.JAR file signature
A Java archive data (JAR) file begins with the bytes 50 4B 03 04 (PK.. in ASCII) at the start of the file. The signature is the same whatever the file is named, which is why a renamed or extensionless JAR file is still recognisable.
Signature
| Offset | Hex | ASCII |
|---|---|---|
| 0 | 50 4B 03 04A ZIP archive containing META-INF/MANIFEST.MF. | PK.. |
MIME type: application/java-archive · usual extensions: .jar
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.jar
file file.jarWindows (PowerShell):
Format-Hex -Path file.jar -Count 32If the bytes do not match
A file named .jar whose first bytes are something else is not a JAR 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 JAR guide explains what a genuine file should contain.