Hello Janakiraman,
Please be aware that the new implementation of frontend methods via WEBGUI (without a Java applet), the method GET_USER_NAME (and a few others) are not supported - as the new browser API does not support such possibility.
While using the applet version, then it works as mentioned by Oisín.
You can use the java trace to obtain more information about the method execution.
Regards,
Cris