/*
    * Licensed to the Apache Software Foundation (ASF) under one
    * or more contributor license agreements.  See the NOTICE file
    * distributed with this work for additional information
    * regarding copyright ownership.  The ASF licenses this file
    * to you under the Apache License, Version 2.0 (the
    * "License"); you may not use this file except in compliance
    * with the License.  You may obtain a copy of the License at
    *
   *     http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing,
   * software distributed under the License is distributed on an
   * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
   * KIND, either express or implied.  See the License for the
   * specific language governing permissions and limitations
   * under the License.
   */
  package org.apache.shiro.authc.pam;
  
  import org.apache.shiro.authc.*;
  import org.apache.shiro.realm.Realm;
  import org.apache.shiro.subject.PrincipalCollection;
  import org.apache.shiro.util.CollectionUtils;
  import org.slf4j.Logger;
  import org.slf4j.LoggerFactory;
  
  import java.util.Collection;
  
  /**
   * A {@code ModularRealmAuthenticator} delegates account lookups to a pluggable (modular) collection of
   * {@link Realm}s.  This enables PAM (Pluggable Authentication Module) behavior in Shiro.
   * In addition to authorization duties, a Shiro Realm can also be thought of a PAM 'module'.
   * <p/>
   * Using this Authenticator allows you to &quot;plug-in&quot; your own
   * {@code Realm}s as you see fit.  Common realms are those based on accessing
   * LDAP, relational databases, file systems, etc.
   * <p/>
   * If only one realm is configured (this is often the case for most applications), authentication success is naturally
   * only dependent upon invoking this one Realm's
   * {@link Realm#getAuthenticationInfo(org.apache.shiro.authc.AuthenticationToken)} method.
   * <p/>
   * But if two or more realms are configured, PAM behavior is implemented by iterating over the collection of realms
   * and interacting with each over the course of the authentication attempt.  As this is more complicated, this
   * authenticator allows customized behavior for interpreting what happens when interacting with multiple realms - for
   * example, you might require all realms to be successful during the attempt, or perhaps only at least one must be
   * successful, or some other interpretation.  This customized behavior can be performed via the use of a
   * {@link #setAuthenticationStrategy(AuthenticationStrategy) AuthenticationStrategy}, which
   * you can inject as a property of this class.
   * <p/>
   * The strategy object provides callback methods that allow you to
   * determine what constitutes a success or failure in a multi-realm (PAM) scenario.  And because this only makes sense
   * in a multi-realm scenario, the strategy object is only utilized when more than one Realm is configured.
   * <p/>
   * As most multi-realm applications require at least one Realm authenticates successfully, the default
   * implementation is the {@link AtLeastOneSuccessfulStrategy}.
   *
   * @see #setRealms
   * @see AtLeastOneSuccessfulStrategy
   * @see AllSuccessfulStrategy
   * @see FirstSuccessfulStrategy
   * @since 0.1
   */
  public class ModularRealmAuthenticator extends AbstractAuthenticator {
  
      /*--------------------------------------------
      |             C O N S T A N T S             |
      ============================================*/
      private static final Logger log = LoggerFactory.getLogger(ModularRealmAuthenticator.class);
  
      /*--------------------------------------------
      |    I N S T A N C E   V A R I A B L E S    |
      ============================================*/
      /**
       * List of realms that will be iterated through when a user authenticates.
       */
      private Collection<Realm> realms;
  
      /**
       * The authentication strategy to use during authentication attempts, defaults to a
       * {@link org.apache.shiro.authc.pam.AtLeastOneSuccessfulStrategy} instance.
       */
      private AuthenticationStrategy authenticationStrategy;
  
      /*--------------------------------------------
      |         C O N S T R U C T O R S           |
      ============================================*/
  
      /**
       * Default no-argument constructor which
       * {@link #setAuthenticationStrategy(AuthenticationStrategy) enables}  an
       * {@link org.apache.shiro.authc.pam.AtLeastOneSuccessfulStrategy} by default.
       */
      public ModularRealmAuthenticator() {
          this.authenticationStrategy = new AtLeastOneSuccessfulStrategy();
      }
  
      /*--------------------------------------------
      |  A C C E S S O R S / M O D I F I E R S    |
     ============================================*/
 
     /**
      * Sets all realms used by this Authenticator, providing PAM (Pluggable Authentication Module) configuration.
      *
      * @param realms the realms to consult during authentication attempts.
      */
     public void setRealms(Collection<Realm> realms) {
         this.realms = realms;
     }
 
     /**
      * Returns the realm(s) used by this {@code Authenticator} during an authentication attempt.
      *
      * @return the realm(s) used by this {@code Authenticator} during an authentication attempt.
      */
     protected Collection<Realm> getRealms() {
         return this.realms;
     }
 
     /**
      * Returns the {@code AuthenticationStrategy} utilized by this modular authenticator during a multi-realm
      * log-in attempt.  This object is only used when two or more Realms are configured.
      * <p/>
      * Unless overridden by
      * the {@link #setAuthenticationStrategy(AuthenticationStrategy)} method, the default implementation
      * is the {@link org.apache.shiro.authc.pam.AtLeastOneSuccessfulStrategy}.
      *
      * @return the {@code AuthenticationStrategy} utilized by this modular authenticator during a log-in attempt.
      * @since 0.2
      */
     public AuthenticationStrategy getAuthenticationStrategy() {
         return authenticationStrategy;
     }
 
     /**
      * Allows overriding the default {@code AuthenticationStrategy} utilized during multi-realm log-in attempts.
      * This object is only used when two or more Realms are configured.
      *
      * @param authenticationStrategy the strategy implementation to use during log-in attempts.
      * @since 0.2
      */
     public void setAuthenticationStrategy(AuthenticationStrategy authenticationStrategy) {
         this.authenticationStrategy = authenticationStrategy;
     }
 
     /*--------------------------------------------
     |               M E T H O D S               |
 
     /**
      * Used by the internal {@link #doAuthenticate} implementation to ensure that the {@code realms} property
      * has been set.  The default implementation ensures the property is not null and not empty.
      *
      * @throws IllegalStateException if the {@code realms} property is configured incorrectly.
      */
 
     protected void assertRealmsConfigured() throws IllegalStateException {
         Collection<Realm> realms = getRealms();
         if (CollectionUtils.isEmpty(realms)) {
             String msg = "Configuration error:  No realms have been configured!  One or more realms must be " +
                     "present to execute an authentication attempt.";
             throw new IllegalStateException(msg);
         }
     }
 
     /**
      * Performs the authentication attempt by interacting with the single configured realm, which is significantly
      * simpler than performing multi-realm logic.
      *
      * @param realm the realm to consult for AuthenticationInfo.
      * @param token the submitted AuthenticationToken representing the subject's (user's) log-in principals and credentials.
      * @return the AuthenticationInfo associated with the user account corresponding to the specified {@code token}
      */
     protected AuthenticationInfo doSingleRealmAuthentication(Realm realm, AuthenticationToken token) {
         if (!realm.supports(token)) {
             String msg = "Realm [" + realm + "] does not support authentication token [" +
                     token + "].  Please ensure that the appropriate Realm implementation is " +
                     "configured correctly or that the realm accepts AuthenticationTokens of this type.";
             throw new UnsupportedTokenException(msg);
         }
         AuthenticationInfo info = realm.getAuthenticationInfo(token);
         if (info == null) {
             String msg = "Realm [" + realm + "] was unable to find account data for the " +
                     "submitted AuthenticationToken [" + token + "].";
             throw new UnknownAccountException(msg);
         }
         return info;
     }
 
     /**
      * Performs the multi-realm authentication attempt by calling back to a {@link AuthenticationStrategy} object
      * as each realm is consulted for {@code AuthenticationInfo} for the specified {@code token}.
      *
      * @param realms the multiple realms configured on this Authenticator instance.
      * @param token  the submitted AuthenticationToken representing the subject's (user's) log-in principals and credentials.
      * @return an aggregated AuthenticationInfo instance representing account data across all the successfully
      *         consulted realms.
      */
     protected AuthenticationInfo doMultiRealmAuthentication(Collection<Realm> realms, AuthenticationToken token) {
 
         AuthenticationStrategy strategy = getAuthenticationStrategy();
 
         AuthenticationInfo aggregate = strategy.beforeAllAttempts(realms, token);
 
         if (log.isTraceEnabled()) {
             log.trace("Iterating through {} realms for PAM authentication", realms.size());
         }
 
         for (Realm realm : realms) {
 
             try {
                 aggregate = strategy.beforeAttempt(realm, token, aggregate);
             } catch (ShortCircuitIterationException shortCircuitSignal) {
                 // Break from continuing with subsequnet realms on receiving 
                 // short circuit signal from strategy
                 break;
             }
 
             if (realm.supports(token)) {
 
                 log.trace("Attempting to authenticate token [{}] using realm [{}]", token, realm);
 
                 AuthenticationInfo info = null;
                 Throwable t = null;
                 try {
                     info = realm.getAuthenticationInfo(token);
                 } catch (Throwable throwable) {
                     t = throwable;
                     if (log.isDebugEnabled()) {
                         String msg = "Realm [" + realm + "] threw an exception during a multi-realm authentication attempt:";
                         log.debug(msg, t);
                     }
                 }
 
                 aggregate = strategy.afterAttempt(realm, token, info, aggregate, t);
 
             } else {
                 log.debug("Realm [{}] does not support token {}.  Skipping realm.", realm, token);
             }
         }
 
         aggregate = strategy.afterAllAttempts(token, aggregate);
 
         return aggregate;
     }
 
 
     /**
      * Attempts to authenticate the given token by iterating over the internal collection of
      * {@link Realm}s.  For each realm, first the {@link Realm#supports(org.apache.shiro.authc.AuthenticationToken)}
      * method will be called to determine if the realm supports the {@code authenticationToken} method argument.
      * <p/>
      * If a realm does support
      * the token, its {@link Realm#getAuthenticationInfo(org.apache.shiro.authc.AuthenticationToken)}
      * method will be called.  If the realm returns a non-null account, the token will be
      * considered authenticated for that realm and the account data recorded.  If the realm returns {@code null},
      * the next realm will be consulted.  If no realms support the token or all supporting realms return null,
      * an {@link AuthenticationException} will be thrown to indicate that the user could not be authenticated.
      * <p/>
      * After all realms have been consulted, the information from each realm is aggregated into a single
      * {@link AuthenticationInfo} object and returned.
      *
      * @param authenticationToken the token containing the authentication principal and credentials for the
      *                            user being authenticated.
      * @return account information attributed to the authenticated user.
      * @throws IllegalStateException   if no realms have been configured at the time this method is invoked
      * @throws AuthenticationException if the user could not be authenticated or the user is denied authentication
      *                                 for the given principal and credentials.
      */
     protected AuthenticationInfo doAuthenticate(AuthenticationToken authenticationToken) throws AuthenticationException {
         assertRealmsConfigured();
         Collection<Realm> realms = getRealms();
         if (realms.size() == 1) {
             return doSingleRealmAuthentication(realms.iterator().next(), authenticationToken);
         } else {
             return doMultiRealmAuthentication(realms, authenticationToken);
         }
     }
 
     /**
      * First calls <code>super.onLogout(principals)</code> to ensure a logout notification is issued, and for each
      * wrapped {@code Realm} that implements the {@link LogoutAware LogoutAware} interface, calls
      * <code>((LogoutAware)realm).onLogout(principals)</code> to allow each realm the opportunity to perform
      * logout/cleanup operations during an user-logout.
      * <p/>
      * Shiro's Realm implementations all implement the {@code LogoutAware} interface by default and can be
      * overridden for realm-specific logout logic.
      *
      * @param principals the application-specific Subject/user identifier.
      */
     public void onLogout(PrincipalCollection principals) {
         super.onLogout(principals);
         Collection<Realm> realms = getRealms();
         if (!CollectionUtils.isEmpty(realms)) {
             for (Realm realm : realms) {
                 if (realm instanceof LogoutAware) {
                     ((LogoutAware) realm).onLogout(principals);
                 }
             }
         }
     }
 }