------------吾亦无他,唯手熟尔，谦卑若愚，好学若饥-------------
本篇博客将讲述Hibernate中的关联查询，及其级联(cascade)操作，以及指定哪一方维护关联关系的(inverse)
一，讲述目录如下：
1.单向一对多：（增加一个区县及其它以下的对应街道）
2.单项一对多：（查询一个区县，以及它下面所有的对应街道）
3.单项多对一：（查询一个指定的街道，并同时展示出其对应的区县）
4.双向一对多（多对一）：（值得注意：toString()套路不对容易引发错误Error------StackOverflowError）
5.inverse的使用和详解：（指定哪一方来维护关联关系）
6.级联删除：(删除主表中的数据，同时将会干掉外键是它的数据，慎用！！！！！！)
注：其实还想讲一下多对多，但是没有准备，下篇博客会详细总结；
二，单向一对多：（增加一个区县及其它以下的对应街道）
1.简介：
我会在此案例下做好全部准备工作，其他的案例会在此案例上做略改操作：
2.实体类的准备：
2.1：District区县实体类：
package cn.dawn.day03.entity;import java.util.ArrayList;import java.util.List;/** * Created by Dawn on 2018/5/30. *//*区县*/public class District { ???private ?Integer id;//区县id ???private ?String name;//区县名称 ???/*一个区县对应多个街道*/ ???private List<Street> streets=new ArrayList<Street>(); ???@Override ???public String toString() { ???????return "District{" + ???????????????"id=" + id + ???????????????", name=‘" + name + ‘\‘‘ + ???????????????", streets=" + streets + ???????????????‘}‘; ???} ???public Integer getId() { ???????return id; ???} ???public void setId(Integer id) { ???????this.id = id; ???} ???public String getName() { ???????return name; ???} ???public void setName(String name) { ???????this.name = name; ???} ???public List<Street> getStreets() { ???????return streets; ???} ???public void setStreets(List<Street> streets) { ???????this.streets = streets; ???} ???public District() { ???} ???public District(Integer id, String name, List<Street> streets) { ???????this.id = id; ???????this.name = name; ???????this.streets = streets; ???} ???public District(Integer id, String name) { ???????this.id = id; ???????this.name = name; ???}}2.2：Street街道类：
package cn.dawn.day03.entity;/** * Created by Dawn on 2018/5/30. *//*街道类*/public class Street { ???private Integer id;//街道id ???private String name;//街道名称 ???private District district; ???@Override ???public String toString() { ???????return "Street{" + ???????????????"id=" + id + ???????????????", name=‘" + name + ‘\‘‘ + ???????????????", district=" + district + ???????????????‘}‘; ???} ???public Street() { ???} ???public Street(Integer id, String name) { ???????this.id = id; ???????this.name = name; ???} ???public District getDistrict() { ???????return district; ???} ???public void setDistrict(District district) { ???????this.district = district; ???} ???public Integer getId() { ???????return id; ???} ???public void setId(Integer id) { ???????this.id = id; ???} ???public String getName() { ???????return name; ???} ???public void setName(String name) { ???????this.name = name; ???}}3.hbm文件的准备：
3.1：District.hbm.xml文件的配置：
<?xml version="1.0"?><!DOCTYPE hibernate-mapping PUBLIC ???????"-//Hibernate/Hibernate Mapping DTD 3.0//EN" ???????"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd"><hibernate-mapping package="cn.dawn.day03.entity"> ???<!--如果上面指定package的话，class的name就不必写全类名--> ???<!--lazy:是否懒加载（延迟加载） ???????默认值是true，延迟加载--> ???<!--<class name="Teacher">--> ???<!--直接加载--> ???<class name="District" lazy="false"> ???????<!--主键--> ???????<id name="id" column="id"> ???????????<!--主键生成策咯 ?assigned程序员自己创建--> ???????????<!--identity是mysql里的自增，一会做增加操作不必再给主键赋值--> ???????????<!--increment是先查最大的主键列，在下一条给主键加一--> ???????????<!--sequence是oracle的主键生成策咯，他一会需要指定序列名字<param name="sequence">序列名</param>--> ???????????<generator class="assigned"></generator> ???????</id> ???????<property name="name" column="name"></property> ???????<!-- 配置一对多的关联管理 ???????name:关联关系属性名 ???????column：数据库中对应的外键 ???????class：关联关系的类型 ???????cascade：对当前对象操作的时候，是否影响关联对象 ???????inverse="true"： 放弃与数据库的交互 ???????--> ???????<bag name="streets" cascade="save-update" > ???????????<key column="districtid"></key> ???????????<one-to-many class="Street"></one-to-many> ???????</bag> ???</class></hibernate-mapping>
3.2:Street.hbm.xml文件配置：
<?xml version="1.0"?><!DOCTYPE hibernate-mapping PUBLIC ???????"-//Hibernate/Hibernate Mapping DTD 3.0//EN" ???????"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd"><hibernate-mapping package="cn.dawn.day03.entity"> ???<!--如果上面指定package的话，class的name就不必写全类名--> ???<!--lazy:是否懒加载（延迟加载） ???????默认值是true，延迟加载--> ???<!--<class name="Teacher">--> ???<!--直接加载--> ???<class name="Street" lazy="false"> ???????<!--主键--> ???????<id name="id" column="id"> ???????????<!--主键生成策咯 ?assigned程序员自己创建--> ???????????<!--identity是mysql里的自增，一会做增加操作不必再给主键赋值--> ???????????<!--increment是先查最大的主键列，在下一条给主键加一--> ???????????<!--sequence是oracle的主键生成策咯，他一会需要指定序列名字<param name="sequence">序列名</param>--> ???????????<generator class="assigned"></generator> ???????</id> ???????<property name="name" column="name"></property> ???????<!--配置多对一的关联关系 ???????name: 本类中 关联关系的属性名 ???????class：关联关系的类型 ???????column：在数据库中两个表的外键 ???????--> ???????<!--<many-to-one ?name="district" class="District" column="districtId"/>--> ???</class></hibernate-mapping>
4.单测方法,增加一个区县及其它以下的对应街道:
???@Test ???/** ????* 单项一对多，增加一个区县及其他下面的对应街道 ????* 千万注意，在hbm中要加：级联 ????* ?cascade="save-update" ????*/ ???public void t1OneToMany(){ ???????/*创建三个街道*/ ???????Street ?street1=new Street(1,"海淀1街"); ???????Street ?street2=new Street(2,"海淀2街"); ???????Street ?street3=new Street(3,"海淀3街"); ???????List<Street> streets=new ArrayList<Street>(); ???????streets.add(street1); ???????streets.add(street2); ???????streets.add(street3); ???????/*添加一个区到数据库*/ ???????District district=new District(1,"测试海淀区"); ???????/*也可以从数据库中获取一个区，推荐第二种*/ ???????/*District district = session.get(District.class, 1);*/ ???????district.setStreets(streets); ???????/*新增街道*/ ???????session.save(district); ???????tr.commit(); ???????/*执行结果 ???????Hibernate: create table District (id integer not null, name varchar(255), primary key (id)) engine=MyISAM ???????Hibernate: create table Street (id integer not null, name varchar(255), districtid integer, primary key (id)) engine=MyISAM ???????Hibernate: alter table Street add constraint FKjuba2team1j6124qeurjyblk1 foreign key (districtid) references District (id) ???????Hibernate: select street_.id, street_.name as name2_1_ from Street street_ where street_.id=? ???????Hibernate: select street_.id, street_.name as name2_1_ from Street street_ where street_.id=? ???????Hibernate: select street_.id, street_.name as name2_1_ from Street street_ where street_.id=? ???????Hibernate: insert into District (name, id) values (?, ?) ???????Hibernate: insert into Street (name, id) values (?, ?) ???????Hibernate: insert into Street (name, id) values (?, ?) ???????Hibernate: insert into Street (name, id) values (?, ?) ???????Hibernate: update Street set districtid=? where id=? ???????Hibernate: update Street set districtid=? where id=? ???????Hibernate: update Street set districtid=? where id=? ???????*/ ???}三，单项一对多：（查询一个区县，以及它下面所有的对应街道）
1.在原有代码不改变的情况下，做如下操作：
2.查询一个区县，以及它下面所有的对应街道：
???@Test ???/*查询指定区县下所有的街道*/ ???public void t2selectStreetByDistrict(){ ???????/*获取id为1的区县*/ ???????District district=session.get(District.class,1); ???????/*获取区县下所有的街道*/ ???????List<Street> streets = district.getStreets(); ???????for (Street street:streets) { ???????????System.out.println(street); ???????} ???????/*运行结果 ???????Hibernate: alter table Street add constraint FKjuba2team1j6124qeurjyblk1 foreign key (districtid) references District (id) ???????Hibernate: select district0_.id as id1_0_0_, district0_.name as name2_0_0_ from District district0_ where district0_.id=? ???????Hibernate: select streets0_.districtid as district3_1_0_, streets0_.id as id1_1_0_, streets0_.id as id1_1_1_, streets0_.name as name2_1_1_ from Street streets0_ where streets0_.districtid=? ???????Street{id=1, name=‘海淀1街‘} ???????Street{id=2, name=‘海淀2街‘} ???????Street{id=3, name=‘海淀3街‘} ???????* */ ???}四，单项多对一：（查询一个指定的街道，并同时展示出其对应的区县）
1.先把二.3.3.1中District.hbm.xml把 <bag节点注释掉，或者直接干掉>
???????<!--<bag name="streets" cascade="save-update" > ???????????<key column="districtid"></key> ???????????<one-to-many class="Street"></one-to-many> ???????</bag>-->
2.其实应该再多的一方，植入一个少的一方的对象，但是我在准备工作中就已经做过了
/*街道类*/public class Street { ???private Integer id;//街道id ???private String name;//街道名称 ???/*重点：：：：：：：：：*/ ???private District district;3.在Street.hbm.xml配置文件中，把我注释掉的那个<many-to-one>的节点放开：
<!--配置多对一的关联关系 ???????name: 本类中 关联关系的属性名 ???????class：关联关系的类型 ???????column：在数据库中两个表的外键 ???????--> ???????<many-to-one ?name="district" class="District" column="districtId"/>
4.书写测试方法：查询一个指定的街道，并同时展示出其对应的区县：
???@Test ???/*查询指定街道所对应的县区*/ ???/*多对一*/ ???/*先把之前配置的那个干掉*/ ???public void t3ManyToOne(){ ???????/*需要在多的一方的实体类加入少的一方对象，并且在hbm配置manytoone*/ ???????Street street = session.get(Street.class, 1); ???????System.out.println(street); ???????/* ???????Hibernate: alter table Street add constraint FKfch89j84iailpfqs5bpkj2rs0 foreign key (districtId) references District (id) ???????Hibernate: select street0_.id as id1_1_0_, street0_.name as name2_1_0_, street0_.districtId as district3_1_0_ from Street street0_ where street0_.id=? ???????Hibernate: select district0_.id as id1_0_0_, district0_.name as name2_0_0_ from District district0_ where district0_.id=? ???????Street{id=1, name=‘海淀1街‘, district=District{id=1, name=‘测试海淀区‘, streets=[]}} ???????*/ ???}五，双向一对多（多对一）：（值得注意：toString()套路不对容易引发错误Error------StackOverflowError）
1.解释：
他们的的关系是相互的，都要参与维护关联关系
2.讲四中注释掉District.hbm.xml中的<bag>标签再放开
???????<bag name="streets" cascade="save-update" > ???????????<key column="districtid"></key> ???????????<one-to-many class="Street"></one-to-many> ???????</bag>
3.值得注意的是，需要修改toString()方法，否者他会互相调用，出现堆栈溢出的错误
4.先不改toString()的时候，书写测试方法：（查询一个街道及其对应的区县）
???@Test ???/*双向的一对多，就是关系是相互的！！！！！！！！*/ ???/*将t3干掉的配置再加回去*/ ???public void t4twoway(){ ???????Street street = session.get(Street.class, 1); ???????System.out.println(street); ???????/*注：toString需要做个手段，否则互相调用的话就死循环，堆栈溢出java.lang.StackOverflowError*/ ???????/* ???????Hibernate: alter table Street add constraint FKfch89j84iailpfqs5bpkj2rs0 foreign key (districtId) references District (id) ???????Hibernate: select street0_.id as id1_1_0_, street0_.name as name2_1_0_, street0_.districtId as district3_1_0_ from Street street0_ where street0_.id=? ???????Hibernate: select district0_.id as id1_0_0_, district0_.name as name2_0_0_ from District district0_ where district0_.id=? ???????Hibernate: select streets0_.districtid as district3_1_0_, streets0_.id as id1_1_0_, streets0_.id as id1_1_1_, streets0_.name as name2_1_1_, streets0_.districtId as district3_1_1_ from Street streets0_ where streets0_.districtid=? ???????java.lang.StackOverflowError ???????...... ???????* */ ???}5.修改任意一toString()方法，使其不再互相无节制的调用：
/*区县*/public class District { ???private ?Integer id;//区县id ???private ?String name;//区县名称 ???/*一个区县对应多个街道*/ ???private List<Street> streets=new ArrayList<Street>(); ???@Override ???public String toString() { ???????return "District{" + ???????????????"id=" + id + ???????????????", name=‘" + name + ‘\‘‘ + ???????????????", streets=" + streets.size() + ???????????????‘}‘; ???}6.重新执行4中的代码，发现不会再出现错误，成功查询
六，inverse的使用和详解：（指定哪一方来维护关联关系）
1.讲解：
（1）.Hibernate中的规则是，多的一方必须维护关联关系，所以，我们只能修改一的一方(one-to-many)
（2）.inverse默认值为false，就是与数据库交互，会维护关联关系，如果为了性能的提升，我们可以放弃一的一方与数据库交互，即放弃维护关联关系，将值改为true即可；
2.District.hbm.xml中的配置修改的如下：
???????<!-- 配置一对多的关联管理 ???????name:关联关系属性名 ???????column：数据库中对应的外键 ???????class：关联关系的类型 ???????cascade：对当前对象操作的时候，是否影响关联对象 ???????inverse="true"： 放弃与数据库的交互 ???????--> ???????<bag name="streets" cascade="save-update" inverse="false"> ???????????<key column="districtid"></key> ???????????<one-to-many class="Street"></one-to-many> ???????</bag>
3.单测方法（先测试一的一方维护关联关系）：
???@Test ???public ??void ?t5inverse(){ ???????//创建区县 ???????District district=new District(2,"大兴区"); ???????//创建街道 ???????Street street1=new Street(4,"大兴3街道"); ???????Street street2=new Street(5,"大兴4街道"); ???????//把街道放进区县的街道集合中 ???????district.getStreets().add(street1); ???????district.getStreets().add(street2); ???????//新增区县 ???????session.save(district); ???????tr.commit(); ???????/** ????????* ??产生了8条sql语句！ ?发现 后面两条update 是无用功！！！！ ????????Hibernate: alter table Street add constraint FKfch89j84iailpfqs5bpkj2rs0 foreign key (districtId) references District (id) ????????Hibernate: select street_.id, street_.name as name2_1_, street_.districtId as district3_1_ from Street street_ where street_.id=? ????????Hibernate: select street_.id, street_.name as name2_1_, street_.districtId as district3_1_ from Street street_ where street_.id=? ????????Hibernate: insert into District (name, id) values (?, ?) ????????Hibernate: insert into Street (name, districtId, id) values (?, ?, ?) ????????Hibernate: insert into Street (name, districtId, id) values (?, ?, ?) ????????Hibernate: update Street set districtid=? where id=? ????????Hibernate: update Street set districtid=? where id=? ????????3条insert是 District产生的！ ????????为什么会产生两条update？？ Street！ ????????hibernate中 规定： ????????01.多的一方 ?many-to-one,必须去维护双方的关系！ ????????因为many-to-one压根就没有inverse这个属性！ ????????02.inverse默认为false! ?不反转！ 我来维护！ ????????03.必须在一的一方 ?设置 inverse="true" 放弃维护的权力！ ????????维护===》是否与数据库产生交互！ ????????*/ ???}4.将inverse改为true，放弃维护的权力，再执行刚才的代码，发现：
???????/*修改inverse=true，删除刚才添加的数据，再次执行单测*/ ???????/* ???????Hibernate: alter table Street add constraint FKfch89j84iailpfqs5bpkj2rs0 foreign key (districtId) references District (id) ???????Hibernate: select street_.id, street_.name as name2_1_, street_.districtId as district3_1_ from Street street_ where street_.id=? ???????Hibernate: select street_.id, street_.name as name2_1_, street_.districtId as district3_1_ from Street street_ where street_.id=? ???????Hibernate: insert into District (name, id) values (?, ?) ???????Hibernate: insert into Street (name, districtId, id) values (?, ?, ?) ???????Hibernate: insert into Street (name, districtId, id) values (?, ?, ?) ???????发现后面的update没有了 ???????*/
七，级联删除（慎用慎用慎用！！！！！！！！！！！！！！！！！！！！！！！）
1.讲述：
如果设置完毕后，当干掉主键所在的那张表中的一台记录，它所关联的外键的那一行记录也会被干掉，所以说很危险，不要使用！！
2.设置District.hbm.xml中的cascade="delete"和inverse="false"（或者inverse不配置）
???????<bag name="streets" cascade="delete" inverse="false"> ???????????<key column="districtid"></key> ???????????<one-to-many class="Street"></one-to-many> ???????</bag>
3.书写测试代码，比如删一个区县：
???@Test ???/*级联删除*/ ???/*cascade="delete" inverse="false"*/ ???public void t6deleteMapping(){ ???????/*cascade="delete" inverse="false"记得改这个参数*/ ???????/*获取id为2的区县*/ ???????District district=session.get(District.class,2); ???????/*删除*/ ???????session.delete(district); ???????tr.commit(); ???????/* ???????Hibernate: alter table Street add constraint FKfch89j84iailpfqs5bpkj2rs0 foreign key (districtId) references District (id) ???????Hibernate: select district0_.id as id1_0_0_, district0_.name as name2_0_0_ from District district0_ where district0_.id=? ???????Hibernate: select streets0_.districtid as district3_1_0_, streets0_.id as id1_1_0_, streets0_.id as id1_1_1_, streets0_.name as name2_1_1_, streets0_.districtId as district3_1_1_ from Street streets0_ where streets0_.districtid=? ???????Hibernate: update Street set districtid=null where districtid=? ???????Hibernate: delete from Street where id=? ???????Hibernate: delete from Street where id=? ???????Hibernate: delete from District where id=? ???????* */ ???????/*他会把俩张表的都干掉了，所以，慎用！！！！！！！！！！！！！*/ ???}作者：晨曦Dawn
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Hibernate-ORM:11.Hibernate中的关联查询
原文地址：https://www.cnblogs.com/DawnCHENXI/p/9123184.html