Hibernate 之QBC

一、Hibernate 中聚合函数的使用

Criteria接口的Projections类主要用于帮助Criteria接口完成数据的分组查询和统计功能:

Listcats = session.createCriteria(Cat. class )
.setProjection(Projections.projectionList()
.add(Projections.rowCount())
.add(Projections.avg( " weight " ))
.add(Projections.max( " weight " ))
.add(Projections.min( " weight " ))
.add(Projections.groupProperty( " color " ))
).addOrder(Order.asc( " color " )).list();

示例代码相当于:

select color,count(*),avg(weight),max(weight),min(weight),min(weight) from cat group by color ordy color asc;

二、条件查询(Criteria Queries)

具有一个直观的、可扩展的条件查询API是Hibernate的特色。

1. 创建一个Criteria 实例

org.hibernate.Criteria接口表示特定持久类的一个查询。Session是 Criteria实例的工厂。

Criteria crit = sess.createCriteria(Cat.class);

crit.setMaxResults(50);

List cats = crit.list();

2. 限制结果集内容

一个单独的查询条件是org.hibernate.criterion.Criterion 接口的一个实例。org.hibernate.criterion.Restrictions类 定义了获得某些内置Criterion类型的工厂方法。

Listcats = sess.createCriteria(Cat. class )
.add(Restrictions.like( " name " , " Fritz% " ))
.add(Restrictions.between( " weight " ,minWeight,maxWeight))
.list();

约束可以按逻辑分组。

Listcats = sess.createCriteria(Cat. class )
.add(Restrictions.like( " name " , " Fritz% " ))
.add(Restrictions.or(
Restrictions.eq( " age " , new Integer( 0 )),
Restrictions.isNull( " age " )
))
.list();

Listcats = sess.createCriteria(Cat. class )
.add(Restrictions.in( " name " , new String[] ... {"Fritz","Izi","Pk"} ))
.add(Restrictions.disjunction()
.add(Restrictions.isNull( " age " ))
.add(Restrictions.eq( " age " , new Integer( 0 )))
.add(Restrictions.eq( " age " , new Integer( 1 )))
.add(Restrictions.eq( " age " , new Integer( 2 )))
))
.list();

Hibernate提供了相当多的内置criterion类型(Restrictions 子类), 但是尤其有用的是可以允许你直接使用SQL。

Listcats = sess.createCriteria(Cat. class )
.add(Restrictions.sql( " lower({alias}.name)likelower(?) " , " Fritz% " ,Hibernate.STRING))
.list();

{alias}占位符应当被替换为被查询实体的列别名。

Property实例是获得一个条件的另外一种途径。你可以通过调用Property.forName() 创建一个Property。

Propertyage = Property.forName( " age " );
Listcats = sess.createCriteria(Cat. class )
.add(Restrictions.disjunction()
.add(age.isNull())
.add(age.eq( new Integer( 0 )))
.add(age.eq( new Integer( 1 )))
.add(age.eq( new Integer( 2 )))
))
.add(Property.forName( " name " ).in( new String[] ... {"Fritz","Izi","Pk"} ))
.list();

3. 结果集排序

你可以使用org.hibernate.criterion.Order来为查询结果排序。

Listcats = sess.createCriteria(Cat. class )
.add(Restrictions.like( " name " , " F% " )
.addOrder(Order.asc( " name " ))
.addOrder(Order.desc( " age " ))
.setMaxResults( 50 )
.list();

Listcats = sess.createCriteria(Cat. class )
.add(Property.forName( " name " ).like( " F% " ))
.addOrder(Property.forName( " name " ).asc())
.addOrder(Property.forName( " age " ).desc())
.setMaxResults( 50 )
.list();

4. 关联

你可以使用createCriteria()非常容易的在互相关联的实体间建立 约束。

Listcats = sess.createCriteria(Cat. class )
.add(Restrictions.like( " name " , " F% " )
.createCriteria( " kittens " )
.add(Restrictions.like( " name " , " F% " )
.list();

注意第二个 createCriteria()返回一个新的 Criteria实例，该实例引用kittens 集合中的元素。

接下来，替换形态在某些情况下也是很有用的。

Listcats = sess.createCriteria(Cat. class )
.createAlias( " kittens " , " kt " )
.createAlias( " mate " , " mt " )
.add(Restrictions.eqProperty( " kt.name " , " mt.name " ))
.list();

(createAlias()并不创建一个新的 Criteria实例。)

Cat实例所保存的之前两次查询所返回的kittens集合是 没有被条件预过滤的。如果你希望只获得符合条件的kittens， 你必须使用returnMaps()。

Listcats = sess.createCriteria(Cat. class )
.createCriteria( " kittens " , " kt " )
.add(Restrictions.eq( " name " , " F% " ))
.returnMaps()
.list();
Iteratoriter = cats.iterator();
while (iter.hasNext()) ... {
Mapmap=(Map)iter.next();
Catcat=(Cat)map.get(Criteria.ROOT_ALIAS);
Catkitten=(Cat)map.get("kt");
}

5. 动态关联抓取

你可以使用setFetchMode()在运行时定义动态关联抓取的语义。

Listcats = sess.createCriteria(Cat. class )
.add(Restrictions.like( " name " , " Fritz% " ))
.setFetchMode( " mate " ,FetchMode.EAGER)
.setFetchMode( " kittens " ,FetchMode.EAGER)
.list();

这个查询可以通过外连接抓取mate和kittens。 查看第 20.1 节 “ 抓取策略(Fetching strategies) ”可以获得更多信息。

6. 查询示例

org.hibernate.criterion.Example类允许你通过一个给定实例 构建一个条件查询。

Cat cat = new Cat();

cat.setSex('F');

cat.setColor(Color.BLACK);

List results = session.createCriteria(Cat.class)

.add( Example.create(cat) )

.list();

版本属性、标识符和关联被忽略。默认情况下值为null的属性将被排除。

你可以自行调整Example使之更实用。

Exampleexample = Example.create(cat)
.excludeZeroes() // excludezerovaluedproperties
.excludeProperty( " color " ) // excludethepropertynamed"color"
.ignoreCase() // performcaseinsensitivestringcomparisons
.enableLike(); // uselikeforstringcomparisons
Listresults = session.createCriteria(Cat. class )
.add(example)
.list();

你甚至可以使用examples在关联对象上放置条件。

Listresults = session.createCriteria(Cat. class )
.add(Example.create(cat))
.createCriteria( " mate " )
.add(Example.create(cat.getMate()))
.list();

7. 投影(Projections)、聚合（aggregation）和分组（grouping）

org.hibernate.criterion.Projections是 Projection 的实例工厂。我们通过调用 setProjection()应用投影到一个查询。

Listresults = session.createCriteria(Cat. class )
.setProjection(Projections.rowCount())
.add(Restrictions.eq( " color " ,Color.BLACK))
.list();

Listresults = session.createCriteria(Cat. class )
.setProjection(Projections.projectionList()
.add(Projections.rowCount())
.add(Projections.avg( " weight " ))
.add(Projections.max( " weight " ))
.add(Projections.groupProperty( " color " ))
)
.list();

在一个条件查询中没有必要显式的使用 "group by" 。某些投影类型就是被定义为 分组投影，他们也出现在SQL的group by子句中。

你可以选择把一个别名指派给一个投影，这样可以使投影值被约束或排序所引用。下面是两种不同的实现方式：

Listresults = session.createCriteria(Cat. class )
.setProjection(Projections.alias(Projections.groupProperty( " color " ), " colr " ))
.addOrder(Order.asc( " colr " ))
.list();

Listresults = session.createCriteria(Cat. class )
.setProjection(Projections.groupProperty( " color " ).as( " colr " ))
.addOrder(Order.asc( " colr " ))
.list();

alias()和as()方法简便的将一个投影实例包装到另外一个 别名的Projection实例中。简而言之，当你添加一个投影到一个投影列表中时 你可以为它指定一个别名：

Listresults = session.createCriteria(Cat. class )
.setProjection(Projections.projectionList()
.add(Projections.rowCount(), " catCountByColor " )
.add(Projections.avg( " weight " ), " avgWeight " )
.add(Projections.max( " weight " ), " maxWeight " )
.add(Projections.groupProperty( " color " ), " color " )
)
.addOrder(Order.desc( " catCountByColor " ))
.addOrder(Order.desc( " avgWeight " ))
.list();

Listresults = session.createCriteria(Domestic. class , " cat " )
.createAlias( " kittens " , " kit " )
.setProjection(Projections.projectionList()
.add(Projections.property( " cat.name " ), " catName " )
.add(Projections.property( " kit.name " ), " kitName " )
)
.addOrder(Order.asc( " catName " ))
.addOrder(Order.asc( " kitName " ))
.list();

你也可以使用Property.forName()来表示投影：

Listresults = session.createCriteria(Cat. class )
.setProjection(Property.forName( " name " ))
.add(Property.forName( " color " ).eq(Color.BLACK))
.list();

Listresults = session.createCriteria(Cat. class )
.setProjection(Projections.projectionList()
.add(Projections.rowCount().as( " catCountByColor " ))
.add(Property.forName( " weight " ).avg().as( " avgWeight " ))
.add(Property.forName( " weight " ).max().as( " maxWeight " ))
.add(Property.forName( " color " ).group().as( " color " )
)
.addOrder(Order.desc( " catCountByColor " ))
.addOrder(Order.desc( " avgWeight " ))
.list();

8. 离线(detached)查询和子查询

DetachedCriteria类使你在一个session范围之外创建一个查询，并且可以使用任意的 Session来执行它。

DetachedCriteriaquery = DetachedCriteria.forClass(Cat. class )
.add(Property.forName( " sex " ).eq( ' F ' ));

Sessionsession = .;
Transactiontxn = session.beginTransaction();
Listresults = query.getExecutableCriteria(session).setMaxResults( 100 ).list();
txn.commit();
session.close();

DetachedCriteria也可以用以表示子查询。条件实例包含子查询可以通过 Subqueries或者Property获得。

DetachedCriteriaavgWeight = DetachedCriteria.forClass(Cat. class )
.setProjection(Property.forName( " weight " ).avg());
session.createCriteria(Cat. class )
.add(Property.forName( " weight).gt(avgWeight))
.list();

DetachedCriteriaweights = DetachedCriteria.forClass(Cat. class )
.setProjection(Property.forName( " weight " ));
session.createCriteria(Cat. class )
.add(Subqueries.geAll( " weight " ,weights))
.list();

甚至相互关联的子查询也是有可能的：

DetachedCriteriaavgWeightForSex = DetachedCriteria.forClass(Cat. class , " cat2 " )
.setProjection(Property.forName( " weight " ).avg())
.add(Property.forName( " cat2.sex " ).eqProperty( " cat.sex " ));
session.createCriteria(Cat. class , " cat " )
.add(Property.forName( " weight).gt(avgWeightForSex))
.list();