Hibernate-ORM:11.Hibernate中的關聯查詢
------------吾亦無他,唯手熟爾,謙卑若愚,好學若饑-------------
本篇博客將講述Hibernate中的關聯查詢,及其級聯(cascade)操作,以及指定哪一方維護關聯關系的(inverse)
一,講述目錄如下:
1.單向一對多:(增加一個區縣及其它以下的對應街道)
2.單項一對多:(查詢一個區縣,以及它下面所有的對應街道)
3.單項多對一:(查詢一個指定的街道,並同時展示出其對應的區縣)
4.雙向一對多(多對一):(值得註意:toString()套路不對容易引發錯誤Error------StackOverflowError)
5.inverse的使用和詳解:(指定哪一方來維護關聯關系)
6.級聯刪除:(刪除主表中的數據,同時將會幹掉外鍵是它的數據,慎用!!!!!!)
註:其實還想講一下多對多,但是沒有準備,下篇博客會詳細總結;
二,單向一對多:(增加一個區縣及其它以下的對應街道)
1.簡介:
我會在此案例下做好全部準備工作,其他的案例會在此案例上做略改操作:
2.實體類的準備:
2.1:District區縣實體類:
package cn.dawn.day03.entity; import java.util.ArrayList; import java.util.List;/** * Created by Dawn on 2018/5/30. */ /*區縣*/ public class District { private Integer id;//區縣id private String name;//區縣名稱 /*一個區縣對應多個街道*/ 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街道類:
package cn.dawn.day03.entity; /** * Created by Dawn on 2018/5/30. */ /*街道類*/ public class Street { private Integer id;//街道id private String name;//街道名稱 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文件的準備:
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的話,class的name就不必寫全類名--> <!--lazy:是否懶加載(延遲加載) 默認值是true,延遲加載--> <!--<class name="Teacher">--> <!--直接加載--> <class name="District" lazy="false"> <!--主鍵--> <id name="id" column="id"> <!--主鍵生成策咯 assigned程序員自己創建--> <!--identity是mysql裏的自增,一會做增加操作不必再給主鍵賦值--> <!--increment是先查最大的主鍵列,在下一條給主鍵加一--> <!--sequence是oracle的主鍵生成策咯,他一會需要指定序列名字<param name="sequence">序列名</param>--> <generator class="assigned"></generator> </id> <property name="name" column="name"></property> <!-- 配置一對多的關聯管理 name:關聯關系屬性名 column:數據庫中對應的外鍵 class:關聯關系的類型 cascade:對當前對象操作的時候,是否影響關聯對象 inverse="true": 放棄與數據庫的交互 --> <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的話,class的name就不必寫全類名--> <!--lazy:是否懶加載(延遲加載) 默認值是true,延遲加載--> <!--<class name="Teacher">--> <!--直接加載--> <class name="Street" lazy="false"> <!--主鍵--> <id name="id" column="id"> <!--主鍵生成策咯 assigned程序員自己創建--> <!--identity是mysql裏的自增,一會做增加操作不必再給主鍵賦值--> <!--increment是先查最大的主鍵列,在下一條給主鍵加一--> <!--sequence是oracle的主鍵生成策咯,他一會需要指定序列名字<param name="sequence">序列名</param>--> <generator class="assigned"></generator> </id> <property name="name" column="name"></property> <!--配置多對一的關聯關系 name: 本類中 關聯關系的屬性名 class:關聯關系的類型 column:在數據庫中兩個表的外鍵 --> <!--<many-to-one name="district" class="District" column="districtId"/>--> </class> </hibernate-mapping>
4.單測方法,增加一個區縣及其它以下的對應街道:
@Test /** * 單項一對多,增加一個區縣及其他下面的對應街道 * 千萬註意,在hbm中要加:級聯 * cascade="save-update" */ public void t1OneToMany(){ /*創建三個街道*/ Street street1=new Street(1,"海澱1街"); Street street2=new Street(2,"海澱2街"); Street street3=new Street(3,"海澱3街"); List<Street> streets=new ArrayList<Street>(); streets.add(street1); streets.add(street2); streets.add(street3); /*添加一個區到數據庫*/ District district=new District(1,"測試海澱區"); /*也可以從數據庫中獲取一個區,推薦第二種*/ /*District district = session.get(District.class, 1);*/ district.setStreets(streets); /*新增街道*/ session.save(district); tr.commit(); /*執行結果 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,單項一對多:(查詢一個區縣,以及它下面所有的對應街道)
1.在原有代碼不改變的情況下,做如下操作:
2.查詢一個區縣,以及它下面所有的對應街道:
@Test /*查詢指定區縣下所有的街道*/ public void t2selectStreetByDistrict(){ /*獲取id為1的區縣*/ District district=session.get(District.class,1); /*獲取區縣下所有的街道*/ List<Street> streets = district.getStreets(); for (Street street:streets) { System.out.println(street); } /*運行結果 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=‘海澱1街‘} Street{id=2, name=‘海澱2街‘} Street{id=3, name=‘海澱3街‘} * */ }
四,單項多對一:(查詢一個指定的街道,並同時展示出其對應的區縣)
1.先把二.3.3.1中District.hbm.xml把 <bag節點註釋掉,或者直接幹掉>
<!--<bag name="streets" cascade="save-update" > <key column="districtid"></key> <one-to-many class="Street"></one-to-many> </bag>-->
2.其實應該再多的一方,植入一個少的一方的對象,但是我在準備工作中就已經做過了
/*街道類*/ public class Street { private Integer id;//街道id private String name;//街道名稱 /*重點:::::::::*/ private District district;
3.在Street.hbm.xml配置文件中,把我註釋掉的那個<many-to-one>的節點放開:
<!--配置多對一的關聯關系 name: 本類中 關聯關系的屬性名 class:關聯關系的類型 column:在數據庫中兩個表的外鍵 --> <many-to-one name="district" class="District" column="districtId"/>
4.書寫測試方法:查詢一個指定的街道,並同時展示出其對應的區縣:
@Test /*查詢指定街道所對應的縣區*/ /*多對一*/ /*先把之前配置的那個幹掉*/ public void t3ManyToOne(){ /*需要在多的一方的實體類加入少的一方對象,並且在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=‘海澱1街‘, district=District{id=1, name=‘測試海澱區‘, streets=[]}} */ }
五,雙向一對多(多對一):(值得註意:toString()套路不對容易引發錯誤Error------StackOverflowError)
1.解釋:
他們的的關系是相互的,都要參與維護關聯關系
2.講四中註釋掉District.hbm.xml中的<bag>標簽再放開
<bag name="streets" cascade="save-update" > <key column="districtid"></key> <one-to-many class="Street"></one-to-many> </bag>
3.值得註意的是,需要修改toString()方法,否者他會互相調用,出現堆棧溢出的錯誤
4.先不改toString()的時候,書寫測試方法:(查詢一個街道及其對應的區縣)
@Test /*雙向的一對多,就是關系是相互的!!!!!!!!*/ /*將t3幹掉的配置再加回去*/ public void t4twoway(){ Street street = session.get(Street.class, 1); System.out.println(street); /*註:toString需要做個手段,否則互相調用的話就死循環,堆棧溢出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,使其不再互相無節制的調用:
/*區縣*/ public class District { private Integer id;//區縣id private String name;//區縣名稱 /*一個區縣對應多個街道*/ private List<Street> streets=new ArrayList<Street>(); @Override public String toString() { return "District{" + "id=" + id + ", name=‘" + name + ‘\‘‘ + ", streets=" + streets.size() + ‘}‘; }
6.重新執行4中的代碼,發現不會再出現錯誤,成功查詢
六,inverse的使用和詳解:(指定哪一方來維護關聯關系)
1.講解:
(1).Hibernate中的規則是,多的一方必須維護關聯關系,所以,我們只能修改一的一方(one-to-many)
(2).inverse默認值為false,就是與數據庫交互,會維護關聯關系,如果為了性能的提升,我們可以放棄一的一方與數據庫交互,即放棄維護關聯關系,將值改為true即可;
2.District.hbm.xml中的配置修改的如下:
<!-- 配置一對多的關聯管理 name:關聯關系屬性名 column:數據庫中對應的外鍵 class:關聯關系的類型 cascade:對當前對象操作的時候,是否影響關聯對象 inverse="true": 放棄與數據庫的交互 --> <bag name="streets" cascade="save-update" inverse="false"> <key column="districtid"></key> <one-to-many class="Street"></one-to-many> </bag>
3.單測方法(先測試一的一方維護關聯關系):
@Test public void t5inverse(){ //創建區縣 District district=new District(2,"大興區"); //創建街道 Street street1=new Street(4,"大興3街道"); Street street2=new Street(5,"大興4街道"); //把街道放進區縣的街道集合中 district.getStreets().add(street1); district.getStreets().add(street2); //新增區縣 session.save(district); tr.commit(); /** * 產生了8條sql語句! 發現 後面兩條update 是無用功!!!! 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條insert是 District產生的! 為什麽會產生兩條update?? Street! hibernate中 規定: 01.多的一方 many-to-one,必須去維護雙方的關系! 因為many-to-one壓根就沒有inverse這個屬性! 02.inverse默認為false! 不反轉! 我來維護! 03.必須在一的一方 設置 inverse="true" 放棄維護的權力! 維護===》是否與數據庫產生交互! */ }
4.將inverse改為true,放棄維護的權力,再執行剛才的代碼,發現:
/*修改inverse=true,刪除剛才添加的數據,再次執行單測*/ /* 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 (?, ?, ?) 發現後面的update沒有了 */
七,級聯刪除(慎用慎用慎用!!!!!!!!!!!!!!!!!!!!!!!)
1.講述:
如果設置完畢後,當幹掉主鍵所在的那張表中的一臺記錄,它所關聯的外鍵的那一行記錄也會被幹掉,所以說很危險,不要使用!!
2.設置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.書寫測試代碼,比如刪一個區縣:
@Test /*級聯刪除*/ /*cascade="delete" inverse="false"*/ public void t6deleteMapping(){ /*cascade="delete" inverse="false"記得改這個參數*/ /*獲取id為2的區縣*/ District district=session.get(District.class,2); /*刪除*/ 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=? * */ /*他會把倆張表的都幹掉了,所以,慎用!!!!!!!!!!!!!*/ }
作者:晨曦Dawn
轉載請註明出處:https://www.cnblogs.com/DawnCHENXI/p/9123184.html
如果有錯誤請您指出,感激不盡!!!!!!!!!!!!!!!!
Hibernate-ORM:11.Hibernate中的關聯查詢