001/* CallbackHandler.java -- base interface for callback handlers.
002   Copyright (C) 2003, Free Software Foundation, Inc.
003
004This file is part of GNU Classpath.
005
006GNU Classpath is free software; you can redistribute it and/or modify
007it under the terms of the GNU General Public License as published by
008the Free Software Foundation; either version 2, or (at your option)
009any later version.
010
011GNU Classpath is distributed in the hope that it will be useful, but
012WITHOUT ANY WARRANTY; without even the implied warranty of
013MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
014General Public License for more details.
015
016You should have received a copy of the GNU General Public License
017along with GNU Classpath; see the file COPYING.  If not, write to the
018Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
01902110-1301 USA.
020
021Linking this library statically or dynamically with other modules is
022making a combined work based on this library.  Thus, the terms and
023conditions of the GNU General Public License cover the whole
024combination.
025
026As a special exception, the copyright holders of this library give you
027permission to link this library with independent modules to produce an
028executable, regardless of the license terms of these independent
029modules, and to copy and distribute the resulting executable under
030terms of your choice, provided that you also meet, for each linked
031independent module, the terms and conditions of the license of that
032module.  An independent module is a module which is not derived from
033or based on this library.  If you modify this library, you may extend
034this exception to your version of the library, but you are not
035obligated to do so.  If you do not wish to do so, delete this
036exception statement from your version. */
037
038
039package javax.security.auth.callback;
040
041import java.io.IOException;
042
043/**
044 * <p>An application implements a <code>CallbackHandler</code> and passes it to
045 * underlying security services so that they may interact with the application
046 * to retrieve specific authentication data, such as usernames and passwords, or
047 * to display certain information, such as error and warning messages.</p>
048 *
049 * <p><code>CallbackHandler</code>s are implemented in an application-dependent
050 * fashion. For example, implementations for an application with a graphical
051 * user interface (GUI) may pop up windows to prompt for requested information
052 * or to display error messages. An implementation may also choose to obtain
053 * requested information from an alternate source without asking the end user.</p>
054 *
055 * <p>Underlying security services make requests for different types of
056 * information by passing individual Callbacks to the <code>CallbackHandler</code>.
057 * The <code>CallbackHandler</code> implementation decides how to retrieve and
058 * display information depending on the {@link Callback}s passed to it. For
059 * example, if the underlying service needs a username and password to
060 * authenticate a user, it uses a {@link NameCallback} and
061 * {@link PasswordCallback}. The <code>CallbackHandler</code> can then choose
062 * to prompt for a username and password serially, or to prompt for both in a
063 * single window.</p>
064 *
065 * <p>A default <code>CallbackHandler</code> class implementation may be
066 * specified in the <code>auth.login.defaultCallbackHandler</code> security
067 * property. The security property can be set in the Java security properties
068 * file located in the file named
069 * <code>&lt;JAVA_HOME>/lib/security/java.security</code>, where
070 * <code>&lt;JAVA_HOME></code> refers to the directory where the SDK was
071 * installed.</p>
072 *
073 * <p>If the security property is set to the fully qualified name of a
074 * <code>CallbackHandler</code> implementation class, then a
075 * <code>LoginContext</code>will load the specified <code>CallbackHandler</code>
076 * and pass it to the underlying <code>LoginModules</code>. The
077 * <code>LoginContext</code> only loads the default handler if one was not
078 * provided.</p>
079 *
080 * <p>All default handler implementations must provide a public zero-argument
081 * constructor.</p>
082 *
083 */
084public interface CallbackHandler
085{
086
087  /**
088   * <p>Retrieve or display the information requested in the provided
089   * {@link Callback}s.</p>
090   *
091   * <p>The <code>handle()</code> method implementation checks the instance(s)
092   * of the {@link Callback} object(s) passed in to retrieve or display the
093   * requested information. The following example is provided to help
094   * demonstrate what an <code>handle()</code> method implementation might look
095   * like. This example code is for guidance only. Many details, including
096   * proper error handling, are left out for simplicity.</p>
097   *
098   * <pre>
099   *public void handle(Callback[] callbacks)
100   *throws IOException, UnsupportedCallbackException {
101   *   for (int i = 0; i < callbacks.length; i++) {
102   *      if (callbacks[i] instanceof TextOutputCallback) {
103   *         // display the message according to the specified type
104   *         TextOutputCallback toc = (TextOutputCallback)callbacks[i];
105   *         switch (toc.getMessageType()) {
106   *         case TextOutputCallback.INFORMATION:
107   *            System.out.println(toc.getMessage());
108   *            break;
109   *         case TextOutputCallback.ERROR:
110   *            System.out.println("ERROR: " + toc.getMessage());
111   *            break;
112   *         case TextOutputCallback.WARNING:
113   *            System.out.println("WARNING: " + toc.getMessage());
114   *            break;
115   *         default:
116   *            throw new IOException("Unsupported message type: "
117   *                  + toc.getMessageType());
118   *         }
119   *      } else if (callbacks[i] instanceof NameCallback) {
120   *         // prompt the user for a username
121   *         NameCallback nc = (NameCallback)callbacks[i];
122   *         // ignore the provided defaultName
123   *         System.err.print(nc.getPrompt());
124   *         System.err.flush();
125   *         nc.setName((new BufferedReader(
126   *               new InputStreamReader(System.in))).readLine());
127   *      } else if (callbacks[i] instanceof PasswordCallback) {
128   *         // prompt the user for sensitive information
129   *         PasswordCallback pc = (PasswordCallback)callbacks[i];
130   *         System.err.print(pc.getPrompt());
131   *         System.err.flush();
132   *         pc.setPassword(readPassword(System.in));
133   *      } else {
134   *         throw new UnsupportedCallbackException(
135   *               callbacks[i], "Unrecognized Callback");
136   *      }
137   *   }
138   *}
139   *
140   * // Reads user password from given input stream.
141   *private char[] readPassword(InputStream in) throws IOException {
142   *   // insert code to read a user password from the input stream
143   *}
144   * </pre>
145   *
146   * @param callbacks an array of {@link Callback} objects provided by an
147   * underlying security service which contains the information requested to
148   * be retrieved or displayed.
149   * @throws IOException if an input or output error occurs.
150   * @throws UnsupportedCallbackException if the implementation of this method
151   * does not support one or more of the Callbacks specified in the
152   * <code>callbacks</code> parameter.
153   */
154  void handle(Callback[] callbacks) throws IOException, UnsupportedCallbackException;
155}