ASP.NET Cache缓存的使用
ASP.NET Cache是提升系统性能的重要方法，它使用了“最近使用”原则（a least-recently-used algorithm）。在数据库访问中经常会用到Cache保存数据库数据。
1.缓存的添加：
Cache的添加方法有Add()或Insert（），两种方法几乎类似，只是Inser方法可以使用可选参数，即使用默认参数，来实现缓存的添加：
Cache.Add(
KeyName,//缓存名
KeyValue,//要缓存的对象
Dependencies,//依赖项
AbsoluteExpiration,//绝对过期时间
SlidingExpiration,//相对过期时间
Priority,//优先级
CacheItemRemovedCallback);//缓存过期引发事件
2. 缓存依赖项：
缓存可以设置的时效性可以通过 文件依赖，其他缓存依赖，数据库依赖和过期时间方法来设置，当文件改变，依赖缓存项改变，数据库改变或时间的到期时，缓存会失效，并可以引发一定事件。
2.1 文件依赖：
CacheDependency fileDepends = new CacheDependency(Server.MapPath("Northwind.xml"));
Cache.Insert("GridViewDataSet", dsGrid, fileDepends);
此例为通过Northiwind.xml文件依赖出来缓存的用法：
2.2 其他缓存项依赖：
string[] fileDependsArray = {Server.MapPath("Northwind.xml")};
string[] cacheDependsArray = {"Depend0", "Depend1", "Depend2"};
CacheDependency cacheDepends = new CacheDependency(fileDependsArray, cacheDependsArray);
Cache.Insert("GridViewDataSet", dsGrid, cacheDepends);
此例设置了Northwind.xml文件依赖和 Depend0,depend1,Depend2缓存项
其中Depend0,depend1,Depend2为另外三个缓存。
如果不需要文件依赖可以设置为NULL。
2.3 过期时间设定：
AbsoluteExpiration可以设置缓存的绝对过期时间，如：
Cache.Insert("GridViewDataSet ", dsGrid, null, DateTime.Now.AddMinutes(30), Cache.NoSlidingExpiration);
缓存会在添加起30分钟后过期。
NoSlidingExpiration可以设置相对过期时间,如果缓存在NoSlidingExpiration设定的时间内没有被访问，缓存过期，如果在这段时间内有访问，则缓存过期时间将会重置为原始值，如NoSlidingExpiration=20
在20分钟内如果没有被访问，缓存过期，如果每次19分钟访问缓存，缓存将永远不会过期。
Cache.Insert("DataGridDataSet", dsGrid, null,Cache.NoAbsoluteExpiration, TimeSpan.FromSeconds(30));
3. 优先级：
Priority属性值和意义：
| Priority value |
Description |
| NotRemovable |
Items with this priority will not be evicted. |
| High |
Items with this priority level are the least likely to be evicted. |
| AboveNormal |
Items with this priority level are less likely to be evicted than items assigned Normal priority. |
| Default |
This is equivalent to Normal. |
| Normal |
The default value. |
| BelowNormal |
Items with this priority level are more likely to be evicted than items assigned Normal priority. |
| Low |
Items with this priority level are the most likely to be evicted. |
4. 缓存失效事件处理：
using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Web.Caching; // necessary for CacheDependency
using System.Xml; // necessary for Xml stuff
public partial class _Default : System.Web.UI.Page
{
public static CacheItemRemovedCallback onRemove = null;
protected void Page_Load(object sender, EventArgs e)
{
CreateGridView( );
}
private void CreateGridView( )
{
DataSet dsGrid;
dsGrid = (DataSet)Cache["GridViewDataSet"];
onRemove = new CacheItemRemovedCallback(this.RemovedCallback);
if (dsGrid == null)
{
dsGrid = GetDataSet( );
string[] fileDependsArray = {Server.MapPath("Northwind.xml")};
string[] cacheDependsArray = {"Depend0", "Depend1", "Depend2"};
CacheDependency cacheDepends = new CacheDependency
(fileDependsArray, cacheDependsArray);
Cache.Insert("GridViewDataSet", dsGrid, cacheDepends,
DateTime.Now.AddSeconds(10),
Cache.NoSlidingExpiration,
CacheItemPriority.Default,
onRemove);
lblMessage.Text = "Data from XML file.";
}
else
{
lblMessage.Text = "Data from cache.";
}
gv.DataSource = dsGrid.Tables[0];
gv.DataBind( );
}
private DataSet GetDataSet( )
{
DataSet dsData = new DataSet( );
XmlDataDocument doc = new XmlDataDocument( );
doc.DataSet.ReadXml(Server.MapPath("Northwind.xml"));
dsData = doc.DataSet;
return dsData;
}
public void RemovedCallback(string cacheKey,
Object cacheObject,
CacheItemRemovedReason reasonToRemove)
{
WriteFile("Cache removed for following reason: " +
reasonToRemove.ToString( ));
}
private void WriteFile(string strText)
{
System.IO.StreamWriter writer = new System.IO.StreamWriter(
@"C:"test.txt", true);
string str;
str = DateTime.Now.ToString( ) + " " + strText;
writer.WriteLine(str);
writer.Close( );
}
protected void btnClear_Click(object sender, EventArgs e)
{
Cache.Remove("GridViewDataSet");
CreateGridView( );
}
protected void btnInit_Click(object sender, EventArgs e)
{
// Initialize caches to depend on.
Cache["Depend0"] = "This is the first dependency.";
Cache["Depend1"] = "This is the 2nd dependency.";
Cache["Depend2"] = "This is the 3rd dependency.";
}
protected void btnKey0_Click(object sender, EventArgs e)
{
Cache["Depend0"] = "This is a changed first dependency.";
}
}
| Table 17-5. Members of the CacheItemRemovedReason enumeration |
|
| Reason |
Description |
| DependencyChanged |
A file or item key dependency has changed. |
| Expired |
The cached item has expired. |
| Removed |
The cached item has been explicitly removed by the Remove method or replaced by another item with the same key. |
| Underused |
The cached item was removed to free up system memory. |