This week's giveaway is in the Java/Jakarta EE forum. We're giving away four copies of Java EE 8 High Performance and have Romain Manni-Bucau on-line! See You expected the Java code to run at the client machine, but it doesn't. It runs at the server machine. That's your misinterpretation here.
If you want to get the client's user name by System.getProperty(), then you obviously need to run Java at the client machine. You can do this using a signed applet or a web start application.