Using dynamic instantiation In Hibernate

Hibernate中用select new 动态构造对象，如果select出的对象不是一个具体mapped对象，则hibernate返回一个

对象数组的list,要对每个数组元素cast，势必代码显得不够简捷和coarse-grained,下面的例子描述了如何实时构造Java对象

Iterator i = session.createQuery(
"select item.id, item.description, bid.amount "
"from Item item join item.bids bid "
"where bid.amount > 100"
)
.list()
.iterator();
while ( i.hasNext() ) {
Object[] row = (Object[]) i.next();
Long id = (Long) row[0];
String description = (String) row[1];
BigDecimal amount = (BigDecimal) row[2];
// ... show values in a report screen
}

Since the previous example was verbose and not very object-oriented (working
with a tabular data representation in arrays), we can define a class to represent
each row of results and use the HQL select new construct:
select new ItemRow( item.id, item.description, bid.amount )
from Item item join item.bids bid
where bid.amount > 100
Assuming that the ItemRow class has an appropriate constructor (you have to write
that class), this query returns newly instantiated (transient) instances of ItemRow,
as you can see in the next example:
Iterator i = session.createQuery(
"select new ItemRow( item.id, item.description, bid.amount ) "
"from Item item join item.bids bid "
"where bid.amount > 100"
)
.list()
.iterator();
while ( i.hasNext() ) {
ItemRow row = (ItemRow) i.next();
// Do something
}
The custom ItemRow class doesn’t have to be a persistent class; it doesn’t have to be
mapped to the database or even be known to Hibernate. ItemRow is therefore only
a data-transfer class, useful in report generation.

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