FULL PRODUCT VERSION :
java version "1.6.0-rc"
Java(TM) SE Runtime Environment (build 1.6.0-rc-b101)
Java HotSpot(TM) Client VM (build 1.6.0-rc-b101, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows XP [Version 5.1.2600]

A DESCRIPTION OF THE PROBLEM :
On Win32 platforms (and also on X-windows apps), it is normal for the top-level application windows to register a classname specific to the application. This allows other programs, such as remote-control applications, to recognise which top window belongs to which application, and can be used for finding windows to send messages to

(see docs for Windows API calls FindWindowEX and RegisterClassEX)

In JRE 1.5 and earlier, the java.awt.Frame class registered its Class name as the window class name.

In JRE 1.6, the java.awt.Frame class always registers the fixed string "SunAwtFrame"

Examination of the JRE source code shows the following:
in j2se/src/windows/native/sun/windows:
RegisterClassEx in awt_Component.cpp calls the virtual function GetClassName()...
In JRE 1.5, awt_Frame::GetClassName() in awt_Frame.cpp gets the Windows class name from the Java class name (it was the only class that did this -- all the others used hardcoded names)
In JRE 1.6, awt_Frame::GetClassName() in awt_Frame.cpp has been substantially changed from 1.5 to use a hardcoded class name...

Note this may have been a fix for bug #4846680


STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Reproducer:

Compile and run the test class attached below with several JRES. Find the Win32 class name of the top level frame.

ACTUAL -
run with JRE 1.4.2 Win32 Class name == "Hello"
run with JRE 1.5 Win32 Class name == "Hello"
run with JRE 1.6 Win32 Class name == "SunAwtFrame"

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import java.awt.*;

public class Hello extends Frame {
  public static void main(String argv[])
  {
    new Hello();
  }
  Hello() {
    Label hello =
             new Label("Hello World");
    add(hello, "Center");
    setSize(200, 200);
    setVisible(true);
  }
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
There is no workaround: the class name has to be specified on window creation.

Release Regression From : mustang
The above release value was the last known release where this
bug was not reproducible. Since then there has been a regression.