/*
    * 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.
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  package org.apache.shiro.subject;
  
  import org.apache.shiro.SecurityUtils;
  import org.apache.shiro.authc.AuthenticationException;
  import org.apache.shiro.authc.AuthenticationToken;
  import org.apache.shiro.authz.AuthorizationException;
  import org.apache.shiro.authz.Permission;
  import org.apache.shiro.mgt.SecurityManager;
  import org.apache.shiro.mgt.SubjectFactory;
  import org.apache.shiro.session.Session;
  import org.apache.shiro.subject.support.DefaultSubjectContext;
  import org.apache.shiro.util.StringUtils;
  
  import java.io.Serializable;
  import java.util.Collection;
  import java.util.List;
  import java.util.concurrent.Callable;
  
  /**
   * A {@code Subject} represents state and security operations for a <em>single</em> application user.
   * These operations include authentication (login/logout), authorization (access control), and
   * session access. It is Shiro's primary mechanism for single-user security functionality.
   * <h3>Acquiring a Subject</h3>
   * To acquire the currently-executing {@code Subject}, application developers will almost always use
   * {@code SecurityUtils}:
   * <pre>
   * {@link SecurityUtils SecurityUtils}.{@link org.apache.shiro.SecurityUtils#getSubject() getSubject()}</pre>
   * Almost all security operations should be performed with the {@code Subject} returned from this method.
   * <h3>Permission methods</h3>
   * Note that there are many *Permission methods in this interface overloaded to accept String arguments instead of
   * {@link Permission Permission} instances. They are a convenience allowing the caller to use a String representation of
   * a {@link Permission Permission} if desired.  The underlying Authorization subsystem implementations will usually
   * simply convert these String values to {@link Permission Permission} instances and then just call the corresponding
   * type-safe method.  (Shiro's default implementations do String-to-Permission conversion for these methods using
   * {@link org.apache.shiro.authz.permission.PermissionResolver PermissionResolver}s.)
   * <p/>
   * These overloaded *Permission methods forgo type-safety for the benefit of convenience and simplicity,
   * so you should choose which ones to use based on your preferences and needs.
   *
   * @since 0.1
   */
  public interface Subject {
  
      /**
       * Returns this Subject's application-wide uniquely identifying principal, or {@code null} if this
       * Subject is anonymous because it doesn't yet have any associated account data (for example,
       * if they haven't logged in).
       * <p/>
       * The term <em>principal</em> is just a fancy security term for any identifying attribute(s) of an application
       * user, such as a username, or user id, or public key, or anything else you might use in your application to
       * identify a user.
       * <h4>Uniqueness</h4>
       * Although given names and family names (first/last) are technically considered principals as well,
       * Shiro expects the object returned from this method to be an identifying attribute unique across
       * your entire application.
       * <p/>
       * This implies that things like given names and family names are usually poor
       * candidates as return values since they are rarely guaranteed to be unique;  Things often used for this value:
       * <ul>
       * <li>A {@code long} RDBMS surrogate primary key</li>
       * <li>An application-unique username</li>
       * <li>A {@link java.util.UUID UUID}</li>
       * <li>An LDAP Unique ID</li>
       * </ul>
       * or any other similar suitable unique mechanism valuable to your application.
       * <p/>
       * Most implementations will simply return
       * <code>{@link #getPrincipals()}.{@link org.apache.shiro.subject.PrincipalCollection#getPrimaryPrincipal() getPrimaryPrincipal()}</code>
       *
       * @return this Subject's application-specific unique identity.
       * @see org.apache.shiro.subject.PrincipalCollection#getPrimaryPrincipal()
       */
      Object getPrincipal();
  
      /**
       * Returns this Subject's principals (identifying attributes) in the form of a {@code PrincipalCollection} or
       * {@code null} if this Subject is anonymous because it doesn't yet have any associated account data (for example,
       * if they haven't logged in).
       * <p/>
       * The word &quot;principals&quot; is nothing more than a fancy security term for identifying attributes associated
       * with a Subject, aka, application user.  For example, user id, a surname (family/last name), given (first) name,
      * social security number, nickname, username, etc, are all examples of a principal.
      *
      * @return all of this Subject's principals (identifying attributes).
      * @see #getPrincipal()
      * @see org.apache.shiro.subject.PrincipalCollection#getPrimaryPrincipal()
      */
     PrincipalCollection getPrincipals();
 
     /**
      * Returns {@code true} if this Subject is permitted to perform an action or access a resource summarized by the
      * specified permission string.
      * <p/>
      * This is an overloaded method for the corresponding type-safe {@link Permission Permission} variant.
      * Please see the class-level JavaDoc for more information on these String-based permission methods.
      *
      * @param permission the String representation of a Permission that is being checked.
      * @return true if this Subject is permitted, false otherwise.
      * @see #isPermitted(Permission permission)
      * @since 0.9
      */
     boolean isPermitted(String permission);
 
     /**
      * Returns {@code true} if this Subject is permitted to perform an action or access a resource summarized by the
      * specified permission.
      * <p/>
      * More specifically, this method determines if any {@code Permission}s associated
      * with the subject {@link Permission#implies(Permission) imply} the specified permission.
      *
      * @param permission the permission that is being checked.
      * @return true if this Subject is permitted, false otherwise.
      */
     boolean isPermitted(Permission permission);
 
     /**
      * Checks if this Subject implies the given permission strings and returns a boolean array indicating which
      * permissions are implied.
      * <p/>
      * This is an overloaded method for the corresponding type-safe {@link Permission Permission} variant.
      * Please see the class-level JavaDoc for more information on these String-based permission methods.
      *
      * @param permissions the String representations of the Permissions that are being checked.
      * @return a boolean array where indices correspond to the index of the
      *         permissions in the given list.  A true value at an index indicates this Subject is permitted for
      *         for the associated {@code Permission} string in the list.  A false value at an index
      *         indicates otherwise.
      * @since 0.9
      */
     boolean[] isPermitted(String... permissions);
 
     /**
      * Checks if this Subject implies the given Permissions and returns a boolean array indicating which permissions
      * are implied.
      * <p/>
      * More specifically, this method should determine if each {@code Permission} in
      * the array is {@link Permission#implies(Permission) implied} by permissions
      * already associated with the subject.
      * <p/>
      * This is primarily a performance-enhancing method to help reduce the number of
      * {@link #isPermitted} invocations over the wire in client/server systems.
      *
      * @param permissions the permissions that are being checked.
      * @return a boolean array where indices correspond to the index of the
      *         permissions in the given list.  A true value at an index indicates this Subject is permitted for
      *         for the associated {@code Permission} object in the list.  A false value at an index
      *         indicates otherwise.
      */
     boolean[] isPermitted(List<Permission> permissions);
 
     /**
      * Returns {@code true} if this Subject implies all of the specified permission strings, {@code false} otherwise.
      * <p/>
      * This is an overloaded method for the corresponding type-safe {@link org.apache.shiro.authz.Permission Permission}
      * variant.  Please see the class-level JavaDoc for more information on these String-based permission methods.
      *
      * @param permissions the String representations of the Permissions that are being checked.
      * @return true if this Subject has all of the specified permissions, false otherwise.
      * @see #isPermittedAll(Collection)
      * @since 0.9
      */
     boolean isPermittedAll(String... permissions);
 
     /**
      * Returns {@code true} if this Subject implies all of the specified permissions, {@code false} otherwise.
      * <p/>
      * More specifically, this method determines if all of the given {@code Permission}s are
      * {@link Permission#implies(Permission) implied by} permissions already associated with this Subject.
      *
      * @param permissions the permissions to check.
      * @return true if this Subject has all of the specified permissions, false otherwise.
      */
     boolean isPermittedAll(Collection<Permission> permissions);
 
     /**
      * Ensures this Subject implies the specified permission String.
      * <p/>
      * If this subject's existing associated permissions do not {@link Permission#implies(Permission)} imply}
      * the given permission, an {@link org.apache.shiro.authz.AuthorizationException} will be thrown.
      * <p/>
      * This is an overloaded method for the corresponding type-safe {@link Permission Permission} variant.
      * Please see the class-level JavaDoc for more information on these String-based permission methods.
      *
      * @param permission the String representation of the Permission to check.
      * @throws org.apache.shiro.authz.AuthorizationException
      *          if the user does not have the permission.
      * @since 0.9
      */
     void checkPermission(String permission) throws AuthorizationException;
 
     /**
      * Ensures this Subject {@link Permission#implies(Permission) implies} the specified {@code Permission}.
      * <p/>
      * If this subject's existing associated permissions do not {@link Permission#implies(Permission) imply}
      * the given permission, an {@link org.apache.shiro.authz.AuthorizationException} will be thrown.
      *
      * @param permission the Permission to check.
      * @throws org.apache.shiro.authz.AuthorizationException
      *          if this Subject does not have the permission.
      */
     void checkPermission(Permission permission) throws AuthorizationException;
 
     /**
      * Ensures this Subject
      * {@link org.apache.shiro.authz.Permission#implies(org.apache.shiro.authz.Permission) implies} all of the
      * specified permission strings.
      * <p/>
      * If this subject's existing associated permissions do not
      * {@link org.apache.shiro.authz.Permission#implies(org.apache.shiro.authz.Permission) imply} all of the given permissions,
      * an {@link org.apache.shiro.authz.AuthorizationException} will be thrown.
      * <p/>
      * This is an overloaded method for the corresponding type-safe {@link Permission Permission} variant.
      * Please see the class-level JavaDoc for more information on these String-based permission methods.
      *
      * @param permissions the string representations of Permissions to check.
      * @throws AuthorizationException if this Subject does not have all of the given permissions.
      * @since 0.9
      */
     void checkPermissions(String... permissions) throws AuthorizationException;
 
     /**
      * Ensures this Subject
      * {@link org.apache.shiro.authz.Permission#implies(org.apache.shiro.authz.Permission) implies} all of the
      * specified permission strings.
      * <p/>
      * If this subject's existing associated permissions do not
      * {@link org.apache.shiro.authz.Permission#implies(org.apache.shiro.authz.Permission) imply} all of the given permissions,
      * an {@link org.apache.shiro.authz.AuthorizationException} will be thrown.
      *
      * @param permissions the Permissions to check.
      * @throws AuthorizationException if this Subject does not have all of the given permissions.
      */
     void checkPermissions(Collection<Permission> permissions) throws AuthorizationException;
 
     /**
      * Returns {@code true} if this Subject has the specified role, {@code false} otherwise.
      *
      * @param roleIdentifier the application-specific role identifier (usually a role id or role name).
      * @return {@code true} if this Subject has the specified role, {@code false} otherwise.
      */
     boolean hasRole(String roleIdentifier);
 
     /**
      * Checks if this Subject has the specified roles, returning a boolean array indicating
      * which roles are associated.
      * <p/>
      * This is primarily a performance-enhancing method to help reduce the number of
      * {@link #hasRole} invocations over the wire in client/server systems.
      *
      * @param roleIdentifiers the application-specific role identifiers to check (usually role ids or role names).
      * @return a boolean array where indices correspond to the index of the
      *         roles in the given identifiers.  A true value indicates this Subject has the
      *         role at that index.  False indicates this Subject does not have the role at that index.
      */
     boolean[] hasRoles(List<String> roleIdentifiers);
 
     /**
      * Returns {@code true} if this Subject has all of the specified roles, {@code false} otherwise.
      *
      * @param roleIdentifiers the application-specific role identifiers to check (usually role ids or role names).
      * @return true if this Subject has all the roles, false otherwise.
      */
     boolean hasAllRoles(Collection<String> roleIdentifiers);
 
     /**
      * Asserts this Subject has the specified role by returning quietly if they do or throwing an
      * {@link org.apache.shiro.authz.AuthorizationException} if they do not.
      *
      * @param roleIdentifier the application-specific role identifier (usually a role id or role name ).
      * @throws org.apache.shiro.authz.AuthorizationException
      *          if this Subject does not have the role.
      */
     void checkRole(String roleIdentifier) throws AuthorizationException;
 
     /**
      * Asserts this Subject has all of the specified roles by returning quietly if they do or throwing an
      * {@link org.apache.shiro.authz.AuthorizationException} if they do not.
      *
      * @param roleIdentifiers the application-specific role identifiers to check (usually role ids or role names).
      * @throws org.apache.shiro.authz.AuthorizationException
      *          if this Subject does not have all of the specified roles.
      */
     void checkRoles(Collection<String> roleIdentifiers) throws AuthorizationException;
 
     /**
      * Same as {@link #checkRoles(Collection<String> roleIdentifiers) checkRoles(Collection<String> roleIdentifiers)} but
      * doesn't require a collection as a an argument.
      * Asserts this Subject has all of the specified roles by returning quietly if they do or throwing an
      * {@link org.apache.shiro.authz.AuthorizationException} if they do not.
      *
      * @param roleIdentifiers roleIdentifiers the application-specific role identifiers to check (usually role ids or role names).
      * @throws AuthorizationException org.apache.shiro.authz.AuthorizationException
      *          if this Subject does not have all of the specified roles.
      * @since 1.1.0
      */
     void checkRoles(String... roleIdentifiers) throws AuthorizationException;
 
     /**
      * Performs a login attempt for this Subject/user.  If unsuccessful,
      * an {@link AuthenticationException} is thrown, the subclass of which identifies why the attempt failed.
      * If successful, the account data associated with the submitted principals/credentials will be
      * associated with this {@code Subject} and the method will return quietly.
      * <p/>
      * Upon returning quietly, this {@code Subject} instance can be considered
      * authenticated and {@link #getPrincipal() getPrincipal()} will be non-null and
      * {@link #isAuthenticated() isAuthenticated()} will be {@code true}.
      *
      * @param token the token encapsulating the subject's principals and credentials to be passed to the
      *              Authentication subsystem for verification.
      * @throws org.apache.shiro.authc.AuthenticationException
      *          if the authentication attempt fails.
      * @since 0.9
      */
     void login(AuthenticationToken token) throws AuthenticationException;
 
     /**
      * Returns {@code true} if this Subject/user proved their identity <em>during their current session</em>
      * by providing valid credentials matching those known to the system, {@code false} otherwise.
      * <p/>
      * Note that even if this Subject's identity has been remembered via 'remember me' services, this method will
      * still return {@code false} unless the user has actually logged in with proper credentials <em>during their
      * current session</em>.  See the {@link #isRemembered() isRemembered()} method JavaDoc for more.
      *
      * @return {@code true} if this Subject proved their identity during their current session
      *         by providing valid credentials matching those known to the system, {@code false} otherwise.
      * @since 0.9
      */
     boolean isAuthenticated();
 
 
     /**
      * Returns {@code true} if this {@code Subject} has an identity (it is not anonymous) and the identity
      * (aka {@link #getPrincipals() principals}) is remembered from a successful authentication during a previous
      * session.
      * <p/>
      * Although the underlying implementation determines exactly how this method functions, most implementations have
      * this method act as the logical equivalent to this code:
      * <pre>
      * {@link #getPrincipal() getPrincipal()} != null && !{@link #isAuthenticated() isAuthenticated()}</pre>
      * <p/>
      * Note as indicated by the above code example, if a {@code Subject} is remembered, they are
      * <em>NOT</em> considered authenticated.  A check against {@link #isAuthenticated() isAuthenticated()} is a more
      * strict check than that reflected by this method.  For example, a check to see if a subject can access financial
      * information should almost always depend on {@link #isAuthenticated() isAuthenticated()} to <em>guarantee</em> a
      * verified identity, and not this method.
      * <p/>
      * Once the subject is authenticated, they are no longer considered only remembered because their identity would
      * have been verified during the current session.
      * <h4>Remembered vs Authenticated</h4>
      * Authentication is the process of <em>proving</em> you are who you say you are.  When a user is only remembered,
      * the remembered identity gives the system an idea who that user probably is, but in reality, has no way of
      * absolutely <em>guaranteeing</em> if the remembered {@code Subject} represents the user currently
      * using the application.
      * <p/>
      * So although many parts of the application can still perform user-specific logic based on the remembered
      * {@link #getPrincipals() principals}, such as customized views, it should never perform highly-sensitive
      * operations until the user has legitimately verified their identity by executing a successful authentication
      * attempt.
      * <p/>
      * We see this paradigm all over the web, and we will use <a href="http://www.amazon.com">Amazon.com</a> as an
      * example:
      * <p/>
      * When you visit Amazon.com and perform a login and ask it to 'remember me', it will set a cookie with your
      * identity.  If you don't log out and your session expires, and you come back, say the next day, Amazon still knows
      * who you <em>probably</em> are: you still see all of your book and movie recommendations and similar user-specific
      * features since these are based on your (remembered) user id.
      * <p/>
      * BUT, if you try to do something sensitive, such as access your account's billing data, Amazon forces you
      * to do an actual log-in, requiring your username and password.
      * <p/>
      * This is because although amazon.com assumed your identity from 'remember me', it recognized that you were not
      * actually authenticated.  The only way to really guarantee you are who you say you are, and therefore allow you
      * access to sensitive account data, is to force you to perform an actual successful authentication.  You can
      * check this guarantee via the {@link #isAuthenticated() isAuthenticated()} method and not via this method.
      *
      * @return {@code true} if this {@code Subject}'s identity (aka {@link #getPrincipals() principals}) is
      *         remembered from a successful authentication during a previous session, {@code false} otherwise.
      * @since 1.0
      */
     boolean isRemembered();
 
     /**
      * Returns the application {@code Session} associated with this Subject.  If no session exists when this
      * method is called, a new session will be created, associated with this Subject, and then returned.
      *
      * @return the application {@code Session} associated with this Subject.
      * @see #getSession(boolean)
      * @since 0.2
      */
     Session getSession();
 
     /**
      * Returns the application {@code Session} associated with this Subject.  Based on the boolean argument,
      * this method functions as follows:
      * <ul>
      * <li>If there is already an existing session associated with this {@code Subject}, it is returned and
      * the {@code create} argument is ignored.</li>
      * <li>If no session exists and {@code create} is {@code true}, a new session will be created, associated with
      * this {@code Subject} and then returned.</li>
      * <li>If no session exists and {@code create} is {@code false}, {@code null} is returned.</li>
      * </ul>
      *
      * @param create boolean argument determining if a new session should be created or not if there is no existing session.
      * @return the application {@code Session} associated with this {@code Subject} or {@code null} based
      *         on the above described logic.
      * @since 0.2
      */
     Session getSession(boolean create);
 
     /**
      * Logs out this Subject and invalidates and/or removes any associated entities,
      * such as a {@link Session Session} and authorization data.  After this method is called, the Subject is
      * considered 'anonymous' and may continue to be used for another log-in if desired.
      * <h3>Web Environment Warning</h3>
      * Calling this method in web environments will usually remove any associated session cookie as part of
      * session invalidation.  Because cookies are part of the HTTP header, and headers can only be set before the
      * response body (html, image, etc) is sent, this method in web environments must be called before <em>any</em>
      * content has been rendered.
      * <p/>
      * The typical approach most applications use in this scenario is to redirect the user to a different
      * location (e.g. home page) immediately after calling this method.  This is an effect of the HTTP protocol
      * itself and not a reflection of Shiro's implementation.
      * <p/>
      * Non-HTTP environments may of course use a logged-out subject for login again if desired.
      */
     void logout();
 
     /**
      * Associates the specified {@code Callable} with this {@code Subject} instance and then executes it on the
      * currently running thread.  If you want to execute the {@code Callable} on a different thread, it is better to
      * use the {@link #associateWith(Callable)} method instead.
      *
      * @param callable the Callable to associate with this subject and then execute.
      * @param <V>      the type of return value the {@code Callable} will return
      * @return the resulting object returned by the {@code Callable}'s execution.
      * @throws ExecutionException if the {@code Callable}'s {@link Callable#call call} method throws an exception.
      * @since 1.0
      */
     <V> V execute(Callable<V> callable) throws ExecutionException;
 
     /**
      * Associates the specified {@code Runnable} with this {@code Subject} instance and then executes it on the
      * currently running thread.  If you want to execute the {@code Runnable} on a different thread, it is better to
      * use the {@link #associateWith(Runnable)} method instead.
      * <p/>
      * <b>Note</b>: This method is primarily provided to execute existing/legacy Runnable implementations.  It is better
      * for new code to use {@link #execute(Callable)} since that supports the ability to return values and catch
      * exceptions.
      *
      * @param runnable the {@code Runnable} to associate with this {@code Subject} and then execute.
      * @since 1.0
      */
     void execute(Runnable runnable);
 
     /**
      * Returns a {@code Callable} instance matching the given argument while additionally ensuring that it will
      * retain and execute under this Subject's identity.  The returned object can be used with an
      * {@link java.util.concurrent.ExecutorService ExecutorService} to execute as this Subject.
      * <p/>
      * This will effectively ensure that any calls to
      * {@code SecurityUtils}.{@link SecurityUtils#getSubject() getSubject()} and related functionality will continue
      * to function properly on any thread that executes the returned {@code Callable} instance.
      *
      * @param callable the callable to execute as this {@code Subject}
      * @param <V>      the {@code Callable}s return value type
      * @return a {@code Callable} that can be run as this {@code Subject}.
      * @since 1.0
      */
     <V> Callable<V> associateWith(Callable<V> callable);
 
     /**
      * Returns a {@code Runnable} instance matching the given argument while additionally ensuring that it will
      * retain and execute under this Subject's identity.  The returned object can be used with an
      * {@link java.util.concurrent.Executor Executor} or another thread to execute as this Subject.
      * <p/>
      * This will effectively ensure that any calls to
      * {@code SecurityUtils}.{@link SecurityUtils#getSubject() getSubject()} and related functionality will continue
      * to function properly on any thread that executes the returned {@code Runnable} instance.
      * <p/>
      * *Note that if you need a return value to be returned as a result of the runnable's execution or if you need to
      * react to any Exceptions, it is highly recommended to use the
      * {@link #associateWith(java.util.concurrent.Callable) createCallable} method instead of this one.
      *
      * @param runnable the runnable to execute as this {@code Subject}
      * @return a {@code Runnable} that can be run as this {@code Subject} on another thread.
      * @see #associateWith (java.util.concurrent.Callable)
      * @since 1.0
      */
     Runnable associateWith(Runnable runnable);
 
     /**
      * Allows this subject to 'run as' or 'assume' another identity indefinitely.  This can only be
      * called when the {@code Subject} instance already has an identity (i.e. they are remembered from a previous
      * log-in or they have authenticated during their current session).
      * <p/>
      * Some notes about {@code runAs}:
      * <ul>
      * <li>You can tell if a {@code Subject} is 'running as' another identity by calling the
      * {@link #isRunAs() isRunAs()} method.</li>
      * <li>If running as another identity, you can determine what the previous 'pre run as' identity
      * was by calling the {@link #getPreviousPrincipals() getPreviousPrincipals()} method.</li>
      * <li>When you want a {@code Subject} to stop running as another identity, you can return to its previous
      * 'pre run as' identity by calling the {@link #releaseRunAs() releaseRunAs()} method.</li>
      * </ul>
      *
      * @param principals the identity to 'run as', aka the identity to <em>assume</em> indefinitely.
      * @throws NullPointerException  if the specified principals collection is {@code null} or empty.
      * @throws IllegalStateException if this {@code Subject} does not yet have an identity of its own.
      * @since 1.0
      */
     void runAs(PrincipalCollection principals) throws NullPointerException, IllegalStateException;
 
     /**
      * Returns {@code true} if this {@code Subject} is 'running as' another identity other than its original one or
      * {@code false} otherwise (normal {@code Subject} state).  See the {@link #runAs runAs} method for more
      * information.
      *
      * @return {@code true} if this {@code Subject} is 'running as' another identity other than its original one or
      *         {@code false} otherwise (normal {@code Subject} state).
      * @see #runAs
      * @since 1.0
      */
     boolean isRunAs();
 
     /**
      * Returns the previous 'pre run as' identity of this {@code Subject} before assuming the current
      * {@link #runAs runAs} identity, or {@code null} if this {@code Subject} is not operating under an assumed
      * identity (normal state). See the {@link #runAs runAs} method for more information.
      *
      * @return the previous 'pre run as' identity of this {@code Subject} before assuming the current
      *         {@link #runAs runAs} identity, or {@code null} if this {@code Subject} is not operating under an assumed
      *         identity (normal state).
      * @see #runAs
      * @since 1.0
      */
     PrincipalCollection getPreviousPrincipals();
 
     /**
      * Releases the current 'run as' (assumed) identity and reverts back to the previous 'pre run as'
      * identity that existed before {@code #runAs runAs} was called.
      * <p/>
      * This method returns 'run as' (assumed) identity being released or {@code null} if this {@code Subject} is not
      * operating under an assumed identity.
      *
      * @return the 'run as' (assumed) identity being released or {@code null} if this {@code Subject} is not operating
      *         under an assumed identity.
      * @see #runAs
      * @since 1.0
      */
     PrincipalCollection releaseRunAs();
 
     /**
      * Builder design pattern implementation for creating {@link Subject} instances in a simplified way without
      * requiring knowledge of Shiro's construction techniques.
      * <p/>
      * <b>NOTE</b>: This is provided for framework development support only and should typically never be used by
      * application developers.  {@code Subject} instances should generally be acquired by using
      * <code>SecurityUtils.{@link SecurityUtils#getSubject() getSubject()}</code>
      * <h4>Usage</h4>
      * The simplest usage of this builder is to construct an anonymous, session-less {@code Subject} instance:
      * <pre>
      * Subject subject = new Subject.{@link #Builder() Builder}().{@link #buildSubject() buildSubject()};</pre>
      * The default, no-arg {@code Subject.Builder()} constructor shown above will use the application's
      * currently accessible {@code SecurityManager} via
      * <code>SecurityUtils.{@link SecurityUtils#getSecurityManager() getSecurityManager()}</code>.  You may also
      * specify the exact {@code SecurityManager} instance to be used by the additional
      * <code>Subject.{@link #Builder(org.apache.shiro.mgt.SecurityManager) Builder(securityManager)}</code>
      * constructor if desired.
      * <p/>
      * All other methods may be called before the {@link #buildSubject() buildSubject()} method to
      * provide context on how to construct the {@code Subject} instance.  For example, if you have a session id and
      * want to acquire the {@code Subject} that 'owns' that session (assuming the session exists and is not expired):
      * <pre>
      * Subject subject = new Subject.Builder().sessionId(sessionId).buildSubject();</pre>
      * <p/>
      * Similarly, if you want a {@code Subject} instance reflecting a certain identity:
      * <pre>
      * PrincipalCollection principals = new SimplePrincipalCollection("username", <em>yourRealmName</em>);
      * Subject subject = new Subject.Builder().principals(principals).build();</pre>
      * <p/>
      * <b>Note*</b> that the returned {@code Subject} instance is <b>not</b> automatically bound to the application (thread)
      * for further use.  That is,
      * {@link org.apache.shiro.SecurityUtils SecurityUtils}.{@link org.apache.shiro.SecurityUtils#getSubject() getSubject()}
      * will not automatically return the same instance as what is returned by the builder.  It is up to the framework
      * developer to bind the built {@code Subject} for continued use if desired.
      *
      * @since 1.0
      */
     public static class Builder {
 
         /**
          * Hold all contextual data via the Builder instance's method invocations to be sent to the
          * {@code SecurityManager} during the {@link #buildSubject} call.
          */
         private final SubjectContext subjectContext;
 
         /**
          * The SecurityManager to invoke during the {@link #buildSubject} call.
          */
         private final SecurityManager securityManager;
 
         /**
          * Constructs a new {@link Subject.Builder} instance, using the {@code SecurityManager} instance available
          * to the calling code as determined by a call to {@link org.apache.shiro.SecurityUtils#getSecurityManager()}
          * to build the {@code Subject} instance.
          */
         public Builder() {
             this(SecurityUtils.getSecurityManager());
         }
 
         /**
          * Constructs a new {@link Subject.Builder} instance which will use the specified {@code SecurityManager} when
          * building the {@code Subject} instance.
          *
          * @param securityManager the {@code SecurityManager} to use when building the {@code Subject} instance.
          */
         public Builder(SecurityManager securityManager) {
             if (securityManager == null) {
                 throw new NullPointerException("SecurityManager method argument cannot be null.");
             }
             this.securityManager = securityManager;
             this.subjectContext = newSubjectContextInstance();
             if (this.subjectContext == null) {
                 throw new IllegalStateException("Subject instance returned from 'newSubjectContextInstance' " +
                         "cannot be null.");
             }
             this.subjectContext.setSecurityManager(securityManager);
         }
 
         /**
          * Creates a new {@code SubjectContext} instance to be used to populate with subject contextual data that
          * will then be sent to the {@code SecurityManager} to create a new {@code Subject} instance.
          *
          * @return a new {@code SubjectContext} instance
          */
         protected SubjectContext newSubjectContextInstance() {
             return new DefaultSubjectContext();
         }
 
         /**
          * Returns the backing context used to build the {@code Subject} instance, available to subclasses
          * since the {@code context} class attribute is marked as {@code private}.
          *
          * @return the backing context used to build the {@code Subject} instance, available to subclasses.
          */
         protected SubjectContext getSubjectContext() {
             return this.subjectContext;
         }
 
         /**
          * Enables building a {@link Subject Subject} instance that owns the {@link Session Session} with the
          * specified {@code sessionId}.
          * <p/>
          * Usually when specifying a {@code sessionId}, no other {@code Builder} methods would be specified because
          * everything else (principals, inet address, etc) can usually be reconstructed based on the referenced
          * session alone.  In other words, this is almost always sufficient:
          * <pre>
          * new Subject.Builder().sessionId(sessionId).buildSubject();</pre>
          * <p/>
          * <b>Although simple in concept, this method provides very powerful functionality previously absent in almost
          * all Java environments:</b>
          * <p/>
          * The ability to reference a {@code Subject} and their server-side session
          * <em>across clients of different mediums</em> such as web applications, Java applets,
          * standalone C# clients over XML-RPC and/or SOAP, and many others. This is a <em>huge</em>
          * benefit in heterogeneous enterprise applications.
          * <p/>
          * To maintain session integrity across client mediums, the {@code sessionId} <b>must</b> be transmitted
          * to all client mediums securely (e.g. over SSL) to prevent man-in-the-middle attacks.  This
          * is nothing new - all web applications are susceptible to the same problem when transmitting
          * {@code Cookie}s or when using URL rewriting.  As long as the
          * {@code sessionId} is transmitted securely, session integrity can be maintained.
          *
          * @param sessionId the id of the session that backs the desired Subject being acquired.
          * @return this {@code Builder} instance for method chaining.
          */
         public Builder sessionId(Serializable sessionId) {
             if (sessionId != null) {
                 this.subjectContext.setSessionId(sessionId);
             }
             return this;
         }
 
         /**
          * Ensures the {@code Subject} being built will reflect the specified host name or IP as its originating
          * location.
          *
          * @param host the host name or IP address to use as the {@code Subject}'s originating location.
          * @return this {@code Builder} instance for method chaining.
          */
         public Builder host(String host) {
             if (StringUtils.hasText(host)) {
                 this.subjectContext.setHost(host);
             }
             return this;
         }
 
         /**
          * Ensures the {@code Subject} being built will use the specified {@link Session} instance.  Note that it is
          * more common to use the {@link #sessionId sessionId} builder method rather than having to construct a
          * {@code Session} instance for this method.
          *
          * @param session the session to use as the {@code Subject}'s {@link Session}
          * @return this {@code Builder} instance for method chaining.
          */
         public Builder session(Session session) {
             if (session != null) {
                 this.subjectContext.setSession(session);
             }
             return this;
         }
 
         /**
          * Ensures the {@code Subject} being built will reflect the specified principals (aka identity).
          * <p/>
          * For example, if your application's unique identifier for users is a {@code String} username, and you wanted
          * to create a {@code Subject} instance that reflected a user whose username is
          * '{@code jsmith}', and you knew the Realm that could acquire {@code jsmith}'s principals based on the username
          * was named &quot;{@code myRealm}&quot;, you might create the '{@code jsmith} {@code Subject} instance this
          * way:
          * <pre>
          * PrincipalCollection identity = new {@link org.apache.shiro.subject.SimplePrincipalCollection#SimplePrincipalCollection(Object, String) SimplePrincipalCollection}(&quot;jsmith&quot;, &quot;myRealm&quot;);
          * Subject jsmith = new Subject.Builder().principals(identity).buildSubject();</pre>
          * <p/>
          * Similarly, if your application's unique identifier for users is a {@code long} value (such as might be used
          * as a primary key in a relational database) and you were using a {@code JDBC}
          * {@code Realm} named, (unimaginatively) &quot;jdbcRealm&quot;, you might create the Subject
          * instance this way:
          * <pre>
          * long userId = //get user ID from somewhere
          * PrincipalCollection userIdentity = new {@link org.apache.shiro.subject.SimplePrincipalCollection#SimplePrincipalCollection(Object, String) SimplePrincipalCollection}(<em>userId</em>, &quot;jdbcRealm&quot;);
          * Subject user = new Subject.Builder().principals(identity).buildSubject();</pre>
          *
          * @param principals the principals to use as the {@code Subject}'s identity.
          * @return this {@code Builder} instance for method chaining.
          */
         public Builder principals(PrincipalCollection principals) {
             if (principals != null && !principals.isEmpty()) {
                 this.subjectContext.setPrincipals(principals);
             }
             return this;
         }
 
         /**
          * Configures whether or not the created Subject instance can create a new {@code Session} if one does not
          * already exist.  If set to {@code false}, any application calls to
          * {@code subject.getSession()} or {@code subject.getSession(true))} will result in a SessionException.
          * <p/>
          * This setting is {@code true} by default, as most applications find value in sessions.
          *
          * @param enabled whether or not the created Subject instance can create a new {@code Session} if one does not
          *                already exist.
          * @return this {@code Builder} instance for method chaining.
          * @since 1.2
          */
         public Builder sessionCreationEnabled(boolean enabled) {
             this.subjectContext.setSessionCreationEnabled(enabled);
             return this;
         }
 
         /**
          * Ensures the {@code Subject} being built will be considered
          * {@link org.apache.shiro.subject.Subject#isAuthenticated() authenticated}.  Per the
          * {@link org.apache.shiro.subject.Subject#isAuthenticated() isAuthenticated()} JavaDoc, be careful
          * when specifying {@code true} - you should know what you are doing and have a good reason for ignoring Shiro's
          * default authentication state mechanisms.
          *
          * @param authenticated whether or not the built {@code Subject} will be considered authenticated.
          * @return this {@code Builder} instance for method chaining.
          * @see org.apache.shiro.subject.Subject#isAuthenticated()
          */
         public Builder authenticated(boolean authenticated) {
             this.subjectContext.setAuthenticated(authenticated);
             return this;
         }
 
         /**
          * Allows custom attributes to be added to the underlying context {@code Map} used to construct the
          * {@link Subject} instance.
          * <p/>
          * A {@code null} key throws an {@link IllegalArgumentException}. A {@code null} value effectively removes
          * any previously stored attribute under the given key from the context map.
          * <p/>
          * <b>*NOTE*:</b> This method is only useful when configuring Shiro with a custom {@link SubjectFactory}
          * implementation.  This method allows end-users to append additional data to the context map which the
          * {@code SubjectFactory} implementation can use when building custom Subject instances. As such, this method
          * is only useful when a custom {@code SubjectFactory} implementation has been configured.
          *
          * @param attributeKey   the key under which the corresponding value will be stored in the context {@code Map}.
          * @param attributeValue the value to store in the context map under the specified {@code attributeKey}.
          * @return this {@code Builder} instance for method chaining.
          * @throws IllegalArgumentException if the {@code attributeKey} is {@code null}.
          * @see SubjectFactory#createSubject(SubjectContext)
          */
         public Builder contextAttribute(String attributeKey, Object attributeValue) {
             if (attributeKey == null) {
                 String msg = "Subject context map key cannot be null.";
                 throw new IllegalArgumentException(msg);
             }
             if (attributeValue == null) {
                 this.subjectContext.remove(attributeKey);
             } else {
                 this.subjectContext.put(attributeKey, attributeValue);
             }
             return this;
         }
 
         /**
          * Creates and returns a new {@code Subject} instance reflecting the cumulative state acquired by the
          * other methods in this class.
          * <p/>
          * This {@code Builder} instance will still retain the underlying state after this method is called - it
          * will not clear it; repeated calls to this method will return multiple {@link Subject} instances, all
          * reflecting the exact same state.  If a new (different) {@code Subject} is to be constructed, a new
          * {@code Builder} instance must be created.
          * <p/>
          * <b>Note</b> that the returned {@code Subject} instance is <b>not</b> automatically bound to the application
          * (thread) for further use.  That is,
          * {@link org.apache.shiro.SecurityUtils SecurityUtils}.{@link org.apache.shiro.SecurityUtils#getSubject() getSubject()}
          * will not automatically return the same instance as what is returned by the builder.  It is up to the
          * framework developer to bind the returned {@code Subject} for continued use if desired.
          *
          * @return a new {@code Subject} instance reflecting the cumulative state acquired by the
          *         other methods in this class.
          */
         public Subject buildSubject() {
             return this.securityManager.createSubject(this.subjectContext);
         }
     }
 
 }