在.NET 4.0中使用MEF来节省我大量的抽象工厂代码和配置gubbins。由于未部署,因此无法移至.net 4.5。MEF实例和多线程
类
/// <summary>
/// Factory relies upon the use of the .net 4.0 MEF framework
/// All processors need to export themselves to make themselves visible to the 'Processors' import property auto MEF population
/// This class is implemented as a singleton
/// </summary>
public class MessageProsessorFactory
{
private static readonly string pluginFilenameFilter = "Connectors.*.dll";
private static CompositionContainer _container;
private static MessageProsessorFactory _instance;
private static object MessageProsessorFactoryLock = new object();
/// <summary>
/// Initializes the <see cref="MessageProsessorFactory" /> class.
/// Loads all MEF imports
/// </summary>
/// <exception cref="System.NotSupportedException"></exception>
private MessageProsessorFactory()
{
lock (MessageProsessorFactoryLock)
{
if (_container == null)
{
RemoveDllSecurityZoneRestrictions();
//Create a thread safe composition container
_container = new CompositionContainer(new DirectoryCatalog(".", pluginFilenameFilter), true, null);
_container.ComposeParts(this);
}
}
}
/// <summary>
/// A list of detected class instances that support IMessageProcessor
/// </summary>
[ImportMany(typeof(IMessageProcessor), RequiredCreationPolicy = CreationPolicy.NonShared)]
private List<Lazy<IMessageProcessor, IMessageProccessorExportMetadata>> Processors { get; set; }
/// <summary>
/// Gets the message factory.
/// </summary>
/// <param name="messageEnvelope">The message envelope.</param>
/// <returns><see cref="IMessageProcessor"/></returns>
/// <exception cref="System.NotSupportedException">The supplied target is not supported: + target</exception>
public static IMessageProcessor GetMessageProcessor(MessageEnvelope messageEnvelope)
{
if (_instance == null)
_instance = new MessageProsessorFactory();
var p = _instance.Processors.FirstOrDefault(
s => s.Metadata.ExpectedType.AssemblyQualifiedName == messageEnvelope.AssemblyQualifiedName);
if (p == null)
throw new NotSupportedException(
"The supplied type is not supported: " + messageEnvelope.AssemblyQualifiedName);
return p.Value;
}
/// <summary>
/// Removes any zone flags otherwise MEF wont load files with
/// a URL zone flag set to anything other than 'MyComputer', we are trusting all pluggins here.
/// http://msdn.microsoft.com/en-us/library/ms537183(v=vs.85).aspx
/// </summary>
private static void RemoveDllSecurityZoneRestrictions()
{
string path = System.IO.Path.GetDirectoryName(
System.Reflection.Assembly.GetExecutingAssembly().Location);
foreach (var filePath in Directory.EnumerateFiles(path, pluginFilenameFilter))
{
var zone = Zone.CreateFromUrl(filePath);
if (zone.SecurityZone != SecurityZone.MyComputer)
{
var fileInfo = new FileInfo(filePath);
fileInfo.DeleteAlternateDataStream("Zone.Identifier");
}
}
}
}
_container.ComposeParts(this);
之后被调用时,处理器会填充找到的所有IMessageProcessor实现。大。
注意
- GetMessageProcessor是由许多线程调用。
- 我们无法控制开发人员如何构造IMessageProcessor的实现类 ,因此我们无法保证 是线程安全的 - 可重入。但是,该类必须使用“导出”属性进行检测。
出口属性
[MetadataAttribute]
[AttributeUsage(AttributeTargets.Class, AllowMultiple = false)]
public class MessageProccessorExportAttribute : ExportAttribute
{
public MessageProccessorExportAttribute()
: base(typeof(IMessageProcessor))
{
}
public Type ExpectedType { get; set; }
}
- ExpectedType是什么笔记 IMessageProcessor.ProcessMessage()预计将处理,纯粹 实现细节只是元数据。
我的问题是,我到处读到每个导入的实例都是Singleton,而不管它的激活策略是通过Lazy <>引用构造的。
因此,我们不能允许从GetMessageProcessor返回MEF中的实例,因为多个线程将获得相同的实例,这是不可取的。唉唉! 我想知道下面的“解决方法”是否是最好的方法,或者我有MEF坚持概念错误。
我的解决方法是将看似毫无意义的RequiredCreationPolicy = CreationPolicy.NonShared
属性设置更改为CreationPolicy.Shared
。
然后改变函数GetMessageProcessor手动创建一个新的实例,真正从MEF中分离出来。使用MEF共享实例prulry作为类型列表。
IMessageProcessor newInstance = (IMessageProcessor)Activator.CreateInstance(p.Value.GetType());
完整的方法;
public static IMessageProcessor GetMessageProcessor(MessageEnvelope messageEnvelope)
{
if (_instance == null)
_instance = new MessageProsessorFactory();
var p = _instance.Processors.FirstOrDefault(
s => s.Metadata.ExpectedType.AssemblyQualifiedName == messageEnvelope.AssemblyQualifiedName);
if (p == null)
throw new NotSupportedException(
"The supplied type is not supported: " + messageEnvelope.AssemblyQualifiedName);
// we need to create a new instance from the singleton instance provided by MEF to assure we get a instance un-associated with the MEF container for
// currently as of .net 4.0 it wants to keep references on objects which may impact memory consumption.
// As we have no control over how a developer will organise there class that exposes an Export,
// this could lead to multithreading issues as an imported lazy instance is a singleton regardless
// of the RequiredCreationPolicy.
// MEF is still invaluable in avoided a tone of abstract factory code and its dynamic detection of all supporting
// Exports conforming to IMessageProcessor means there is no factory config for us to maintain.
IMessageProcessor newInstance = (IMessageProcessor)Activator.CreateInstance(p.Value.GetType());
return newInstance;
}
不会这种方法失败,如果一个'IMessageProcessor'实例都有一个'ImportingConstructor'而不是默认的构造函数?此外,它不会满足新实例的任何导入。 –