Session.Abandon和Session.Clear的实现和区别

来源：互联网发布：开店是淘宝还是天猫好编辑：程序博客网时间：2024/05/23 00:10

我在网上找了一个比较贴切的描述：

Session.Clear()就是把Session对象中的所有项目都删除了，Session对象里面啥都没有。但是Session对象还保留。Session.Abandon()就是把当前Session对象删除了，下一次就是新的Session了。(下一次新会话开始貌似并不会产生新的SESSIONID)主要的不同之处在于当使用Session.Abandon时，会调用Session_End方法（InProc模式下）。当下一个请求到来时将激发Session_Start方法。而Session.Clear只是清除Session中的所有数据并不会中止该Session，因此也不会调用那些方法。其中标红也不是很正确。

首先我们来看一下code：

Dictionary<string, HttpSessionState> dict = new Dictionary<string, HttpSessionState>();
// Abandon
dict.Remove(key);
//Clear()
dict[key].Clear();

假设session存储一个字典结构，key就是我们客户端的sessionID，value 就是我们的SessionSate（它也是一个集合），调用Abandon后就把key删除了，但是clear是value里面的内容清空了。这2个方法的实现大家可以参考HttpSessionStateContainer的实现，Clear 清空集合，Abandon设置变量使其属性IsAbandoned为true。

真正的核心code在SessionStateModule类的OnReleaseState方法里面，在有Session的Http请求中，有一个地方加载我们的session，也有一个地方保存我们的session。

  /// <devdoc>        ///    <para>[To be supplied.]</para>        /// </devdoc>        void OnReleaseState(Object source, EventArgs eventArgs) {            HttpApplication             app;            HttpContext                 context;            bool                        setItemCalled = false;            Debug.Trace("SessionStateOnReleaseState", "Beginning SessionStateModule::OnReleaseState");            Debug.Assert(!(_rqAddedCookie && !_rqIsNewSession),                "If session id was added to the cookie, it must be a new session.");            // !!!            // Please note that due to InProc session id optimization, this function should not            // use _rqId directly because it can still be null.  Instead, use DelayedGetSessionId().            _releaseCalled = true;            app = (HttpApplication)source;            context = app.Context;            ChangeImpersonation(context, false);            try {                if (_rqSessionState != null) {                    bool delayedSessionState = (_rqSessionState == s_delayedSessionState);                    Debug.Trace("SessionStateOnReleaseState", "Remove session state from context");                    SessionStateUtility.RemoveHttpSessionStateFromContext(_rqContext, delayedSessionState);                    /*                     * Don't store untouched new sessions.                     */                    if (                                // The store doesn't have the session state.                                // ( Please note we aren't checking _rqIsNewSession because _rqIsNewSession                                // is lalso true if the item is converted from temp to perm in a GetItemXXX() call.)                                _rqSessionStateNotFound                                // OnStart is not defined                               && _sessionStartEventHandler == null                               // Nothing has been stored in session state                               && (delayedSessionState || !_rqSessionItems.Dirty)                               && (delayedSessionState || _rqStaticObjects == null || _rqStaticObjects.NeverAccessed)                        ) {                        Debug.Trace("SessionStateOnReleaseState", "Not storing unused new session.");                    }                    else if (_rqSessionState.IsAbandoned) {                        Debug.Trace("SessionStateOnReleaseState", "Removing session due to abandonment, SessionId=" + _rqId);                        if (_rqSessionStateNotFound) {                            // The store provider doesn't have it, and so we don't need to remove it from the store.                            // However, if the store provider supports session expiry, and we have a Session_End in global.asax,                            // we need to explicitly call Session_End.                            if (_supportSessionExpiry) {                                if (delayedSessionState) {                                    Debug.Assert(s_allowDelayedStateStoreItemCreation, "s_allowDelayedStateStoreItemCreation");                                    Debug.Assert(_rqItem == null, "_rqItem == null");                                    InitStateStoreItem(false /*addToContext*/);                                }                                _onEndTarget.RaiseSessionOnEnd(ReleaseStateGetSessionID(), _rqItem);                            }                        }                        else {                            Debug.Assert(_rqItem != null, "_rqItem cannot null if it's not a new session");                            // Remove it from the store because the session is abandoned.                            _store.RemoveItem(_rqContext, ReleaseStateGetSessionID(), _rqLockId, _rqItem);                        }                    }                    else if (!_rqReadonly ||                             (_rqReadonly &&                              _rqIsNewSession &&                              _sessionStartEventHandler != null &&                              !SessionIDManagerUseCookieless)) {                        // We need to save it since it isn't read-only                        // See Dev10 588711: Issuing a redirect from inside of Session_Start event                         // triggers an infinite loop when using pages with read-only session state                        // We save it only if there is no error, and if something has changed (unless it's a new session)                        if (    context.Error == null   // no error                                && (    _rqSessionStateNotFound                                    || _rqSessionItems.Dirty    // SessionItems has changed.                                    || (_rqStaticObjects != null && !_rqStaticObjects.NeverAccessed) // Static objects have been accessed                                    || _rqItem.Timeout != _rqSessionState.Timeout   // Timeout value has changed                                    )                            ) {                            if (delayedSessionState) {                                Debug.Assert(_rqIsNewSession, "Saving a session and delayedSessionState is true: _rqIsNewSession must be true");                                Debug.Assert(s_allowDelayedStateStoreItemCreation, "Saving a session and delayedSessionState is true: s_allowDelayedStateStoreItemCreation");                                Debug.Assert(_rqItem == null, "Saving a session and delayedSessionState is true: _rqItem == null");                                InitStateStoreItem(false /*addToContext*/);                            }#if DBG                            if (_rqSessionItems.Dirty) {                                Debug.Trace("SessionStateOnReleaseState", "Setting new session due to dirty SessionItems, SessionId=" + _rqId);                            }                            else if (_rqStaticObjects != null && !_rqStaticObjects.NeverAccessed) {                                Debug.Trace("SessionStateOnReleaseState", "Setting new session due to accessed Static Objects, SessionId=" + _rqId);                            }                            else if (_rqSessionStateNotFound) {                                Debug.Trace("SessionStateOnReleaseState", "Setting new session because it's not found, SessionId=" + _rqId);                            }                            else {                                Debug.Trace("SessionStateOnReleaseState", "Setting new session due to options change, SessionId=" + _rqId +                                            "\n\t_rq.timeout=" + _rqItem.Timeout.ToString(CultureInfo.InvariantCulture) +                                            ", _rqSessionState.timeout=" + _rqSessionState.Timeout.ToString(CultureInfo.InvariantCulture));                            }#endif                            if (_rqItem.Timeout != _rqSessionState.Timeout) {                                _rqItem.Timeout = _rqSessionState.Timeout;                            }                            s_sessionEverSet = true;                            setItemCalled = true;                            _store.SetAndReleaseItemExclusive(_rqContext, ReleaseStateGetSessionID(), _rqItem, _rqLockId, _rqSessionStateNotFound);                        }                        else {                            // Can't save it because of various reason.  Just release our exclusive lock on it.                            Debug.Trace("SessionStateOnReleaseState", "Release exclusive lock on session, SessionId=" + _rqId);                            if (!_rqSessionStateNotFound) {                                Debug.Assert(_rqItem != null, "_rqItem cannot null if it's not a new session");                                _store.ReleaseItemExclusive(_rqContext, ReleaseStateGetSessionID(), _rqLockId);                            }                        }                    }#if DBG                    else {                        Debug.Trace("SessionStateOnReleaseState", "Session is read-only, ignoring SessionId=" + _rqId);                    }#endif                    Debug.Trace("SessionStateOnReleaseState", "Returning from SessionStateModule::OnReleaseState");                }                if (_rqAddedCookie && !setItemCalled && context.Response.IsBuffered()) {                    _idManager.RemoveSessionID(_rqContext);                }            }            finally {                RestoreImpersonation();            }            // WOS 1679798: PERF: Session State Module should disable EndRequest on successful cleanup            bool implementsIRequiresSessionState = context.RequiresSessionState;            if (HttpRuntime.UseIntegratedPipeline                 && (context.NotificationContext.CurrentNotification == RequestNotification.ReleaseRequestState)                 && (s_canSkipEndRequestCall || !implementsIRequiresSessionState)) {                context.DisableNotifications(RequestNotification.EndRequest, 0 /*postNotifications*/);                _acquireCalled = false;                _releaseCalled = false;                ResetPerRequestFields();            }        }

其中主要的code是：

 else if (_rqSessionState.IsAbandoned) {                        Debug.Trace("SessionStateOnReleaseState", "Removing session due to abandonment, SessionId=" + _rqId);                        if (_rqSessionStateNotFound) {                            // The store provider doesn't have it, and so we don't need to remove it from the store.                            // However, if the store provider supports session expiry, and we have a Session_End in global.asax,                            // we need to explicitly call Session_End.                            if (_supportSessionExpiry) {                                if (delayedSessionState) {                                    Debug.Assert(s_allowDelayedStateStoreItemCreation, "s_allowDelayedStateStoreItemCreation");                                    Debug.Assert(_rqItem == null, "_rqItem == null");                                    InitStateStoreItem(false /*addToContext*/);                                }                                _onEndTarget.RaiseSessionOnEnd(ReleaseStateGetSessionID(), _rqItem);                            }                        }                        else {                            Debug.Assert(_rqItem != null, "_rqItem cannot null if it's not a new session");                            // Remove it from the store because the session is abandoned.                            _store.RemoveItem(_rqContext, ReleaseStateGetSessionID(), _rqLockId, _rqItem);                        }                    }

}

if (_rqAddedCookie && !setItemCalled && context.Response.IsBuffered()) {
_idManager.RemoveSessionID(_rqContext);
}

_rqAddedCookie 一般情况下为false，所以SessionID 默认也就没有被移除。也就是调用Abandon方法默认Session ID是不会改变的。 _onEndTarget.RaiseSessionOnEnd(ReleaseStateGetSessionID(), _rqItem);就是调用我们的Session_End方法。看到这里大家应该知道什么情况调用Session_End，什么有调用 _store.RemoveItem(_rqContext, ReleaseStateGetSessionID(), _rqLockId, _rqItem); 还有后面 _store.SetAndReleaseItemExclusive(_rqContext, ReleaseStateGetSessionID(), _rqItem, _rqLockId, _rqSessionStateNotFound);这个方法也很重要哦，就是保存我们的Session大家可以参考 asp.net mvc Session RedisSessionStateProvider锁的实现

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