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.
- Since:
- 1.4
- See Also:
-
Method Summary
-
Method Details
-
handle
Retrieve or display the information requested in the provided Callbacks.
The
handle
method implementation checks the instance(s) of theCallback
object(s) passed in to retrieve or display the requested information. The following example is provided to help demonstrate what anhandle
method implementation might look like. This example code is for guidance only. Many details, including proper error handling, are left out for simplicity.public void handle(Callback[] callbacks) throws IOException, UnsupportedCallbackException { for (int i = 0; i < callbacks.length; i++) { if (callbacks[i] instanceof TextOutputCallback) { // display the message according to the specified type TextOutputCallback toc = (TextOutputCallback)callbacks[i]; switch (toc.getMessageType()) { case TextOutputCallback.INFORMATION: System.out.println(toc.getMessage()); break; case TextOutputCallback.ERROR: System.out.println("ERROR: " + toc.getMessage()); break; case TextOutputCallback.WARNING: System.out.println("WARNING: " + toc.getMessage()); break; default: throw new IOException("Unsupported message type: " + toc.getMessageType()); } } else if (callbacks[i] instanceof NameCallback) { // prompt the user for a username NameCallback nc = (NameCallback)callbacks[i]; // ignore the provided defaultName System.err.print(nc.getPrompt()); System.err.flush(); nc.setName((new BufferedReader (new InputStreamReader(System.in))).readLine()); } else if (callbacks[i] instanceof PasswordCallback) { // prompt the user for sensitive information PasswordCallback pc = (PasswordCallback)callbacks[i]; System.err.print(pc.getPrompt()); System.err.flush(); pc.setPassword(readPassword(System.in)); } else { throw new UnsupportedCallbackException (callbacks[i], "Unrecognized Callback"); } } } // Reads user password from given input stream. private char[] readPassword(InputStream in) throws IOException { // insert code to read a user password from the input stream }
- Parameters:
callbacks
- an array ofCallback
objects provided by an underlying security service which contains the information requested to be retrieved or displayed.- Throws:
IOException
- if an input or output error occurs.UnsupportedCallbackException
- if the implementation of this method does not support one or more of the Callbacks specified in thecallbacks
parameter.
-