What is a .jar file?

About the format

A .jar file is a Java archive — structurally a ZIP containing compiled Java classes plus a manifest, functionally a program (or library) that runs anywhere a Java runtime is installed. Double-clicking a runnable JAR launches it via javaw.exe when Java is present and associated; the command-line form is java -jar file.jar. Which is the point to absorb: a .jar is not a document, it's cross-platform executable code, and it deserves exactly the caution an .exe gets. Minecraft's ecosystem makes this concrete — mods and plugins ship as JARs, and downloading them from rehosting sites rather than the developer's page is a well-worn infection route for exactly the audience least suspicious of a .jar file. If Java isn't installed, JARs simply don't run (modern desktop Java apps mostly bundle their own runtime instead of relying on a system Java). To inspect one without running it, any ZIP tool opens it — the manifest and class names give a sense of what it is.

How to open a .jar file

Is a .jar file dangerous?

Yes — it's a program in a ZIP suit. Source trust matters exactly as for .exe files.

Related file types