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Java 2 Platform EE v1.3: Interface CallbackHandler
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javax.security.auth.callback</FONT>
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Interface CallbackHandler</H2>
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<DT>public interface <B>CallbackHandler</B></DL>
<P>
<p> An application implements a <code>CallbackHandler</code> 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.
<p> 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.
<p> Underlying security services make requests for different types
of information by passing individual Callbacks to the
<code>CallbackHandler</code>. The <code>CallbackHandler</code>
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 <code>NameCallback</code> and
<code>PasswordCallback</code>. The <code>CallbackHandler</code>
can then choose to prompt for a username and password serially,
or to prompt for both in a single window.
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<CODE> void</CODE></FONT></TD>
<TD><CODE><B><A HREF="../../../../javax/security/auth/callback/CallbackHandler.html#handle(javax.security.auth.callback.Callback[])">handle</A></B>(<A HREF="../../../../javax/security/auth/callback/Callback.html">Callback</A>[] callbacks)</CODE>
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Retrieve or display the information requested in the
provided Callbacks.</TD>
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<A NAME="handle(javax.security.auth.callback.Callback[])"><!-- --></A><H3>
handle</H3>
<PRE>
public void <B>handle</B>(<A HREF="../../../../javax/security/auth/callback/Callback.html">Callback</A>[] callbacks)
throws java.io.IOException,
<A HREF="../../../../javax/security/auth/callback/UnsupportedCallbackException.html">UnsupportedCallbackException</A></PRE>
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<DD><p> Retrieve or display the information requested in the
provided Callbacks.
<p> The <code>handle</code> method implementation checks the
instance(s) of the <code>Callback</code> object(s) passed in
to retrieve or display the requested information.
The following example is provided to help demonstrate what an
<code>handle</code> method implementation might look like.
This example code is for guidance only. Many details,
including proper error handling, are left out for simplicity.
<pre>
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(td.getMessage());
break;
case TextOutputCallback.ERROR:
System.out.println("ERROR: " + td.getMessage());
break;
case TextOutputCallback.WARNING:
System.out.println("WARNING: " + td.getMessage());
break;
default:
throw new IOException("Unsupported message type: " +
td.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
}
</pre><DD><DL>
<DT><B>Parameters:</B><DD><CODE>callbacks</CODE> - an array of <code>Callback</code> objects provided
by an underlying security service which contains
the information requested to be retrieved or displayed.<DT><B>Throws:</B><DD><CODE>java.io.IOException</CODE> - if an input or output error occurs. <p><DD><CODE><A HREF="../../../../javax/security/auth/callback/UnsupportedCallbackException.html">UnsupportedCallbackException</A></CODE> - if the implementation of this
method does not support one or more of the Callbacks
specified in the <code>callbacks</code> parameter.</DL>
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