Every Java developer has been there. You have a .jar file—maybe a legacy library with lost documentation, a dependency that’s misbehaving, or even a competitor’s intriguing tool. You need to see the source code. But all you have are compiled .class files—bytecode, not human-readable.
The golden rule remains: Don’t upload what you can’t afford to lose. Instead, run a local decompiler—it’s just one command line away. Have a JAR you need to peek into? Download CFR, unzip it, and run the command above. Your source code stays where it belongs: on your machine.
A hard drive crash wiped your source, but you still have the compiled JAR? An online decompiler can recover 90-99% of the original logic, though comments and local variable names will be lost.
Students can peek into standard library behavior. Developers can verify if a third-party library does what it claims—no hidden network calls or data exfiltration.
