Returns any existing {@code Subject} that may be in use at the time the new {@code Subject} instance is
being created.
<p/>
This is typically used in the case where the existing {@code Subject} instance returned by
this method is unauthenticated and a new {@code Subject} instance is being created to reflect a successful
authentication - you want to return most of the state of the previous {@code Subject} instance when creating the
newly authenticated instance.
@return any existing {@code Subject} that may be in use at the time the new {@code Subject} instance is
being created.
Returns any existing {@code Subject} that may be in use at the time the new {@code Subject} instance is being created. <p/> This is typically used in the case where the existing {@code Subject} instance returned by this method is unauthenticated and a new {@code Subject} instance is being created to reflect a successful authentication - you want to return most of the state of the previous {@code Subject} instance when creating the newly authenticated instance.
@return any existing {@code Subject} that may be in use at the time the new {@code Subject} instance is being created.