NHibernate 2.1.2 参考文档速览

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"这篇文档是关于NHibernate 2.1.2的参考指南，涵盖了从快速入门到高级特性的各种内容，旨在帮助开发者理解和使用这个流行的ORM框架。" NHibernate是一个开源的对象关系映射（ORM）框架，它允许.NET开发者将数据库操作与对象模型无缝集成，从而减轻了在关系型数据库和面向对象编程之间转换的工作负担。在这个2.1.2版本的文档中，主要介绍了以下几个关键知识点： 1. **快速入门**：这部分内容向开发者展示了如何在IIS和Microsoft SQL Server环境下开始使用NHibernate。它包括了从创建第一个持久化类、映射数据库表、操作数据的基本步骤，以及一个简单的示例——如何管理和操作“Cat”（猫）实体。 2. **架构**：深入讲解了NHibernate的架构设计，概述了其核心组件和工作原理。提到了对象的状态（如瞬时、持久、脱管），以及上下文会话的概念，这在处理数据库连接和事务管理时至关重要。 3. **SessionFactory配置**：详细阐述了如何配置SessionFactory，这是NHibernate的核心工厂类，用于创建ISession实例。这部分讨论了程序化配置、SessionFactory的获取方式，以及如何提供自定义的ADO.NET连接。还提到了可选的配置属性，如SQL方言、外键加载策略、缓存提供者、查询语言替换等。 4. **持久化类**：介绍了如何定义和使用持久化类。文档以一个简单的POCO（Plain Old CLR Object）为例，指导开发者如何声明字段访问器和 mutators，实现默认构造函数，设置标识符属性（可选），以及推荐的非密封类和虚拟方法的设计，以支持更灵活的继承。 5. **其他主题**：除了上述内容，文档可能还涵盖了其他高级特性，如继承映射、集合映射、懒加载、事件监听、查询API（HQL和 Criteria API）以及事务处理等。这些特性让开发者能够更高效、更灵活地使用NHibernate进行数据库操作。这个NHibernate 2.1.2的帮助文档是开发者深入了解和熟练掌握ORM框架的宝贵资源，通过学习和实践，可以提升开发效率，降低维护成本，并使代码更加面向对象，易于理解。

between transactions, at a process- or cluster-level.

ISession (NHibernate.ISession)

A single-threaded, short-lived object representing a conversation between the application and the persistent

store. Wraps an ADO.NET connection. Factory for ITransaction. Holds a mandatory (first-level) cache of

persistent objects, used when navigating the object graph or looking up objects by identifier.

Persistent Objects and Collections

Short-lived, single threaded objects containing persistent state and business function. These might be ordin-

ary POCOs, the only special thing about them is that they are currently associated with (exactly one) ISes-

sion. As soon as the Session is closed, they will be detached and free to use in any application layer (e.g.

directly as data transfer objects to and from presentation).

Transient Objects and Collections

Instances of persistent classes that are not currently associated with a ISession. They may have been in-

stantiated by the application and not (yet) persisted or they may have been instantiated by a closed ISes-

sion.

ITransaction (NHibernate.ITransaction)

(Optional) A single-threaded, short-lived object used by the application to specify atomic units of work.

Abstracts application from underlying ADO.NET transaction. An ISession might span several ITransac-

tions in some cases.

IConnectionProvider (NHibernate.Connection.IConnectionProvider)

(Optional) A factory for ADO.NET connections and commands. Abstracts application from the concrete

vendor-specific implementations of IDbConnection and IDbCommand. Not exposed to application, but can

be extended/implemented by the developer.

IDriver (NHibernate.Driver.IDriver)

(Optional) An interface encapsulating differences between ADO.NET providers, such as parameter naming

conventions and supported ADO.NET features.

ITransactionFactory (NHibernate.Transaction.ITransactionFactory)

(Optional) A factory for ITransaction instances. Not exposed to the application, but can be extended/

implemented by the developer.

Given a "lite" architecture, the application bypasses the ITransaction/ITransactionFactory and/or IConnec-

tionProvider APIs to talk to ADO.NET directly.

2.2. Instance states

An instance of a persistent classes may be in one of three different states, which are defined with respect to a

persistence context. The NHibernate ISession object is the persistence context:

transient

The instance is not, and has never been associated with any persistence context. It has no persistent identity

(primary key value).

persistent

The instance is currently associated with a persistence context. It has a persistent identity (primary key

value) and, perhaps, a corresponding row in the database. For a particular persistence context, NHibernate

guarantees that persistent identity is equivalent to CLR identity (in-memory location of the object).

Architecture

NHibernate 2.1.0 9

detached

The instance was once associated with a persistence context, but that context was closed, or the instance

was serialized to another process. It has a persistent identity and, perhaps, a corrsponding row in the data-

base. For detached instances, NHibernate makes no guarantees about the relationship between persistent

identity and CLR identity.

2.3. Contextual Sessions

Most applications using NHibernate need some form of "contextual" sessions, where a given session is in effect

throughout the scope of a given context. However, across applications the definition of what constitutes a con-

text is typically different; and different contexts define different scopes to the notion of current.

Starting with version 1.2, NHibernate added the ISessionFactory.GetCurrentSession() method. The pro-

cessing behind ISessionFactory.GetCurrentSession() is pluggable. An extension interface (NHibern-

ate.Context.ICurrentSessionContext) and a new configuration parameter (hibern-

ate.current_session_context_class) have been added to allow pluggability of the scope and context of de-

fining current sessions.

See the API documentation for the NHibernate.Context.ICurrentSessionContext interface for a detailed dis-

cussion of its contract. It defines a single method, CurrentSession(), by which the implementation is respons-

ible for tracking the current contextual session. Out-of-the-box, NHibernate 1.2.1 comes with several imple-

mentations of this interface:

• NHibernate.Context.ManagedWebSessionContext - current sessions are tracked by HttpContext.

However, you are responsible to bind and unbind an ISession instance with static methods on this class, it

never opens, flushes, or closes an ISession itself.

• NHibernate.Context.CallSessionContext - current sessions are tracked by CallContext. You are re-

sponsible to bind and unbind an ISession instance with static methods of class CurrentSessionContext .

• NHibernate.Context.ThreadStaticSessionContext - current session is stored in a thread-static variable.

This context only supports one session factory. You are responsible to bind and unbind an ISession in-

stance with static methods of class CurrentSessionContext.

• NHibernate.Context.WebSessionContext - analogous to ManagedWebSessionContext above, stores the

current session in HttpContext. You are responsible to bind and unbind an ISession instance with static

methods of class CurrentSessionContext.

The hibernate.current_session_context_class configuration parameter defines which NHibern-

ate.Context.ICurrentSessionContext implementation should be used. Typically, the value of this parameter

would just name the implementation class to use (including the assembly name); for the out-of-the-box imple-

mentations, however, there are corresponding short names: "managed_web", "call", "thread_static", and "web",

respectively.

Architecture

NHibernate 2.1.0 10

Chapter 3. ISessionFactory Configuration

Because NHibernate is designed to operate in many different environments, there are a large number of config-

uration parameters. Fortunately, most have sensible default values and NHibernate is distributed with an ex-

ample App.config file (found in src\NHibernate.Test) that shows the various options. You usually only have

to put that file in your project and customize it.

3.1. Programmatic Configuration

An instance of NHibernate.Cfg.Configuration represents an entire set of mappings of an application's .NET

types to a SQL database. The Configuration is used to build an (immutable) ISessionFactory. The mappings

are compiled from various XML mapping files.

You may obtain a Configuration instance by instantiating it directly. Heres an example of setting up a data-

store from mappings defined in two XML configuration files:

Configuration cfg = new Configuration()

.AddFile("Item.hbm.xml")

.AddFile("Bid.hbm.xml");

An alternative (sometimes better) way is to let NHibernate load a mapping file from an embedded resource:

Configuration cfg = new Configuration()

.AddClass(typeof(NHibernate.Auction.Item))

.AddClass(typeof(NHibernate.Auction.Bid));

Then NHibernate will look for mapping files named NHibernate.Auction.Item.hbm.xml and NHibern-

ate.Auction.Bid.hbm.xml embedded as resources in the assembly that the types are contained in. This ap-

proach eliminates any hardcoded filenames.

Another alternative (probably the best) way is to let NHibernate load all of the mapping files contained in an

Assembly:

Configuration cfg = new Configuration()

.AddAssembly( "NHibernate.Auction" );

Then NHibernate will look through the assembly for any resources that end with .hbm.xml. This approach

eliminates any hardcoded filenames and ensures the mapping files in the assembly get added.

If a tool like Visual Studio .NET or NAnt is used to build the assembly, then make sure that the .hbm.xml files

are compiled into the assembly as Embedded Resources.

A Configuration also specifies various optional properties:

IDictionary props = new Hashtable();

...

Configuration cfg = new Configuration()

.AddClass(typeof(NHibernate.Auction.Item))

.AddClass(typeof(NHibernate.Auction.Bind))

.SetProperties(props);

A Configuration is intended as a configuration-time object, to be discarded once an ISessionFactory is built.

NHibernate 2.1.0 11

3.2. Obtaining an ISessionFactory

When all mappings have been parsed by the Configuration, the application must obtain a factory for ISession

instances. This factory is intended to be shared by all application threads:

ISessionFactory sessions = cfg.BuildSessionFactory();

However, NHibernate does allow your application to instantiate more than one ISessionFactory. This is use-

ful if you are using more than one database.

3.3. User provided ADO.NET connection

An ISessionFactory may open an ISession on a user-provided ADO.NET connection. This design choice

frees the application to obtain ADO.NET connections wherever it pleases:

IDbConnection conn = myApp.GetOpenConnection();

ISession session = sessions.OpenSession(conn);

// do some data access work

The application must be careful not to open two concurrent ISessions on the same ADO.NET connection!

3.4. NHibernate provided ADO.NET connection

Alternatively, you can have the ISessionFactory open connections for you. The ISessionFactory must be

provided with ADO.NET connection properties in one of the following ways:

1. Pass an instance of IDictionary mapping property names to property values to Configura-

tion.SetProperties().

2. Add the properties to a configuration section in the application configuration file. The section should be

named nhibernate and its handler set to System.Configuration.NameValueSectionHandler.

3. Include <property> elements in a configuration section in the application configuration file. The section

should be named hibernate-configuration and its handler set to NHibern-

ate.Cfg.ConfigurationSectionHandler. The XML namespace of the section should be set to

urn:nhibernate-configuration-2.2.

4. Include <property> elements in hibernate.cfg.xml (discussed later).

If you take this approach, opening an ISession is as simple as:

ISession session = sessions.OpenSession(); // open a new Session

// do some data access work, an ADO.NET connection will be used on demand

All NHibernate property names and semantics are defined on the class NHibernate.Cfg.Environment. We will

now describe the most important settings for ADO.NET connection configuration.

NHibernate will obtain (and pool) connections using an ADO.NET data provider if you set the following prop-

erties:

Table 3.1. NHibernate ADO.NET Properties

ISessionFactory Configuration

NHibernate 2.1.0 12

Property name Purpose

connection.provider_class The type of a custom IConnectionProvider.

eg. full.classname.of.ConnectionProvider if the

Provider is built into NHibernate, or

full.classname.of.ConnectionProvider, as-

sembly if using an implementation of IConnection-

Provider not included in NHibernate.

connection.driver_class The type of a custom IDriver, if using DriverCon-

nectionProvider.

full.classname.of.Driver if the Driver is built into

NHibernate, or full.classname.of.Driver, as-

sembly if using an implementation of IDriver not in-

cluded in NHibernate.

This is usually not needed, most of the time the dia-

lect will take care of setting the IDriver using a

sensible default. See the API documentation of the

specific dialect for the defaults.

connection.connection_string Connection string to use to obtain the connection.

connection.connection_string_name The name of the connection string (defined in

<connectionStrings> configuration file element) to

use to obtain the connection.

connection.isolation Set the ADO.NET transaction isolation level. Check

System.Data.IsolationLevel for meaningful values

and the database's documentation to ensure that level

is supported.

eg. Chaos, ReadCommitted, ReadUncommitted, Re-

peatableRead, Serializable, Unspecified

connection.release_mode Specify when NHibernate should release ADO.NET

connections. See Section 10.7, “Connection Release

Modes”.

eg. auto (default) | on_close | after_transaction

Note that this setting only affects ISessions returned

from ISessionFactory.OpenSession. For ISessions

obtained through ISessionFact-

ory.GetCurrentSession, the ICurrentSessionCon-

text implementation configured for use controls the

connection release mode for those ISessions. See

Section 2.3, “Contextual Sessions”.

command_timeout Specify the default timeout of IDbCommands generated

by NHibernate.

adonet.batch_size Specify the batch size to use when batching update

statements. Setting this to 0 (the default) disables the

functionality. See Section 17.6, “Batch updates”.

ISessionFactory Configuration

NHibernate 2.1.0 13

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