/*
    * 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.realm;
  
  import org.apache.shiro.authc.AuthenticationException;
  import org.apache.shiro.authc.AuthenticationInfo;
  import org.apache.shiro.authc.AuthenticationToken;
  import org.apache.shiro.authc.IncorrectCredentialsException;
  import org.apache.shiro.authc.UsernamePasswordToken;
  import org.apache.shiro.authc.credential.AllowAllCredentialsMatcher;
  import org.apache.shiro.authc.credential.CredentialsMatcher;
  import org.apache.shiro.authc.credential.SimpleCredentialsMatcher;
  import org.apache.shiro.cache.Cache;
  import org.apache.shiro.cache.CacheManager;
  import org.apache.shiro.subject.PrincipalCollection;
  import org.apache.shiro.util.Initializable;
  import org.slf4j.Logger;
  import org.slf4j.LoggerFactory;
  
  import java.util.concurrent.atomic.AtomicInteger;
  
  
  /**
   * A top-level abstract implementation of the <tt>Realm</tt> interface that only implements authentication support
   * (log-in) operations and leaves authorization (access control) behavior to subclasses.
   * <h2>Authentication Caching</h2>
   * For applications that perform frequent repeated authentication of the same accounts (e.g. as is often done in
   * REST or Soap applications that authenticate on every request), it might be prudent to enable authentication
   * caching to alleviate constant load on any back-end data sources.
   * <p/>
   * This feature is disabled by default to retain backwards-compatibility with Shiro 1.1 and earlier.  It may be
   * enabled by setting {@link #setAuthenticationCachingEnabled(boolean) authenticationCachingEnabled} = {@code true}
   * (and configuring Shiro with a {@link CacheManager} of course), but <b>NOTE:</b>
   * <p/>
   * <b>ONLY enable authentication caching if either of the following is true for your realm implementation:</b>
   * <ul>
   * <li>The {@link #doGetAuthenticationInfo(org.apache.shiro.authc.AuthenticationToken) doGetAuthenticationInfo}
   * implementation returns {@code AuthenticationInfo} instances where the
   * {@link org.apache.shiro.authc.AuthenticationInfo#getCredentials() credentials} are securely obfuscated and NOT
   * plaintext (raw) credentials. For example,
   * if your realm references accounts with passwords, that the {@code AuthenticationInfo}'s
   * {@link org.apache.shiro.authc.AuthenticationInfo#getCredentials() credentials} are safely hashed and salted or otherwise
   * fully encrypted.<br/><br/></li>
   * <li>The {@link #doGetAuthenticationInfo(org.apache.shiro.authc.AuthenticationToken) doGetAuthenticationInfo}
   * implementation returns {@code AuthenticationInfo} instances where the
   * {@link org.apache.shiro.authc.AuthenticationInfo#getCredentials() credentials} are plaintext (raw) <b>AND</b> the
   * cache region storing the {@code AuthenticationInfo} instances WILL NOT overflow to disk and WILL NOT transmit cache
   * entries over an unprotected (non TLS/SSL) network (as might be the case with a networked/distributed enterprise cache).
   * This should be the case even in private/trusted/corporate networks.</li>
   * </ul>
   * <p/>
   * These points are very important because if authentication caching is enabled, this abstract class implementation
   * will place AuthenticationInfo instances returned from the subclass implementations directly into the cache, for
   * example:
   * <pre>
   * cache.put(cacheKey, subclassAuthenticationInfoInstance);
   * </pre>
   * <p/>
   * Enabling authentication caching is ONLY safe to do if the above two scenarios apply.  It is NOT safe to enable under
   * any other scenario.
   * <p/>
   * When possible, always represent and store credentials in a safe form (hash+salt or encrypted) to eliminate plaintext
   * visibility.
   * <h3>Authentication Cache Invalidation on Logout</h3>
   * If authentication caching is enabled, this implementation will attempt to evict (remove) cached authentication data
   * for an account during logout.  This can only occur if the
   * {@link #getAuthenticationCacheKey(org.apache.shiro.authc.AuthenticationToken)} and
   * {@link #getAuthenticationCacheKey(org.apache.shiro.subject.PrincipalCollection)} methods return the exact same value.
   * <p/>
   * The default implementations of these methods expect that the
   * {@link org.apache.shiro.authc.AuthenticationToken#getPrincipal()} (what the user submits during login) and
   * {@link #getAvailablePrincipal(org.apache.shiro.subject.PrincipalCollection) getAvailablePrincipal} (what is returned
   * by the realm after account lookup) return
   * the same exact value.  For example, the user submitted username is also the primary account identifier.
   * <p/>
   * However, if your application uses, say, a username for end-user login, but returns a primary key ID as the
   * primary principal after authentication, then you will need to override either
   * {@link #getAuthenticationCacheKey(org.apache.shiro.authc.AuthenticationToken) getAuthenticationCacheKey(token)} or
   * {@link #getAuthenticationCacheKey(org.apache.shiro.subject.PrincipalCollection) getAuthenticationCacheKey(principals)}
   * (or both) to ensure that the same cache key can be used for either object.
   * <p/>
   * This guarantees that the same cache key used to cache the data during authentication (derived from the
   * {@code AuthenticationToken}) will be used to remove the cached data during logout (derived from the
  * {@code PrincipalCollection}).
  * <h4>Unmatching Cache Key Values</h4>
  * If the return values from {@link #getAuthenticationCacheKey(org.apache.shiro.authc.AuthenticationToken)} and
  * {@link #getAuthenticationCacheKey(org.apache.shiro.subject.PrincipalCollection)} are not identical, cached
  * authentication data removal is at the mercy of your cache provider settings.  For example, often cache
  * implementations will evict cache entries based on a timeToIdle or timeToLive (TTL) value.
  * <p/>
  * If this lazy eviction capability of the cache product is not sufficient and you want discrete behavior
  * (highly recommended for authentication data), ensure that the return values from those two methods are identical in
  * the subclass implementation.
  *
  * @since 0.2
  */
 public abstract class AuthenticatingRealm extends CachingRealm implements Initializable {
 
     //TODO - complete JavaDoc
 
     private static final Logger log = LoggerFactory.getLogger(AuthenticatingRealm.class);
 
     private static final AtomicInteger INSTANCE_COUNT = new AtomicInteger();
 
     /**
      * The default suffix appended to the realm name used for caching authentication data.
      *
      * @since 1.2
      */
     private static final String DEFAULT_AUTHENTICATION_CACHE_SUFFIX = ".authenticationCache";
 
     /**
      * Credentials matcher used to determine if the provided credentials match the credentials stored in the data store.
      */
     private CredentialsMatcher credentialsMatcher;
 
 
     private Cache<Object, AuthenticationInfo> authenticationCache;
 
     private boolean authenticationCachingEnabled;
     private String authenticationCacheName;
 
     /**
      * The class that this realm supports for authentication tokens.  This is used by the
      * default implementation of the {@link Realm#supports(org.apache.shiro.authc.AuthenticationToken)} method to
      * determine whether or not the given authentication token is supported by this realm.
      */
     private Class<? extends AuthenticationToken> authenticationTokenClass;
 
     /*-------------------------------------------
     |         C O N S T R U C T O R S           |
     ============================================*/
     public AuthenticatingRealm() {
         this(null, new SimpleCredentialsMatcher());
     }
 
     public AuthenticatingRealm(CacheManager cacheManager) {
         this(cacheManager, new SimpleCredentialsMatcher());
     }
 
     public AuthenticatingRealm(CredentialsMatcher matcher) {
         this(null, matcher);
     }
 
     public AuthenticatingRealm(CacheManager cacheManager, CredentialsMatcher matcher) {
         authenticationTokenClass = UsernamePasswordToken.class;
 
         //retain backwards compatibility for Shiro 1.1 and earlier.  Setting to true by default will probably cause
         //unexpected results for existing applications:
         this.authenticationCachingEnabled = false;
 
         int instanceNumber = INSTANCE_COUNT.getAndIncrement();
         this.authenticationCacheName = getClass().getName() + DEFAULT_AUTHENTICATION_CACHE_SUFFIX;
         if (instanceNumber > 0) {
             this.authenticationCacheName = this.authenticationCacheName + "." + instanceNumber;
         }
 
         if (cacheManager != null) {
             setCacheManager(cacheManager);
         }
         if (matcher != null) {
             setCredentialsMatcher(matcher);
         }
     }
 
     /*--------------------------------------------
     |  A C C E S S O R S / M O D I F I E R S    |
     ============================================*/
 
     /**
      * Returns the <code>CredentialsMatcher</code> used during an authentication attempt to verify submitted
      * credentials with those stored in the system.
      * <p/>
      * <p>Unless overridden by the {@link #setCredentialsMatcher setCredentialsMatcher} method, the default
      * value is a {@link org.apache.shiro.authc.credential.SimpleCredentialsMatcher SimpleCredentialsMatcher} instance.
      *
      * @return the <code>CredentialsMatcher</code> used during an authentication attempt to verify submitted
      *         credentials with those stored in the system.
      */
     public CredentialsMatcher getCredentialsMatcher() {
         return credentialsMatcher;
     }
 
     /**
      * Sets the CrendialsMatcher used during an authentication attempt to verify submitted credentials with those
      * stored in the system.  The implementation of this matcher can be switched via configuration to
      * support any number of schemes, including plain text comparisons, hashing comparisons, and others.
      * <p/>
      * <p>Unless overridden by this method, the default value is a
      * {@link org.apache.shiro.authc.credential.SimpleCredentialsMatcher} instance.
      *
      * @param credentialsMatcher the matcher to use.
      */
     public void setCredentialsMatcher(CredentialsMatcher credentialsMatcher) {
         this.credentialsMatcher = credentialsMatcher;
     }
 
     /**
      * Returns the authenticationToken class supported by this realm.
      * <p/>
      * <p>The default value is <tt>{@link org.apache.shiro.authc.UsernamePasswordToken UsernamePasswordToken.class}</tt>, since
      * about 90% of realms use username/password authentication, regardless of their protocol (e.g. over jdbc, ldap,
      * kerberos, http, etc).
      * <p/>
      * <p>If subclasses haven't already overridden the {@link Realm#supports Realm.supports(AuthenticationToken)} method,
      * they must {@link #setAuthenticationTokenClass(Class) set a new class} if they won't support
      * <tt>UsernamePasswordToken</tt> authentication token submissions.
      *
      * @return the authenticationToken class supported by this realm.
      * @see #setAuthenticationTokenClass
      */
     public Class getAuthenticationTokenClass() {
         return authenticationTokenClass;
     }
 
     /**
      * Sets the authenticationToken class supported by this realm.
      * <p/>
      * <p>Unless overridden by this method, the default value is
      * {@link org.apache.shiro.authc.UsernamePasswordToken UsernamePasswordToken.class} to support the majority of applications.
      *
      * @param authenticationTokenClass the class of authentication token instances supported by this realm.
      * @see #getAuthenticationTokenClass getAuthenticationTokenClass() for more explanation.
      */
     public void setAuthenticationTokenClass(Class<? extends AuthenticationToken> authenticationTokenClass) {
         this.authenticationTokenClass = authenticationTokenClass;
     }
 
     /**
      * Sets an explicit {@link Cache} instance to use for authentication caching.  If not set and authentication
      * caching is {@link #isAuthenticationCachingEnabled() enabled}, any available
      * {@link #getCacheManager() cacheManager} will be used to acquire the cache instance if available.
      * <p/>
      * <b>WARNING:</b> Only set this property if safe caching conditions apply, as documented at the top
      * of this page in the class-level JavaDoc.
      *
      * @param authenticationCache an explicit {@link Cache} instance to use for authentication caching or
      *                            {@code null} if the cache should possibly be obtained another way.
      * @see #isAuthenticationCachingEnabled()
      * @since 1.2
      */
     public void setAuthenticationCache(Cache<Object, AuthenticationInfo> authenticationCache) {
         this.authenticationCache = authenticationCache;
     }
 
     /**
      * Returns a {@link Cache} instance to use for authentication caching, or {@code null} if no cache has been
      * set.
      *
      * @return a {@link Cache} instance to use for authentication caching, or {@code null} if no cache has been
      *         set.
      * @see #setAuthenticationCache(org.apache.shiro.cache.Cache)
      * @see #isAuthenticationCachingEnabled()
      * @since 1.2
      */
     public Cache<Object, AuthenticationInfo> getAuthenticationCache() {
         return this.authenticationCache;
     }
 
     /**
      * Returns the name of a {@link Cache} to lookup from any available {@link #getCacheManager() cacheManager} if
      * a cache is not explicitly configured via {@link #setAuthenticationCache(org.apache.shiro.cache.Cache)}.
      * <p/>
      * This name will only be used to look up a cache if authentication caching is
      * {@link #isAuthenticationCachingEnabled() enabled}.
      * <p/>
      * <b>WARNING:</b> Only set this property if safe caching conditions apply, as documented at the top
      * of this page in the class-level JavaDoc.
      *
      * @return the name of a {@link Cache} to lookup from any available {@link #getCacheManager() cacheManager} if
      *         a cache is not explicitly configured via {@link #setAuthenticationCache(org.apache.shiro.cache.Cache)}.
      * @see #isAuthenticationCachingEnabled()
      * @since 1.2
      */
     public String getAuthenticationCacheName() {
         return this.authenticationCacheName;
     }
 
     /**
      * Sets the name of a {@link Cache} to lookup from any available {@link #getCacheManager() cacheManager} if
      * a cache is not explicitly configured via {@link #setAuthenticationCache(org.apache.shiro.cache.Cache)}.
      * <p/>
      * This name will only be used to look up a cache if authentication caching is
      * {@link #isAuthenticationCachingEnabled() enabled}.
      *
      * @param authenticationCacheName the name of a {@link Cache} to lookup from any available
      *                                {@link #getCacheManager() cacheManager} if a cache is not explicitly configured
      *                                via {@link #setAuthenticationCache(org.apache.shiro.cache.Cache)}.
      * @see #isAuthenticationCachingEnabled()
      * @since 1.2
      */
     public void setAuthenticationCacheName(String authenticationCacheName) {
         this.authenticationCacheName = authenticationCacheName;
     }
 
     /**
      * Returns {@code true} if authentication caching should be utilized if a {@link CacheManager} has been
      * {@link #setCacheManager(org.apache.shiro.cache.CacheManager) configured}, {@code false} otherwise.
      * <p/>
      * The default value is {@code true}.
      *
      * @return {@code true} if authentication caching should be utilized, {@code false} otherwise.
      */
     public boolean isAuthenticationCachingEnabled() {
         return this.authenticationCachingEnabled && isCachingEnabled();
     }
 
     /**
      * Sets whether or not authentication caching should be utilized if a {@link CacheManager} has been
      * {@link #setCacheManager(org.apache.shiro.cache.CacheManager) configured}, {@code false} otherwise.
      * <p/>
      * The default value is {@code false} to retain backwards compatibility with Shiro 1.1 and earlier.
      * <p/>
      * <b>WARNING:</b> Only set this property to {@code true} if safe caching conditions apply, as documented at the top
      * of this page in the class-level JavaDoc.
      *
      * @param authenticationCachingEnabled the value to set
      */
     @SuppressWarnings({"UnusedDeclaration"})
     public void setAuthenticationCachingEnabled(boolean authenticationCachingEnabled) {
         this.authenticationCachingEnabled = authenticationCachingEnabled;
         if (authenticationCachingEnabled) {
             setCachingEnabled(true);
         }
     }
 
     public void setName(String name) {
         super.setName(name);
         String authcCacheName = this.authenticationCacheName;
         if (authcCacheName != null && authcCacheName.startsWith(getClass().getName())) {
             //get rid of the default heuristically-created cache name.  Create a more meaningful one
             //based on the application-unique Realm name:
             this.authenticationCacheName = name + DEFAULT_AUTHENTICATION_CACHE_SUFFIX;
         }
     }
 
 
     /*--------------------------------------------
     |               M E T H O D S               |
     ============================================*/
 
     /**
      * Convenience implementation that returns
      * <tt>getAuthenticationTokenClass().isAssignableFrom( token.getClass() );</tt>.  Can be overridden
      * by subclasses for more complex token checking.
      * <p>Most configurations will only need to set a different class via
      * {@link #setAuthenticationTokenClass}, as opposed to overriding this method.
      *
      * @param token the token being submitted for authentication.
      * @return true if this authentication realm can process the submitted token instance of the class, false otherwise.
      */
     public boolean supports(AuthenticationToken token) {
         return token != null && getAuthenticationTokenClass().isAssignableFrom(token.getClass());
     }
 
     /**
      * Initializes this realm and potentially enables an authentication cache, depending on configuration.  Based on
      * the availability of an authentication cache, this class functions as follows:
      * <ol>
      * <li>If the {@link #setAuthenticationCache cache} property has been set, it will be
      * used to cache the AuthenticationInfo objects returned from {@link #getAuthenticationInfo}
      * method invocations.
      * All future calls to {@link #getAuthenticationInfo} will attempt to use this cache first
      * to alleviate any potentially unnecessary calls to an underlying data store.</li>
      * <li>If the {@link #setAuthenticationCache cache} property has <b>not</b> been set,
      * the {@link #setCacheManager cacheManager} property will be checked.
      * If a {@code cacheManager} has been set, it will be used to eagerly acquire an authentication
      * {@code cache}, and this cache which will be used as specified in #1.</li>
      * <li>If neither the {@link #setAuthenticationCache (org.apache.shiro.cache.Cache) authenticationCache}
      * or {@link #setCacheManager(org.apache.shiro.cache.CacheManager) cacheManager}
      * properties are set, caching will not be utilized and authentication look-ups will be delegated to
      * subclass implementations for each authentication attempt.</li>
      * </ol>
      * <p/>
      * This method finishes by calling {@link #onInit()} is to allow subclasses to perform any init behavior desired.
      *
      * @since 1.2
      */
     public final void init() {
         //trigger obtaining the authorization cache if possible
         getAvailableAuthenticationCache();
         onInit();
     }
 
     /**
      * Template method for subclasses to implement any initialization logic.  Called from
      * {@link #init()}.
      *
      * @since 1.2
      */
     protected void onInit() {
     }
 
     /**
      * This implementation attempts to acquire an authentication cache if one is not already configured.
      *
      * @since 1.2
      */
     protected void afterCacheManagerSet() {
         //trigger obtaining the authorization cache if possible
         getAvailableAuthenticationCache();
     }
 
     /**
      * Returns any available {@link Cache} instance to use for authentication caching.  This functions as follows:
      * <ol>
      * <li>If an {@link #setAuthenticationCache(org.apache.shiro.cache.Cache) authenticationCache} has been explicitly
      * configured (it is not null), it is returned.</li>
      * <li>If there is no {@link #getAuthenticationCache() authenticationCache} configured:
      * <ol>
      * <li>If authentication caching is {@link #isAuthenticationCachingEnabled() enabled}, any available
      * {@link #getCacheManager() cacheManager} will be consulted to obtain an available authentication cache.
      * </li>
      * <li>If authentication caching is disabled, this implementation does nothing.</li>
      * </ol>
      * </li>
      * </ol>
      *
      * @return any available {@link Cache} instance to use for authentication caching.
      */
     private Cache<Object, AuthenticationInfo> getAvailableAuthenticationCache() {
         Cache<Object, AuthenticationInfo> cache = getAuthenticationCache();
         boolean authcCachingEnabled = isAuthenticationCachingEnabled();
         if (cache == null && authcCachingEnabled) {
             cache = getAuthenticationCacheLazy();
         }
         return cache;
     }
 
     /**
      * Checks to see if the authenticationCache class attribute is null, and if so, attempts to acquire one from
      * any configured {@link #getCacheManager() cacheManager}.  If one is acquired, it is set as the class attribute.
      * The class attribute is then returned.
      *
      * @return an available cache instance to be used for authentication caching or {@code null} if one is not available.
      * @since 1.2
      */
     private Cache<Object, AuthenticationInfo> getAuthenticationCacheLazy() {
 
         if (this.authenticationCache == null) {
 
             log.trace("No authenticationCache instance set.  Checking for a cacheManager...");
 
             CacheManager cacheManager = getCacheManager();
 
             if (cacheManager != null) {
                 String cacheName = getAuthenticationCacheName();
                 log.debug("CacheManager [{}] configured.  Building authentication cache '{}'", cacheManager, cacheName);
                 this.authenticationCache = cacheManager.getCache(cacheName);
             }
         }
 
         return this.authenticationCache;
     }
 
     /**
      * Returns any cached AuthenticationInfo corresponding to the specified token or {@code null} if there currently
      * isn't any cached data.
      *
      * @param token the token submitted during the authentication attempt.
      * @return any cached AuthenticationInfo corresponding to the specified token or {@code null} if there currently
      *         isn't any cached data.
      * @since 1.2
      */
     private AuthenticationInfo getCachedAuthenticationInfo(AuthenticationToken token) {
         AuthenticationInfo info = null;
 
         Cache<Object, AuthenticationInfo> cache = getAvailableAuthenticationCache();
         if (cache != null && token != null) {
             log.trace("Attempting to retrieve the AuthenticationInfo from cache.");
             Object key = getAuthenticationCacheKey(token);
             info = cache.get(key);
             if (info == null) {
                 log.trace("No AuthorizationInfo found in cache for key [{}]", key);
             } else {
                 log.trace("Found cached AuthorizationInfo for key [{}]", key);
             }
         }
 
         return info;
     }
 
     /**
      * Caches the specified info if authentication caching
      * {@link #isAuthenticationCachingEnabled(org.apache.shiro.authc.AuthenticationToken, org.apache.shiro.authc.AuthenticationInfo) isEnabled}
      * for the specific token/info pair and a cache instance is available to be used.
      *
      * @param token the authentication token submitted which resulted in a successful authentication attempt.
      * @param info  the AuthenticationInfo to cache as a result of the successful authentication attempt.
      * @since 1.2
      */
     private void cacheAuthenticationInfoIfPossible(AuthenticationToken token, AuthenticationInfo info) {
         if (!isAuthenticationCachingEnabled(token, info)) {
             log.debug("AuthenticationInfo caching is disabled for info [{}].  Submitted token: [{}].", info, token);
             //return quietly, caching is disabled for this token/info pair:
             return;
         }
 
         Cache<Object, AuthenticationInfo> cache = getAvailableAuthenticationCache();
         if (cache != null) {
             Object key = getAuthenticationCacheKey(token);
             cache.put(key, info);
             log.trace("Cached AuthenticationInfo for continued authentication.  key=[{}], value=[{}].", key, info);
         }
     }
 
     /**
      * Returns {@code true} if authentication caching should be utilized based on the specified
      * {@link AuthenticationToken} and/or {@link AuthenticationInfo}, {@code false} otherwise.
      * <p/>
      * The default implementation simply delegates to {@link #isAuthenticationCachingEnabled()}, the general-case
      * authentication caching setting.  Subclasses can override this to turn on or off caching at runtime
      * based on the specific submitted runtime values.
      *
      * @param token the submitted authentication token
      * @param info  the {@code AuthenticationInfo} acquired from data source lookup via
      *              {@link #doGetAuthenticationInfo(org.apache.shiro.authc.AuthenticationToken)}
      * @return {@code true} if authentication caching should be utilized based on the specified
      *         {@link AuthenticationToken} and/or {@link AuthenticationInfo}, {@code false} otherwise.
      * @since 1.2
      */
     protected boolean isAuthenticationCachingEnabled(AuthenticationToken token, AuthenticationInfo info) {
         return isAuthenticationCachingEnabled();
     }
 
     /**
      * This implementation functions as follows:
      * <ol>
      * <li>It attempts to acquire any cached {@link AuthenticationInfo} corresponding to the specified
      * {@link AuthenticationToken} argument.  If a cached value is found, it will be used for credentials matching,
      * alleviating the need to perform any lookups with a data source.</li>
      * <li>If there is no cached {@link AuthenticationInfo} found, delegate to the
      * {@link #doGetAuthenticationInfo(org.apache.shiro.authc.AuthenticationToken)} method to perform the actual
      * lookup.  If authentication caching is enabled and possible, any returned info object will be
      * {@link #cacheAuthenticationInfoIfPossible(org.apache.shiro.authc.AuthenticationToken, org.apache.shiro.authc.AuthenticationInfo) cached}
      * to be used in future authentication attempts.</li>
      * <li>If an AuthenticationInfo instance is not found in the cache or by lookup, {@code null} is returned to
      * indicate an account cannot be found.</li>
      * <li>If an AuthenticationInfo instance is found (either cached or via lookup), ensure the submitted
      * AuthenticationToken's credentials match the expected {@code AuthenticationInfo}'s credentials using the
      * {@link #getCredentialsMatcher() credentialsMatcher}.  This means that credentials are always verified
      * for an authentication attempt.</li>
      * </ol>
      *
      * @param token the submitted account principal and credentials.
      * @return the AuthenticationInfo corresponding to the given {@code token}, or {@code null} if no
      *         AuthenticationInfo could be found.
      * @throws AuthenticationException if authentication failed.
      */
     public final AuthenticationInfo getAuthenticationInfo(AuthenticationToken token) throws AuthenticationException {
 
         AuthenticationInfo info = getCachedAuthenticationInfo(token);
         if (info == null) {
             //otherwise not cached, perform the lookup:
             info = doGetAuthenticationInfo(token);
             log.debug("Looked up AuthenticationInfo [{}] from doGetAuthenticationInfo", info);
             if (token != null && info != null) {
                 cacheAuthenticationInfoIfPossible(token, info);
             }
         } else {
             log.debug("Using cached authentication info [{}] to perform credentials matching.", info);
         }
 
         if (info != null) {
             assertCredentialsMatch(token, info);
         } else {
             log.debug("No AuthenticationInfo found for submitted AuthenticationToken [{}].  Returning null.", token);
         }
 
         return info;
     }
 
     /**
      * Asserts that the submitted {@code AuthenticationToken}'s credentials match the stored account
      * {@code AuthenticationInfo}'s credentials, and if not, throws an {@link AuthenticationException}.
      *
      * @param token the submitted authentication token
      * @param info  the AuthenticationInfo corresponding to the given {@code token}
      * @throws AuthenticationException if the token's credentials do not match the stored account credentials.
      */
     protected void assertCredentialsMatch(AuthenticationToken token, AuthenticationInfo info) throws AuthenticationException {
         CredentialsMatcher cm = getCredentialsMatcher();
         if (cm != null) {
             if (!cm.doCredentialsMatch(token, info)) {
                 //not successful - throw an exception to indicate this:
                 String msg = "Submitted credentials for token [" + token + "] did not match the expected credentials.";
                 throw new IncorrectCredentialsException(msg);
             }
         } else {
             throw new AuthenticationException("A CredentialsMatcher must be configured in order to verify " +
                     "credentials during authentication.  If you do not wish for credentials to be examined, you " +
                     "can configure an " + AllowAllCredentialsMatcher.class.getName() + " instance.");
         }
     }
 
     /**
      * Returns the key under which {@link AuthenticationInfo} instances are cached if authentication caching is enabled.
      * This implementation defaults to returning the token's
      * {@link org.apache.shiro.authc.AuthenticationToken#getPrincipal() principal}, which is usually a username in
      * most applications.
      * <h3>Cache Invalidation on Logout</h3>
      * <b>NOTE:</b> If you want to be able to invalidate an account's cached {@code AuthenticationInfo} on logout, you
      * must ensure the {@link #getAuthenticationCacheKey(org.apache.shiro.subject.PrincipalCollection)} method returns
      * the same value as this method.
      *
      * @param token the authentication token for which any successful authentication will be cached.
      * @return the cache key to use to cache the associated {@link AuthenticationInfo} after a successful authentication.
      * @since 1.2
      */
     protected Object getAuthenticationCacheKey(AuthenticationToken token) {
         return token != null ? token.getPrincipal() : null;
     }
 
     /**
      * Returns the key under which {@link AuthenticationInfo} instances are cached if authentication caching is enabled.
      * This implementation delegates to
      * {@link #getAvailablePrincipal(org.apache.shiro.subject.PrincipalCollection)}, which returns the primary principal
      * associated with this particular Realm.
      * <h3>Cache Invalidation on Logout</h3>
      * <b>NOTE:</b> If you want to be able to invalidate an account's cached {@code AuthenticationInfo} on logout, you
      * must ensure that this method returns the same value as the
      * {@link #getAuthenticationCacheKey(org.apache.shiro.authc.AuthenticationToken)} method!
      *
      * @param principals the principals of the account for which to set or remove cached {@code AuthenticationInfo}.
      * @return the cache key to use when looking up cached {@link AuthenticationInfo} instances.
      * @since 1.2
      */
     protected Object getAuthenticationCacheKey(PrincipalCollection principals) {
         return getAvailablePrincipal(principals);
     }
 
     /**
      * This implementation clears out any cached authentication data by calling
      * {@link #clearCachedAuthenticationInfo(org.apache.shiro.subject.PrincipalCollection)}.
      * If overriding in a subclass, be sure to call {@code super.doClearCache} to ensure this behavior is maintained.
      *
      * @param principals principals the principals of the account for which to clear any cached data.
      * @since 1.2
      */
     @Override
     protected void doClearCache(PrincipalCollection principals) {
         super.doClearCache(principals);
         clearCachedAuthenticationInfo(principals);
     }
 
     private static boolean isEmpty(PrincipalCollection pc) {
         return pc == null || pc.isEmpty();
     }
 
     /**
      * Clears out the AuthenticationInfo cache entry for the specified account.
      * <p/>
      * This method is provided as a convenience to subclasses so they can invalidate a cache entry when they
      * change an account's authentication data (e.g. reset password) during runtime.  Because an account's
      * AuthenticationInfo can be cached, there needs to be a way to invalidate the cache for only that account so that
      * subsequent authentication operations don't used the (old) cached value if account data changes.
      * <p/>
      * After this method is called, the next authentication for that same account will result in a call to
      * {@link #doGetAuthenticationInfo(org.apache.shiro.authc.AuthenticationToken) doGetAuthenticationInfo}, and the
      * resulting return value will be cached before being returned so it can be reused for later authentications.
      * <p/>
      * If you wish to clear out all associated cached data (and not just authentication data), use the
      * {@link #clearCache(org.apache.shiro.subject.PrincipalCollection)} method instead (which will in turn call this
      * method by default).
      *
      * @param principals the principals of the account for which to clear the cached AuthorizationInfo.
      * @see #clearCache(org.apache.shiro.subject.PrincipalCollection)
      * @since 1.2
      */
     protected void clearCachedAuthenticationInfo(PrincipalCollection principals) {
         if (!isEmpty(principals)) {
             Cache<Object, AuthenticationInfo> cache = getAvailableAuthenticationCache();
             //cache instance will be non-null if caching is enabled:
             if (cache != null) {
                 Object key = getAuthenticationCacheKey(principals);
                 cache.remove(key);
             }
         }
     }
 
     /**
      * Retrieves authentication data from an implementation-specific datasource (RDBMS, LDAP, etc) for the given
      * authentication token.
      * <p/>
      * For most datasources, this means just 'pulling' authentication data for an associated subject/user and nothing
      * more and letting Shiro do the rest.  But in some systems, this method could actually perform EIS specific
      * log-in logic in addition to just retrieving data - it is up to the Realm implementation.
      * <p/>
      * A {@code null} return value means that no account could be associated with the specified token.
      *
      * @param token the authentication token containing the user's principal and credentials.
      * @return an {@link AuthenticationInfo} object containing account data resulting from the
      *         authentication ONLY if the lookup is successful (i.e. account exists and is valid, etc.)
      * @throws AuthenticationException if there is an error acquiring data or performing
      *                                 realm-specific authentication logic for the specified <tt>token</tt>
      */
     protected abstract AuthenticationInfo doGetAuthenticationInfo(AuthenticationToken token) throws AuthenticationException;
 
 }