hibernate session接口

hibernate session接口

Session接口是hibernate向应用程序提供的操纵数据库的最主要的接口，提供了保存、更新、删除和加载Java对象的方法。

session具有一个缓存，位于缓存中的对象称为持久化对象，和数据库中的相关记录对应。session能够在某些时间点，按照缓存中对象的变化来执行相关的SQL语句，来同步更新数据库，这一过程称为刷新缓存(flush)。

hibernate把对象分为4种状态，持久化状态，临时状态，游离状态，删除状态。session的特定方法可以使对象进行状态转换

session缓存

session实例没有结束生命周期，且没有清理缓存，则存放在session缓存中的对象也不会结束生命周期，session缓存可以减少访问数据库的频率。

操作session缓存

flush方法

缓存中对象同步到数据库(会插入或更新数据库)，使数据库中的状态与缓存中一致

注意：

session在以下情况下会刷新缓存

• hibernate在事务提交之前会执行flush()方法，然后再向数据库提交事务

• 显式调用session.flush()方法

• 在执行HQL或者QBC查询，会先进行flush()操作，以得到数据表最新的记录

refresh方法

将数据库同步到缓存中(会查询数据库)，使缓存中的状态与数据库一致

session.refresh(); 

clear方法

清理缓存，可以将session中的缓存清除

session.clear();

四种状态的转换

临时状态(Transient)

• 在使用代理主键的情况下，OID通常为null
• 不处于session缓存中
• 在数据库中也没有对应的记录，此时刚new了一个实体对象，还存在于保存临时数据的内存区域中

持久化状态(Persist)

• OID不为null
• 位于session缓存中
• 若在数据库中已经有和其对应的记录，持久化对象和数据库中的相关记录对应
• session在flush缓存时，会根据持久化对象的属性变化，来同步数据库
• 在同一个session实例的缓存中，数据库表中每条记录只对应唯一的持久化对象
• 持久化对象的id不可以被修改，因为hibernate是根据id去进行比较的

删除状态(Removed)

• 在数据库中没有和其OID对应的记录
• 不再处于session缓存中

游离状态(Detached)

• OID不为null

• 不再处于session缓存中

• 一般情况下，游离对象是由持久化对象转变过来的(session进行close、clear、evict等情况)，数据库中可能还存在它对应的记录，但是因为会话已经消失，对象不在持久化管理之内，所以处于游离状态

  

save方法和persist方法的区别

在调用persist()方法时如果存在id，则会抛出异常，而save方法则可以正常执行

org.hibernate.PersistentObjectException: detached entity passed to persist

get方法和load方法的区别

• 执行get方法会立即加载对象，执行load方法若不使用该对象，则不会立即查询，而是返回一个代理对象(延时加载)

• 若数据表中没有对应的记录，get方法返回null，load方法抛出异常

  org.hibernate.ObjectNotFoundException: No row with the given identifier exists

• load方法可能会抛出懒加载异常

  org.hibernate.LazyInitializationException: could not initialize proxy - no Session

注意：在session缓存中不能够有两个相同OID的对象，否则会报异常

public static void testOid(Session session){
  User user = (User) session.get(User.class,1);
  System.out.println(user);
  User user1  = new User();
  user1.setId(1);
  user1.setName("王五");
  session.saveOrUpdate(user1);
}

org.hibernate.NonUniqueObjectException: A different object with the same identifier value was already associated with the session

evict方法

从session缓存中将指定的持久化对象移除

hibernate获取原生JDBC连接进行操作

可以使用doWork或者doReturnWork来使用原生JDBC操作数据

session.doWork(new Work() {
  @Override
  public void execute(Connection connection) throws SQLException {

  }
});

session.doReturningWork(new ReturningWork<Object>() {
  @Override
  public Object execute(Connection connection) throws SQLException {
    return null;
  }
});

session方法说明

public interface Session extends SharedSessionContract {
   /**
    * Obtain a {@link Session} builder with the ability to grab certain information from this session.
    *
    * @return The session builder
    */
   public SharedSessionBuilder sessionWithOptions();

   /**
    * Force this session to flush. Must be called at the end of a
    * unit of work, before committing the transaction and closing the
    * session (depending on {@link #setFlushMode(FlushMode)},
    * {@link Transaction#commit()} calls this method).
    * <p/>
    * <i>Flushing</i> is the process of synchronizing the underlying persistent
    * store with persistable state held in memory.
    *
    * @throws HibernateException Indicates problems flushing the session or
    * talking to the database.
    */
   public void flush() throws HibernateException;

   // 获取创建该会话的sessionFactory
   public SessionFactory getSessionFactory();

   // 关闭数据库连接
   public Connection close() throws HibernateException;

   // 取消当前查询的执行
   public void cancelQuery() throws HibernateException;

   // 当前session是否开启
   public boolean isOpen();

   // 当前session是否连接
   public boolean isConnected();

   // 该session中是否包含必须与数据库痛的变化
   public boolean isDirty() throws HibernateException;

   // 返回与当前实体关联的会话标识符
   public Serializable getIdentifier(Object object);

   /**
    * Check if this instance is associated with this <tt>Session</tt>.
    *
    * @param object an instance of a persistent class
    * @return true if the given instance is associated with this <tt>Session</tt>
    */
   public boolean contains(Object object);

   /**
    * Remove this instance from the session cache. Changes to the instance will
    * not be synchronized with the database. This operation cascades to associated
    * instances if the association is mapped with <tt>cascade="evict"</tt>.
    *
    * @param object The entity to evict
    *
    * @throws NullPointerException if the passed object is {@code null}
    * @throws IllegalArgumentException if the passed object is not defined as an entity
    */
   public void evict(Object object);

   /**
    * Return the persistent instance of the given entity class with the given identifier,
    * assuming that the instance exists. This method might return a proxied instance that
    * is initialized on-demand, when a non-identifier method is accessed.
    * <br><br>
    * You should not use this method to determine if an instance exists (use <tt>get()</tt>
    * instead). Use this only to retrieve an instance that you assume exists, where non-existence
    * would be an actual error.
    *
    * @param theClass a persistent class
    * @param id a valid identifier of an existing persistent instance of the class
    *
    * @return the persistent instance or proxy
    */
   public Object load(Class theClass, Serializable id);

   /**
    * Persist the state of the given detached instance, reusing the current
    * identifier value.  This operation cascades to associated instances if
    * the association is mapped with {@code cascade="replicate"}
    *
    * @param object a detached instance of a persistent class
    * @param replicationMode The replication mode to use
    */
   public void replicate(Object object, ReplicationMode replicationMode);

   /**
    * Persist the state of the given detached instance, reusing the current
    * identifier value.  This operation cascades to associated instances if
    * the association is mapped with {@code cascade="replicate"}
    *
    * @param entityName The entity name
    * @param object a detached instance of a persistent class
    * @param replicationMode The replication mode to use
    */
   public void replicate(String entityName, Object object, ReplicationMode replicationMode) ;

   // 保存对象，生成标识，变为持久化状态
   public Serializable save(Object object);

   // 保存对象，生成标识，变为持久化状态
   public Serializable save(String entityName, Object object);

   // 保存或更新对象
   public void saveOrUpdate(Object object);

   // 保存或更新对象
   public void saveOrUpdate(String entityName, Object object);

   // 更新该标识符所对应的对象
   public void update(Object object);
   
      // 更新该标识符所对应的对象
   public void update(String entityName, Object object);

   /**
    * Copy the state of the given object onto the persistent object with the same
    * identifier. If there is no persistent instance currently associated with
    * the session, it will be loaded. Return the persistent instance. If the
    * given instance is unsaved, save a copy of and return it as a newly persistent
    * instance. The given instance does not become associated with the session.
    * This operation cascades to associated instances if the association is mapped
    * with {@code cascade="merge"}
    * <p/>
    * The semantics of this method are defined by JSR-220.
    *
    * @param object a detached instance with state to be copied
    *
    * @return an updated persistent instance
    */
   public Object merge(Object object);

   /**
    * Make a transient instance persistent. This operation cascades to associated
    * instances if the association is mapped with {@code cascade="persist"}
    * <p/>
    * The semantics of this method are defined by JSR-220.
    *
    * @param object a transient instance to be made persistent
    */
   public void persist(Object object);
   

   // 删除该持久化对象
   public void delete(Object object);

   // 删除该持久化对象
   public void delete(String entityName, Object object);

   // 从数据库中重新读取该对象
   public void refresh(Object object);

   /**
    * Re-read the state of the given instance from the underlying database. It is
    * inadvisable to use this to implement long-running sessions that span many
    * business tasks. This method is, however, useful in certain special circumstances.
    * For example
    * <ul>
    * <li>where a database trigger alters the object state upon insert or update
    * <li>after executing direct SQL (eg. a mass update) in the same session
    * <li>after inserting a <tt>Blob</tt> or <tt>Clob</tt>
    * </ul>
    *
    * @param entityName a persistent class
    * @param object a persistent or detached instance
    */
   public void refresh(String entityName, Object object);

   // 为给定集合和查询条件创建查询实例
   public Query createFilter(Object collection, String queryString);

   // 清除该会话
   public void clear();

   // 返回给定命名和标识符的持久化对象实例
   public Object get(Class clazz, Serializable id);

   // 返回给定命名和标识符的持久化对象实例
   public Object get(String entityName, Serializable id);

   /**
    * Return the entity name for a persistent entity.
    *   
    * @param object a persistent entity
    *
    * @return the entity name
    */
   public String getEntityName(Object object);

   /**
    * Create an {@link IdentifierLoadAccess} instance to retrieve the specified entity by
    * primary key.
    *
    * @param entityClass The entity type to be retrieved
    *
    * @return load delegate for loading the specified entity type by primary key
    *
    * @throws HibernateException If the specified Class cannot be resolved as a mapped entity
    */
   public IdentifierLoadAccess byId(Class entityClass);
   
   /**
    * Get the statistics for this session.
    *
    * @return The session statistics being collected for this session
    */
   public SessionStatistics getStatistics();

   /**
    * Controller for allowing users to perform JDBC related work using the Connection managed by this Session.
    *
    * @param work The work to be performed.
    * @throws HibernateException Generally indicates wrapped {@link java.sql.SQLException}
    */
   public void doWork(Work work) throws HibernateException;

   /**
    * Controller for allowing users to perform JDBC related work using the Connection managed by this Session.  After
    * execution returns the result of the {@link ReturningWork#execute} call.
    *
    * @param work The work to be performed.
    * @param <T> The type of the result returned from the work
    *
    * @return the result from calling {@link ReturningWork#execute}.
    *
    * @throws HibernateException Generally indicates wrapped {@link java.sql.SQLException}
    */
   public <T> T doReturningWork(ReturningWork<T> work) throws HibernateException;

   /**
    * Add one or more listeners to the Session
    *
    * @param listeners The listener(s) to add
    */
   public void addEventListeners(SessionEventListener... listeners);
}