1、如何指定使用hibernate.properties还是 hibernate.cfg.xml？
//使用hibernate.properties
//通过addClass装入实体类
Configuration cfg = new Configuration().addClass(Department.class);
cfg.setProperty("hibernate.show_sql", "true");//指定属性
//使用hibernate.cfg.xml
Configuration cfg = new Configuration().configure();
2、在Oracle表中指定使用Sequence
在Department.hbm.xml中使用如下描述：
<id name="id" type="long" column="DEPTNO">
<generator class="sequence">
<param name="sequence">DEPTNO_sequence</param>
<!--DEPTNO_sequence是序列的名称-->
</generator>
</id>
3、多表操作
配置 hibernate.cfg.xml 时注意元素书写次序：
<mapping resource= ...../>
<mapping resource= ...../>
<class-cache class=....../>
<class-cache class=....../>
表中外键定义 <many-to-one name="deptNo" not-null="true"/>
表中被参照的键 <bag name="emp" lazy="true" inverse="true" cascade="all">
<key column="deptNo"/>
<one-to-many class="Emp"/>
</bag>
4、自动产生数据库表
Configuration cfg = new Configuration()
.setProperty(Environment.HBM2DDL_AUTO, "create");
5、查询结果中包含null的项
对于一些数字项包含null，定义该项时必须使用wrapper类型，而不能是primitive类型，例如使用Double、Float等，而不能是double、float否则会抛出如下错误：
org.hibernate.PropertyAccessException: exception setting property value with CGLIB (set hibernate.cglib.use_reflection_optimizer=false for more info) setter of org.hibernate.scott.Emp.setComm
at org.hibernate.tuple.PojoTuplizer.setPropertyValuesWithOptimizer(PojoTuplizer.java:203)
at org.hibernate.tuple.PojoTuplizer.setPropertyValues(PojoTuplizer.java:173)
at org.hibernate.persister.entity.BasicEntityPersister.setPropertyValues(BasicEntityPersister.java:2900)
at org.hibernate.engine.TwoPhaseLoad.initializeEntity(TwoPhaseLoad.java:113)
at org.hibernate.loader.Loader.initializeEntitiesAndCollections(Loader.java:506)
at org.hibernate.loader.Loader.doQuery(Loader.java:415)
at org.hibernate.loader.Loader.doQueryAndInitializeNonLazyCollections(Loader.java:210)
at org.hibernate.loader.Loader.doList(Loader.java:1557)
at org.hibernate.loader.Loader.list(Loader.java:1540)
at org.hibernate.loader.hql.QueryLoader.list(QueryLoader.java:375)
at org.hibernate.hql.ast.QueryTranslatorImpl.list(QueryTranslatorImpl.java:271)
at org.hibernate.impl.SessionImpl.list(SessionImpl.java:791)
at org.hibernate.impl.QueryImpl.list(QueryImpl.java:74)
at org.hibernate.scott.Main.viewAllScottSlow(Main.java:89)
at org.hibernate.scott.Main.main(Main.java:129)
Caused by: net.sf.cglib.beans.BulkBeanException
at org.hibernate.scott.Emp$$BulkBeanByCGLIB$$9815f405.setPropertyValues(<generated>)
at org.hibernate.tuple.PojoTuplizer.setPropertyValuesWithOptimizer(PojoTuplizer.java:200)
... 14 more
Caused by: java.lang.NullPointerException
... 16 more
