Use on activity fields to retain instances that are intensive to
compute, on configuration changes.
in the Android Documentation.
Caution: While you can annotate any field, you should never annotate a field
that is tied to the Activity, such as a Drawable, an Adapter, a View or any
other object that's associated with a Context. If you do, it will leak all
the views and resources of the original activity instance. (Leaking resources
means that your application maintains a hold on them and they cannot be
garbage-collected, so lots of memory can be lost.)
This caution doesn't apply to beans annotated with
AndroidAnnotations automatically takes care of rebinding their context.