Hibernate DML 操作记录

http://developer.51cto.com/art/200906/130045.htm

 

网上流传的很老的一段文字：

红框标记的文字使用hibernate 源码来解释：

hibernate-3.6.0.final/core/src/main/java/org/hibernate/loader/Loader.java

 

请查看代码Loader.java　1341行代码：

 

 

/***
	 * Resolve any IDs for currently loaded objects, duplications within the
	 * <tt>ResultSet</tt>, etc. Instantiate empty objects to be initialized from the
	 * <tt>ResultSet</tt>. Return an array of objects (a row of results) and an
	 * array of booleans (by side-effect) that determine whether the corresponding
	 * object should be initialized.
	 */
	private Object[] getRow(
	        final ResultSet rs,
	        final Loadable[] persisters,
	        final EntityKey[] keys,
	        final Object optionalObject,
	        final EntityKey optionalObjectKey,
	        final LockMode[] lockModes,
	        final List hydratedObjects,
	        final SessionImplementor session) 
	throws HibernateException, SQLException {

		final int cols = persisters.length;
		final EntityAliases[] descriptors = getEntityAliases();

		if ( log.isDebugEnabled() ) {
			log.debug( 
					"result row: " + 
					StringHelper.toString( keys ) 
				);
		}

		final Object[] rowResults = new Object[cols];

		for ( int i = 0; i < cols; i++ ) {

			Object object = null;
			EntityKey key = keys[i];

			if ( keys[i] == null ) {
				//do nothing
			}
			else {//如果对象已经加载，则直接从session缓存中获取
				//If the object is already loaded, return the loaded one
				object = session.getEntityUsingInterceptor( key );
				if ( object != null ) {
					//its already loaded so don't need to hydrate it
					instanceAlreadyLoaded( 
							rs,
							i,
							persisters[i],
							key,
							object,
							lockModes[i],
							session 
						);
				}
				else {//没有直接从数据库查询
					object = instanceNotYetLoaded( 
							rs,
							i,
							persisters[i],
							descriptors[i].getRowIdAlias(),
							key,
							lockModes[i],
							optionalObjectKey,
							optionalObject,
							hydratedObjects,
							session 
						);
				}

			}

			rowResults[i] = object;

		}

		return rowResults;
	}

 

举个例：

账户对象：Account.java

 

public class Account{
 private Long Id; 
 private Long balance;
}

 

 

两个方法：

//通过用户编号查询用户

getAccountById();

//DML操作更新账户余额

updateAccountBalance();

 

 

session.createQuery("update Account a set a.balance=? where a.id = ?").setParameter(0, -100).setParameter(1, 1)      
.executeUpdate();

 

 

 

在AccountService中同一个事务中执行如下操作：

 

 

场景1 

(1) Account account = getAccountById(1);
//更新编号1的余额
(2) updateAccountBalance();
//再次查询
(3)account = getAccountById(1);
此时account对象的余额是没有改变的，因为结果来自session缓存

 如果将Hibernate 中设置show_sql = true ,

(1)和(3)都会输入查询SQL，需注意是(3)虽然输出了SQL但数据并不是来自数据库。

 

 

 

场景2

//更新编号1的余额
updateAccountBalance();
//再次查询 
Account account  = getAccountById(1);
此时account对象的余额是更新后的，因为结果来源数据库