/// This class can be extended by applications that wish to trap authentication related exceptions and automatically retry the exceptional operation with different credentials.
/// </summary>
/// <remarks>
/// This class can be extended by applications that wish to trap authentication related exceptions and automatically retry the exceptional operation with different credentials. Read <a href="../../../authhandler.html">jCIFS Exceptions and NtlmAuthenticator</a> for complete details.
/// <summary>Set the default <tt>NtlmAuthenticator</tt>.</summary>
/// <remarks>
/// Set the default <tt>NtlmAuthenticator</tt>. Once the default authenticator is set it cannot be changed. Calling this metho again will have no effect.
/// <summary>An application extending this class must provide an implementation for this method that returns new user credentials try try when accessing SMB resources described by the <tt>getRequestingURL</tt> and <tt>getRequestingException</tt> methods.
/// </summary>
/// <remarks>
/// An application extending this class must provide an implementation for this method that returns new user credentials try try when accessing SMB resources described by the <tt>getRequestingURL</tt> and <tt>getRequestingException</tt> methods.
/// If this method returns <tt>null</tt> the <tt>SmbAuthException</tt> that triggered the authenticator check will simply be rethrown. The default implementation returns <tt>null</tt>.