早在年前的时候就已经在CSAI博客发表了上一篇文章:《仓储的实现:基础篇》。苦于日夜奔波于工作与生活之间,一直没有能够抽空继续探讨仓储的实现细节,也让很多关注EntityFramework和领域驱动设计的朋友们备感失望。
闲话不多说,现在继续考虑,如何让仓储的操作在相同的事物处理上下文中进行。DDD引入仓储模式,其目的之一就是能够通过仓储隐藏对象持久化的技术细节,使得领域模型变得更为“纯净”。由此可见,仓储的实现是需要基础结构层的组件支持的,表现为对数据库的操作。在传统的关系型数据库操作中,事务处理是一个很重要的概念,虽然从目前某些大型项目看,事务处理会降低效率,但它保证了数据的完整性。关系型数据库仍然是目前数据持久化机制的主流,事务处理的实现还是很有必要的。
为了迎合仓储模式,就需要对经典的ObjectContext使用方式作一些调整。比如,原本我们可以非常简单地使用using (EntitiesContainer ec = new EntitiesContainer())语句来界定LINQ to Entities的操作范围,并使用ObjectContext的SaveChanges成员方法提交事务,而在引入了仓储的实现中,就不能继续采用这种经典的使用方式。这让EntityFramework看上去变得很奇怪,也很牵强,我相信很多网友会批评我的做法,因为我把问题复杂化了。
其实,这应该是关注点不同罢了。关注EntityFramework的开发人员,自然觉得经典的调用方式简单明了,而从DDD的角度看呢?只能把关注点放在仓储上,而把EntityFramework当成是仓储的一种技术选型(当然从DDD角度讲,我们完全可以不选择EntityFramework,而去选择其它技术)。所以本文暂且抛开EntityFramework,继续在上文的基础上,讨论仓储的实现。
前面提到,仓储的实现需要考虑事务处理,而且根据DDD的经验,针对每一个聚合根,都需要有个仓储对其进行持久化以及对象重新组装等操作。为此,我的想法是,将仓储操作“界定”在某一个事务处理上下文(Context)中,仓储的实例是由Context获得的,这有点像EntityFramework中ObjectContext与EntityObject的关系那样。由于仓储是来自于transaction context,所以它知道目前处于哪个事务上下文中。我定义的这个transaction context如下:
public interface IRepositoryTransactionContext : IDisposable
{
IRepository<TEntity> GetRepository<TEntity>()
where TEntity : EntityObject, IAggregateRoot;
void BeginTransaction();
void Commit();
void Rollback();
}
上面第三行代码定义了一个接口方法,这个方法的主要作用就是返回一个针对指定聚合根实体的仓储实例。剩下那三行代码就很明显了,那是标准的transaction操作:启动事务、提交事务以及回滚事务。
在设计上,可以根据需要,选择合适的技术来实现IRepositoryTransactionContext。我们现在讨论的是EntityFramework,所以我将给出EntityFramework的具体实现。当然,如果你不选用EntityFramework,而是用NHibernate实现数据持久化,这样的设计同样能够使你达到目的。以下是基于EntityFramework的实现:EdmRepositoryTransactionContext的伪代码。
internal class EdmRepositoryTransactionContext : IRepositoryTransactionContext
{
private ObjectContext objContext;
private Dictionary<Type, object> repositoryCache = new Dictionary<Type, object>();
public EdmRepositoryTransactionContext(ObjectContext objContext)
{
this.objContext = objContext;
}
#region IRepositoryTransactionContext Members
public IRepository<TEntity> GetRepository<TEntity>() where TEntity : EntityObject, IAggregateRoot
{
if (repositoryCache.ContainsKey(typeof(TEntity)))
{
return (IRepository<TEntity>)repositoryCache[typeof(TEntity)];
}
IRepository<TEntity> repository = new EdmRepository<TEntity>(this.objContext);
this.repositoryCache.Add(typeof(TEntity), repository);
return repository;
}
public void BeginTransaction()
{
// We do not need to begin a transaction here because the object context,
// which would handle the transaction, was created and injected into the
// constructor by Castle Windsor framework.
}
public void Commit()
{
this.objContext.SaveChanges();
}
public void Rollback()
{
// We also do not need to perform the rollback operation because
// entity framework will handle this for us, just when the execution
// point is stepping out of the using scope.
}
#endregion
#region IDisposable Members
public void Dispose()
{
this.repositoryCache.Clear();
this.objContext.Dispose();
}
#endregion
}
EdmRepositoryTransactionContext被定义为internal,这个设计是合理的,因为Domain层是不需要知道事务上下文的具体实现,它将会被IoC/DI容器注入到Domain层中(本系列文章采用Castle Windsor框架)。在EdmRepositoryTransactionContext的构造函数中,它需要EntityFramework的ObjectContext对象来初始化实例。同样,由于IoC/DI的使用,我们在代码中也是不需要去创建这个ObjectContext的,交给Castle Windsor就OK了。第13行的GetRepository方法简单地采用了Dictionary对象来实现缓存仓储实例的效果,当然这种做法还有待改进。
EdmRepositoryTransactionContext是不需要BeginTransaction的,我们将方法置空,因为EntityFramework的事务会由ObjectContext来管理,同理,Rollback也被置空。
EdmRepository的实现就比较显而易见了,请参见上文。
此外,我们还可以针对NHibernate实现仓储模式,只需要实现IRepositoryTransactionContext和IRepository接口即可,比如:
internal class NHibernateRepositoryTransactionContext : IRepositoryTransactionContext
{
ITransaction transaction;
Dictionary<Type, object> repositoryCache = new Dictionary<Type, object>();
public ISession Session { get { return DatabaseSessionFactory.Instance.Session; } }
#region IRepositoryTransactionContext Members
public IRepository<TEntity> GetRepository<TEntity>()
where TEntity : EntityObject, IAggregateRoot
{
if (repositoryCache.ContainsKey(typeof(TEntity)))
{
return (IRepository<TEntity>)repositoryCache[typeof(TEntity)];
}
IRepository<TEntity> repository = new NHibernateRepository<TEntity>(this.Session);
this.repositoryCache.Add(typeof(TEntity), repository);
return repository;
}
public void BeginTransaction()
{
transaction = DatabaseSessionFactory.Instance.Session.BeginTransaction();
}
public void Commit()
{
transaction.Commit();
}
public void Rollback()
{
transaction.Rollback();
}
#endregion
#region IDisposable Members
public void Dispose()
{
transaction.Dispose();
ISession dbSession = DatabaseSessionFactory.Instance.Session;
if (dbSession != null && dbSession.IsOpen)
dbSession.Close();
}
#endregion
}
internal class NHibernateRepository<TEntity> : IRepository<TEntity>
where TEntity : EntityObject, IAggregateRoot
{
ISession session;
public NHibernateRepository(ISession session)
{
this.session = session;
}
#region IRepository<TEntity> Members
public void Add(TEntity entity)
{
this.session.Save(entity);
}
public TEntity GetByKey(int id)
{
return (TEntity)this.session.Get(typeof(TEntity), id);
}
public IEnumerable<TEntity> FindBySpecification(Func<TEntity, bool> spec)
{
throw new NotImplementedException();
}
public void Remove(TEntity entity)
{
this.session.Delete(entity);
}
public void Update(TEntity entity)
{
this.session.SaveOrUpdate(entity);
}
#endregion
}
在NHibernateRepositoryTransactionContext中使用了一个DatabaseSessionFactory的类,该类主要用于管理NHibernate的Session对象,具体实现如下(该实现已被用于我的Apworks应用开发框架原型中):
/// <summary>
/// Represents the factory singleton for database session.
/// </summary>
internal sealed class DatabaseSessionFactory
{
#region Private Static Fields
/// <summary>
/// The singleton instance of the database session factory.
/// </summary>
private static readonly DatabaseSessionFactory databaseSessionFactory = new DatabaseSessionFactory();
#endregion
#region Private Fields
/// <summary>
/// The session factory instance.
/// </summary>
private ISessionFactory sessionFactory = null;
/// <summary>
/// The session instance.
/// </summary>
private ISession session = null;
#endregion
#region Constructors
/// <summary>
/// Privately constructs the database session factory instance, configures the
/// NHibernate framework by using the assemblies listed in the configured spaces(paths)
/// that are decorated by <see cref="EntityVisibleAttribute"/>.
/// </summary>
private DatabaseSessionFactory()
{
Configuration nhibernateConfig = new Configuration();
nhibernateConfig.Configure();
nhibernateConfig.AddAssembly(typeof(IAggregateRoot).Assembly);
sessionFactory = nhibernateConfig.BuildSessionFactory();
}
#endregion
#region Public Properties
/// <summary>
/// Gets the singleton instance of the database session factory.
/// </summary>
public static DatabaseSessionFactory Instance
{
get
{
return databaseSessionFactory;
}
}
/// <summary>
/// Gets the singleton instance of the session. If the session has not been
/// initialized or opened, it will return a newly opened session from the session factory.
/// </summary>
public ISession Session
{
get
{
ISession result = session;
if (result != null && result.IsOpen)
return result;
return OpenSession();
}
}
#endregion
#region Public Methods
/// <summary>
/// Always opens a new session from the session factory.
/// </summary>
/// <returns>The newly opened session.</returns>
public ISession OpenSession()
{
this.session = sessionFactory.OpenSession();
return this.session;
}
#endregion
}
简单小结一下。通过上面的例子可以看到,仓储的实现是不能依赖于任何技术细节的,因为领域模型并不关心技术问题。这并不是DDD一书中怎么说,我们就得怎么做。事实上的确如此,因为DDD的思想,使得我们应该把关注点放在业务分析与领域建模上,而仓储实现的分离正是这一思想的具体表现形式。不管怎么样,采用其它的手段也罢,我们还是应该遵循“将关注点放在领域”这一宗旨。
接下来看如何在领域层结合IoC框架使用仓储。仍然以Castle Windsor为例。配置文件如下(超长部分我用省略号去掉了):
<castle>
<components>
<!-- Object Context for Entity Data Model -->
<component id="ObjectContext"
service="System.Data.Objects.ObjectContext, System.Data.Entity, Version=4.0.0.0,......"
type="EasyCommerce.Domain.Model.EntitiesContainer, EasyCommerce.Domain"/>
<component id="GeneralRepository"
service="EasyCommerce.Domain.IRepository`1[[EasyCommerce.Domain.Model.Customer, ......"
type="EasyCommerce.Infrastructure.Repositories.EdmRepositories.EdmRepository`1[[EasyCo......">
<objContext>${ObjectContext}</objContext>
</component>
<component id="TransactionContext"
service="EasyCommerce.Domain.IRepositoryTransactionContext, EasyCommerce.Domain......"
type="EasyCommerce.Infrastructure.Repositories.EdmRepositories.EdmRepositoryTransactionContext, ...">
</component>
</components>
</castle>
以下是调用代码:
[TestMethod]
public void TestCreateCustomer()
{
IWindsorContainer container = new WindsorContainer(new XmlInterpreter());
using (IRepositoryTransactionContext tx = container.GetService<IRepositoryTransactionContext>())
{
tx.BeginTransaction();
try
{
Customer customer = Customer.CreateCustomer("daxnet", "12345",
new Name { FirstName = "Sunny", LastName = "Chen" },
new Address(), new Address(), DateTime.Now.AddYears(-29));
IRepository<Customer> customerRepository = tx.GetRepository<Customer>();
customerRepository.Add(customer);
tx.Commit();
}
catch
{
tx.Rollback();
throw;
}
}
}
测试结果及数据库数据结果:
【注意】:在使用NHibernate的仓储实现时,由于NHibernate的延迟加载特性,需要将实体的属性设置为virtual,以便由NHibernate产生Proxy Class进而实现延迟加载;但是由EntityFramework自动生成的源代码并不会将实体属性设置为virtual,而采用partial class也无法解决这个问题。因此需要在代码生成技术上做文章。我的做法是,针对edmx产生一个基于T4的代码生成模板,然后修改这个模板,分别在WritePrimitiveTypeProperty和WriteComplexTypeProperty方法中的适当位置加上virtual关键字:
private void WritePrimitiveTypeProperty(EdmProperty primitiveProperty, CodeGenerationTools code)
{
MetadataTools ef = new MetadataTools(this);
#>
/// <summary>
/// <#=SummaryComment(primitiveProperty)#>
/// </summary><#=LongDescriptionCommentElement(primitiveProperty, 1)#>
[EdmScalarPropertyAttribute(EntityKeyProperty=<#=code.CreateLiteral(ef.IsKey(primitiveProperty))#>,
IsNullable=<#=code.CreateLiteral(ef.IsNullable(primitiveProperty))#>)]
[DataMemberAttribute()]
<#=code.SpaceAfter(NewModifier(primitiveProperty))#><#=Accessibility.ForProperty(primitiveProperty)#> virtual
<#=code.Escape(primitiveProperty.TypeUsage)#> <#=code.Escape(primitiveProperty)#>
{
<#=code.SpaceAfter(Accessibility.ForGetter(primitiveProperty))#>get
{
<#+ if (ef.ClrType(primitiveProperty.TypeUsage) == typeof(byte[]))
{
#>
return StructuralObject.GetValidValue(<#=code.FieldName(primitiveProperty)#>);
private void WriteComplexTypeProperty(EdmProperty complexProperty, CodeGenerationTools code)
{
#>
/// <summary>
/// <#=SummaryComment(complexProperty)#>
/// </summary><#=LongDescriptionCommentElement(complexProperty, 1)#>
[EdmComplexPropertyAttribute()]
[DesignerSerializationVisibility(DesignerSerializationVisibility.Content)]
[XmlElement(IsNullable=true)]
[SoapElement(IsNullable=true)]
[DataMemberAttribute()]
<#=code.SpaceAfter(NewModifier(complexProperty))#><#=Accessibility.ForProperty(complexProperty)#> virtual
<#=MultiSchemaEscape(complexProperty.TypeUsage, code)#><#=code.Escape(complexProperty)#>
{
<#=code.SpaceAfter(Accessibility.ForGetter(complexProperty))#>get
{
<#=code.FieldName(complexProperty)#> = GetValidValue(<#=code.FieldName(complexProperty)#>,
"<#=complexProperty.Name#>",
false, <#=InitializedTrackingField(complexProperty, code)#>);
<#=InitializedTrackingField(complexProperty, code)#> = true;
始终坚持一个原则:不要在生成的代码上直接修改,一是工作量巨大,另一方面就是,代码是自动生成的,今后模型修改了,代码将会重新生成。