Interface CallbackHandler

Known Implementing Classes:
AbstractCallbackHandler, AWTCallbackHandler, ConsoleCallbackHandler, DefaultCallbackHandler, SwingCallbackHandler

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/, where <JAVA_HOME> refers to the directory where the SDK was installed.

If the security property is set to the fully qualified name of a CallbackHandler implementation class, then a LoginContextwill load the specified CallbackHandler and pass it to the underlying LoginModules. The LoginContext only loads the default handler if one was not provided.

All default handler implementations must provide a public zero-argument constructor.

Method Summary

handle(Callback[] callbacks)
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.

Method Details


public void handle(Callback[] callbacks)
            throws IOException,
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 <32callbacks.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:
         case TextOutputCallback.ERROR:
            System.out.println("ERROR: " + toc.getMessage());
         case TextOutputCallback.WARNING:
            System.out.println("WARNING: " + toc.getMessage());
            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
         nc.setName((new BufferedReader(
               new InputStreamReader(;
      } else if (callbacks[i] instanceof PasswordCallback) {
         // prompt the user for sensitive information
         PasswordCallback pc = (PasswordCallback)callbacks[i];
      } 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. -- base interface for callback handlers. Copyright (C) 2003, Free Software Foundation, Inc. This file is part of GNU Classpath. GNU Classpath is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2, or (at your option) any later version. GNU Classpath is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with GNU Classpath; see the file COPYING. If not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA. Linking this library statically or dynamically with other modules is making a combined work based on this library. Thus, the terms and conditions of the GNU General Public License cover the whole combination. As a special exception, the copyright holders of this library give you permission to link this library with independent modules to produce an executable, regardless of the license terms of these independent modules, and to copy and distribute the resulting executable under terms of your choice, provided that you also meet, for each linked independent module, the terms and conditions of the license of that module. An independent module is a module which is not derived from or based on this library. If you modify this library, you may extend this exception to your version of the library, but you are not obligated to do so. If you do not wish to do so, delete this exception statement from your version.