= GDC

The GDC class implements a copy of the Global Diagnostic Context, which
is not part of the Apache Log4j library, as of this writing (10 Jan 2009).

The GDC is like the NDC and MDC classes, only it is global to the
application (see NDC and MDC for details on those classes).

The GDC is local to the main thread, and any new threads will return
the value of the current GDC set in the main thread.

Only the main thread can set the GDC, any other threads that
attempt to will raise an exception.