Hibernate的architecture(转载自Hibernate的4.3.4.Final的官方文档)

Hibernate的完整结构如下图所示：



下面是文档中的一些解释（英文的不作翻译了）：

2.1.3. Basic APIs

Here are quick discussions about some of the API objects depicted in the preceding diagrams (you will see them again in more detail in later chapters).

SessionFactory (

org.hibernate.SessionFactory

)
A thread-safe, immutable cache of compiled mappings for a single database. A factory for

org.hibernate.Session

instances. A client of

org.hibernate.connection.ConnectionProvider

. Optionally maintains a

second level cache

of data that is reusable between transactions at a process or cluster level.

Session (

org.hibernate.Session

)
A single-threaded, short-lived object representing a conversation between the application and the persistent store. Wraps a JDBC

java.sql.Connection

. Factory for

org.hibernate.Transaction

. Maintains a

first level cache

of persistent the application's persistent objects and collections; this cache is 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 can be ordinary JavaBeans/POJOs. They are associated with exactly one

org.hibernate.Session

. Once the

org.hibernate.Session

is closed, they will be detached and free to use in any application layer (for example, directly as data transfer objects to and from presentation).
Chapter 11,
Working with objects discusses transient, persistent and detached object states.

Transient and detached objects and collections
Instances of persistent classes that are not currently associated with a

org.hibernate.Session

. They may have been instantiated by the application and not yet persisted, or they may have been instantiated by a closed

org.hibernate.Session

.
Chapter 11, Working with objects discusses transient, persistent and detached object states.

Transaction (

org.hibernate.Transaction

)
(Optional) A single-threaded, short-lived object used by the application to specify atomic units of work. It abstracts the application from the underlying JDBC, JTA or CORBA transaction. A

org.hibernate.Session

might span several

org.hibernate.Transaction

s in some cases. However, transaction demarcation, either using the underlying API or

org.hibernate.Transaction

, is never optional.

ConnectionProvider (

org.hibernate.connection.ConnectionProvider

)
(Optional) A factory for, and pool of, JDBC connections. It abstracts the application from underlying

javax.sql.DataSource

or

java.sql.DriverManager

. It is not exposed to application, but it can be extended and/or implemented by the developer.

TransactionFactory (

org.hibernate.TransactionFactory

)
(Optional) A factory for

org.hibernate.Transaction

instances. It is not exposed to the application, but it can be extended and/or implemented by the developer.

Extension Interfaces
Hibernate offers a range of optional extension interfaces you can implement to customize the behavior of your persistence layer. See the API documentation for details.

2.2. Contextual sessions

Most applications using Hibernate need some form of "contextual" session, where a given session is in effect throughout the scope of a given context. However, across applications the definition of what constitutes a context is typically different; different
contexts define different scopes to the notion of current. Applications using Hibernate prior to version 3.0 tended to utilize either home-grown

ThreadLocal

-based contextual sessions, helper classes such as

HibernateUtil

, or utilized third-party frameworks, such as Spring or Pico, which provided proxy/interception-based contextual sessions.

Starting with version 3.0.1, Hibernate added the

SessionFactory.getCurrentSession()

method. Initially, this assumed usage of

JTA

transactions, where the

JTA

transaction defined both the scope and context of a current session. Given the maturity of the numerous stand-alone

JTA TransactionManager

implementations, most, if not all, applications should be using

JTA

transaction management, whether or not they are deployed into a

J2EE

container. Based on that, the

JTA

-based contextual sessions are all you need to use.

However, as of version 3.1, the processing behind

SessionFactory.getCurrentSession()

is now pluggable. To that end, a new extension interface,

org.hibernate.context.spi.CurrentSessionContext

, and a new configuration parameter,

hibernate.current_session_context_class

, have been added to allow pluggability of the scope and context of defining current sessions.

See the Javadocs for the

org.hibernate.context.spi.CurrentSessionContext

interface for a detailed discussion of its contract. It defines a single method,

currentSession()

, by which the implementation is responsible for tracking the current contextual session. Out-of-the-box, Hibernate comes with three implementations of this interface:

org.hibernate.context.internal.JTASessionContext

: current sessions are tracked and scoped by a

JTA

transaction. The processing here is exactly the same as in the older JTA-only approach. See the Javadocs for details.

org.hibernate.context.internal.ThreadLocalSessionContext

:current sessions are tracked by thread of execution. See the Javadocs for details.

org.hibernate.context.internal.ManagedSessionContext

: current sessions are tracked by thread of execution. However, you are responsible to bind and unbind a

Session

instance with static methods on this class: it does not open, flush, or close a

Session

.

The first two implementations provide a "one session - one database transaction" programming model. This is also known and used as
session-per-request. The beginning and end of a Hibernate session is defined by the duration of a database transaction. If you use programmatic transaction demarcation in plain JSE without JTA, you are advised to use the
Hibernate

Transaction

API to hide the underlying transaction system from your code. If you use JTA, you can utilize the JTA interfaces to demarcate transactions. If you execute in an EJB container that supports CMT, transaction
boundaries are defined declaratively and you do not need any transaction or session demarcation operations in your code. Refer to
Chapter 13,
Transactions and Concurrency for more information and code examples.

The

hibernate.current_session_context_class

configuration parameter defines which

org.hibernate.context.spi.CurrentSessionContext

implementation should be used. For backwards compatibility, if this configuration parameter is not set but a

org.hibernate.engine.transaction.jta.platform.spi.JtaPlatform

is configured, Hibernate will use the

org.hibernate.context.internal.JTASessionContext

. Typically, the value of this parameter would just name the implementation class to use. For the three out-of-the-box implementations, however, there are three corresponding short
names: "jta", "thread", and "managed".