LoginModule describes the interface
implemented by authentication technology providers.  LoginModules
are plugged in under applications to provide a particular type of
authentication.

 While applications write to the LoginContext API,
authentication technology providers implement the
LoginModule interface.
A Configuration specifies the LoginModule(s)
to be used with a particular login application.  Therefore different
LoginModules can be plugged in under the application without
requiring any modifications to the application itself.

 The LoginContext is responsible for reading the
Configuration and instantiating the appropriate
LoginModules.  Each LoginModule is initialized with
a Subject, a CallbackHandler, shared
LoginModule state, and LoginModule-specific options.

The Subject represents the
Subject currently being authenticated and is updated
with relevant Credentials if authentication succeeds.
LoginModules use the CallbackHandler to
communicate with users.  The CallbackHandler may be
used to prompt for usernames and passwords, for example.
Note that the CallbackHandler may be null.  LoginModules
which absolutely require a CallbackHandler to authenticate
the Subject may throw a LoginException.
LoginModules optionally use the shared state to share information
or data among themselves.

 The LoginModule-specific options represent the options
configured for this LoginModule by an administrator or user
in the login Configuration.
The options are defined by the LoginModule itself
and control the behavior within it.  For example, a
LoginModule may define options to support debugging/testing
capabilities.  Options are defined using a key-value syntax,
such as debug=true.  The LoginModule
stores the options as a Map so that the values may
be retrieved using the key.  Note that there is no limit to the number
of options a LoginModule chooses to define.

 The calling application sees the authentication process as a single
operation.  However, the authentication process within the
LoginModule proceeds in two distinct phases.
In the first phase, the LoginModule's
login method gets invoked by the LoginContext's
login method.  The login
method for the LoginModule then performs
the actual authentication (prompt for and verify a password for example)
and saves its authentication status as private state
information.  Once finished, the LoginModule's login
method either returns true (if it succeeded) or
false (if it should be ignored), or throws a
LoginException to specify a failure.
In the failure case, the LoginModule must not retry the
authentication or introduce delays.  The responsibility of such tasks
belongs to the application.  If the application attempts to retry
the authentication, the LoginModule's login method will be
called again.

 In the second phase, if the LoginContext's overall authentication
succeeded (the relevant REQUIRED, REQUISITE, SUFFICIENT and OPTIONAL
LoginModules succeeded), then the commit
method for the LoginModule gets invoked.
The commit method for a LoginModule checks its
privately saved state to see if its own authentication succeeded.
If the overall LoginContext authentication succeeded
and the LoginModule's own authentication succeeded, then the
commit method associates the relevant
Principals (authenticated identities) and Credentials (authentication data
such as cryptographic keys) with the Subject
located within the LoginModule.

 If the LoginContext's overall authentication failed (the relevant
REQUIRED, REQUISITE, SUFFICIENT and OPTIONAL LoginModules did not succeed),
then the abort method for each LoginModule
gets invoked.  In this case, the LoginModule removes/destroys
any authentication state originally saved.

 Logging out a Subject involves only one phase.
The LoginContext invokes the LoginModule's logout
method.  The logout method for the LoginModule
then performs the logout procedures, such as removing Principals or
Credentials from the Subject or logging session information.

 A LoginModule implementation must have a constructor with
no arguments.  This allows classes which load the LoginModule
to instantiate it.