SessionFactory是线程安全的,Session是非线程安全的，怎么正确理解

public final class SessionFactoryImpl implements SessionFactory, SessionFactoryImplementor {    private static final Logger log = LoggerFactory.getLogger(SessionFactoryImpl.class);    private static final IdentifierGenerator UUID_GENERATOR = new UUIDHexGenerator();    private final String name;    private final String uuid;    private final transient Map entityPersisters;    private final transient Map classMetadata;    private final transient Map collectionPersisters;    private final transient Map collectionMetadata;    private final transient Map collectionRolesByEntityParticipant;    private final transient Map identifierGenerators;    private final transient Map namedQueries;    private final transient Map namedSqlQueries;    private final transient Map sqlResultSetMappings;    private final transient Map filters;    private final transient Map imports;    private final transient Interceptor interceptor;    private final transient Settings settings;    private final transient Properties properties;    private transient SchemaExport schemaExport;    private final transient TransactionManager transactionManager;    private final transient QueryCache queryCache;    private final transient UpdateTimestampsCache updateTimestampsCache;    private final transient Map queryCaches;    private final transient Map allCacheRegions = new HashMap();    private final transient StatisticsImpl statistics = new StatisticsImpl(this);    private final transient EventListeners eventListeners;    private final transient CurrentSessionContext currentSessionContext;    private final transient EntityNotFoundDelegate entityNotFoundDelegate;    private final transient SQLFunctionRegistry sqlFunctionRegistry;    private final transient SessionFactoryObserver observer;    private final transient HashMap entityNameResolvers = new HashMap();    private final QueryPlanCache queryPlanCache = new QueryPlanCache( this );    private transient boolean isClosed = false;

public final class SessionImpl extends AbstractSessionImpl        implements EventSource, org.hibernate.classic.Session, JDBCContext.Context {    // todo : need to find a clean way to handle the "event source" role    // a seperate classs responsible for generating/dispatching events just duplicates most of the Session methods...    // passing around seperate reto interceptor, factory, actionQueue, and persistentContext is not manageable...    private static final Logger log = LoggerFactory.getLogger(SessionImpl.class);    private transient EntityMode entityMode = EntityMode.POJO;    private transient boolean autoClear; //for EJB3        private transient long timestamp;    private transient FlushMode flushMode = FlushMode.AUTO;    private transient CacheMode cacheMode = CacheMode.NORMAL;    private transient Interceptor interceptor;    private transient int dontFlushFromFind = 0;    private transient ActionQueue actionQueue;    private transient StatefulPersistenceContext persistenceContext;    private transient JDBCContext jdbcContext;    private transient EventListeners listeners;    private transient boolean flushBeforeCompletionEnabled;    private transient boolean autoCloseSessionEnabled;    private transient ConnectionReleaseMode connectionReleaseMode;        private transient String fetchProfile;    private transient Map enabledFilters = new HashMap();    private transient Session rootSession;    private transient Map childSessionsByEntityMode;    private EntityNameResolver entityNameResolver = new CoordinatingEntityNameResolver();