public interface CallbackHandler
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 in the auth.login.defaultCallbackHandler
security property. The security property can be set
in the Java security properties file located in the file named
<JAVA_HOME>/lib/security/java.security.
<JAVA_HOME> refers to the value of the java.home system property,
and specifies the directory where the JRE is installed.
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.
void handle(Callback[] callbacks) throws IOException, UnsupportedCallbackException
Retrieve or display the information requested in the provided Callbacks.
The handle method implementation checks the
instance(s) of the Callback object(s) passed in
to retrieve or display the requested information.
The following example is provided to help demonstrate what an
handle 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
}
callbacks - an array of Callback objects provided
by an underlying security service which contains
the information requested to be retrieved or displayed.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 the callbacks parameter.