Implementations of this interface are passed to a
CallbackHandler, allowing underlying security services
the ability to interact with a calling application to retrieve specific
authentication data such as usernames and passwords, or to display
certain information, such as error and warning messages.

 Callback implementations do not retrieve or
display the information requested by underlying security services.
Callback implementations simply provide the means
to pass such requests to applications, and for applications,
if appropriate, to return requested information back to the
underlying security services.

 An application implements a CallbackHandler and passes
it to underlying security services so that they may interact with
the application to retrieve specific authentication data,
such as usernames and passwords, or to display certain information,
such as error and warning messages.

 CallbackHandlers are implemented in an application-dependent fashion.
For example, implementations for an application with a graphical user
interface (GUI) may pop up windows to prompt for requested information
or to display error messages.  An implementation may also choose to obtain
requested information from an alternate source without asking the end user.

 Underlying security services make requests for different types
of information by passing individual Callbacks to the
CallbackHandler.  The CallbackHandler
implementation decides how to retrieve and display information
depending on the Callbacks passed to it.  For example,
if the underlying service needs a username and password to
authenticate a user, it uses a NameCallback and
PasswordCallback.  The CallbackHandler
can then choose to prompt for a username and password serially,
or to prompt for both in a single window.

 A default CallbackHandler class implementation
may be specified by setting the value of the
auth.login.defaultCallbackHandler security property.

 If the security property is set to the fully qualified name of a
CallbackHandler implementation class,
then a LoginContext will load the specified
CallbackHandler and pass it to the underlying LoginModules.
The LoginContext only loads the default handler
if it was not provided one.

 All default handler implementations must provide a public
zero-argument constructor.

 Underlying security services instantiate and pass a
ChoiceCallback to the handle
method of a CallbackHandler to display a list of choices
and to retrieve the selected choice(s).

 Underlying security services instantiate and pass a
ConfirmationCallback to the handle
method of a CallbackHandler to ask for YES/NO,
OK/CANCEL, YES/NO/CANCEL or other similar confirmations.

 Underlying security services instantiate and pass a
LanguageCallback to the handle
method of a CallbackHandler to retrieve the Locale
used for localizing text.

 Underlying security services instantiate and pass a
NameCallback to the handle
method of a CallbackHandler to retrieve name information.

 Underlying security services instantiate and pass a
PasswordCallback to the handle
method of a CallbackHandler to retrieve password information.

 Underlying security services instantiate and pass a
TextInputCallback to the handle
method of a CallbackHandler to retrieve generic text
information.

 Underlying security services instantiate and pass a
TextOutputCallback to the handle
method of a CallbackHandler to display information messages,
warning messages and error messages.

Signals that a CallbackHandler does not
recognize a particular Callback.