what the bytes say; the JAVABYTECODE guide explains what a genuine file should contain.