Hibernate 简明教程

Hibernate - Sessions

使用会话来与数据库获得物理连接。会话对象是轻量的，并且设计为在需要与数据库进行交互时进行实例化。持久性对象通过会话对象保存并检索。

A Session is used to get a physical connection with a database. The Session object is lightweight and designed to be instantiated each time an interaction is needed with the database. Persistent objects are saved and retrieved through a Session object.

会话对象不应该长时间保持打开状态，因为它们通常不是线程安全的，并且应该根据需要创建和销毁它们。会话的主要功能是为映射实体类的实例提供、创建、读取和删除操作。

The session objects should not be kept open for a long time because they are not usually thread safe and they should be created and destroyed them as needed. The main function of the Session is to offer, create, read, and delete operations for instances of mapped entity classes.

实例在给定的时间点可以处于以下三个状态之一 -

Instances may exist in one of the following three states at a given point in time −

transient − A new instance of a persistent class, which is not associated with a Session and has no representation in the database and no identifier value is considered transient by Hibernate.
persistent − You can make a transient instance persistent by associating it with a Session. A persistent instance has a representation in the database, an identifier value and is associated with a Session.
detached − Once we close the Hibernate Session, the persistent instance will become a detached instance.

如果其持久类是可序列化的，则会话实例是可序列化的。一个典型的交易应该使用以下习语 -

A Session instance is serializable if its persistent classes are serializable. A typical transaction should use the following idiom −

Session session = factory.openSession();
Transaction tx = null;

try {
   tx = session.beginTransaction();
   // do some work
   ...
   tx.commit();
}

catch (Exception e) {
   if (tx!=null) tx.rollback();
   e.printStackTrace();
} finally {
   session.close();
}

如果会话抛出异常，则必须回滚事务并放弃会话。

If the Session throws an exception, the transaction must be rolled back and the session must be discarded.

Session Interface Methods

Session 接口提供了一些方法，但我只列出几个重要的方法，我们将在本教程中使用它们。您可以检查 Hibernate 文档以获取与 Session 和 SessionFactory 相关联的方法的完整列表。

There are number of methods provided by the Session interface, but I’m going to list down a few important methods only, which we will use in this tutorial. You can check Hibernate documentation for a complete list of methods associated with Session and SessionFactory.

Sr.No.	Session Methods & Description
1	Transaction beginTransaction() Begin a unit of work and return the associated Transaction object.
2	void cancelQuery() Cancel the execution of the current query.
3	void clear() Completely clear the session.
4	Connection close() End the session by releasing the JDBC connection and cleaning up.
5	Criteria createCriteria(Class persistentClass) Create a new Criteria instance, for the given entity class, or a superclass of an entity class.
6	Criteria createCriteria(String entityName) Create a new Criteria instance, for the given entity name.
7	Serializable getIdentifier(Object object) Return the identifier value of the given entity as associated with this session.
8	Query createFilter(Object collection, String queryString) Create a new instance of Query for the given collection and filter string.
9	Query createQuery(String queryString) Create a new instance of Query for the given HQL query string.
10	SQLQuery createSQLQuery(String queryString) Create a new instance of SQLQuery for the given SQL query string.
11	void delete(Object object) Remove a persistent instance from the datastore.
12	void delete(String entityName, Object object) Remove a persistent instance from the datastore.
13	Session get(String entityName, Serializable id) Return the persistent instance of the given named entity with the given identifier, or null if there is no such persistent instance.
14	SessionFactory getSessionFactory() Get the session factory which created this session.
15	void refresh(Object object) Re-read the state of the given instance from the underlying database.
16	Transaction getTransaction() Get the Transaction instance associated with this session.
17	boolean isConnected() Check if the session is currently connected.
18	boolean isDirty() Does this session contain any changes which must be synchronized with the database?
19	boolean isOpen() Check if the session is still open.
20	Serializable save(Object object) Persist the given transient instance, first assigning a generated identifier.
21	void saveOrUpdate(Object object) Either save(Object) or update(Object) the given instance.
22	void update(Object object) Update the persistent instance with the identifier of the given detached instance.
23	void update(String entityName, Object object) Update the persistent instance with the identifier of the given detached instance.