Hibernate 超簡單的一對多和多對一查詢
阿新 • • 發佈：2019-02-02
這裡使用的Teacher類和Student類（假設一個Teacher對應多個學生，一個學生對應一個老師）
所需jar包
開始建表
1（表名 teacher）
2（表名 student）
主鍵都為自增長
建立實體類
Teacher類
package com.bright.po; import java.util.Set; public class Teacher { private Integer id; private String name; private Integer age; private Set<Student> students; 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 Integer getAge() { return age; } public void setAge(Integer age) { this.age = age; } public Set<Student> getStudents() { return students; } public void setStudents(Set<Student> students) { this.students = students; } @Override public String toString() { return "Teacher [id=" + id + ", name=" + name + ", age=" + age + ", students=" + students + "]"; } }
Student類
package com.bright.po; public class Student { private Integer id; private String name; private Teacher teacher; public Student() { super(); } public Student(String name, Teacher teacher) { super(); this.name = name; this.teacher = teacher; } 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 Teacher getTeacher() { return teacher; } public void setTeacher(Teacher teacher) { this.teacher = teacher; } @Override public String toString() { return "Student [id=" + id + ", name=" + name + "]"; } }
配置Teacher類的對映檔案
<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd"> <hibernate-mapping package="com.bright.po"> <!-- name：即實體類的全名 table：對映到資料庫裡面的那個表的名稱 catalog：資料庫的名稱 --> <class name="Teacher" table="teacher" catalog="me"> <!-- class下必須要有一個id的子元素 --> <!-- id是用於描述主鍵的 --> <id name="id" column="id"> <!-- 主鍵生成策略 --> <generator class="native"></generator> </id> <!-- 使用property來描述屬性與欄位的對應關係 如果length忽略不寫，且你的表是自動建立這種方案，那麼length的預設長度是255 --> <property name="name" column="name" length="20"></property> <property name="age" column="age" length="20"></property> <!-- 一對多關聯對映配置（通過部門管理到員工） Teacher對映關鍵點: 1.指定對映的集合屬性：students 2.集合屬性對應的集合表：student 3.集合表的外來鍵欄位：t_id 4.集合元素的型別--> <set name = "students" table = "student"> <key column="t_id"></key> <one-to-many class="Student"></one-to-many> </set> </class> </hibernate-mapping>
配置Student類的對映檔案
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.bright.po">
<!--
name：即實體類的全名
table：對映到資料庫裡面的那個表的名稱
catalog：資料庫的名稱
-->
<class name="Student" table="student" catalog="me">
<!-- class下必須要有一個id的子元素 -->
<!-- id是用於描述主鍵的 -->
<id name="id" column="id">
<!-- 主鍵生成策略 -->
<generator class="native"></generator>
</id>
<!--
使用property來描述屬性與欄位的對應關係
如果length忽略不寫，且你的表是自動建立這種方案，那麼length的預設長度是255
-->
<property name="name" column="name" length="20"></property>
<!-- 一對多關聯對映配置（通過部門管理到員工）
Student對映關鍵點:
1.指定對映的屬性：teacher
2.集合表的外來鍵欄位：t_id
3.元素的型別:teacher-->
<many-to-one name="teacher" column="t_Id" class="Teacher"></many-to-one>
</class>
</hibernate-mapping>配置Hibrnate的配置檔案
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
"http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
<session-factory>
<!-- 配置關於資料庫連線的四個項：driverClass url username password -->
<property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
<property name="hibernate.connection.url">jdbc:mysql:///me</property>
<property name="hibernate.connection.username">root</property>
<property name="hibernate.connection.password">root</property>
<!-- 可以將向資料庫傳送的SQL語句顯示出來 -->
<property name="hibernate.show_sql">true</property>
<!-- 格式化SQL語句 -->
<property name="hibernate.format_sql">true</property>
<!-- hibernate的方言 -->
<property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
<!-- 配置hibernate的對映檔案所在的位置 -->
<mapping resource="com/bright/po/Teacher.hbm.xml" />
<mapping resource="com/bright/po/Student.hbm.xml" />
</session-factory>
</hibernate-configuration>表中插入幾條資料
teacher表
student表
開始測試
首先寫個獲取Session的工具類
package com.bright.utils;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;
public class SessionUtils {
public static Session getSession(){
Configuration cfg = new Configuration().configure();
SessionFactory factory = cfg.buildSessionFactory();
Session session = factory.openSession();
session.beginTransaction();
return session;
}
public static void closeSession(Session session){
if(session!=null){
session.getTransaction().commit();
session.close();
}
}
}具體測試
package com.bright.test;
import java.util.List;
import org.hibernate.Session;
import com.bright.po.Student;
import com.bright.po.Teacher;
import com.bright.utils.SessionUtils;
public class TestTeacher {
public static void main(String[] args) {
Session session = SessionUtils.getSession();
List<Teacher> list = session.createQuery("from Teacher").list();
for(Teacher t:list){
System.out.println(t);
}
SessionUtils.closeSession(session);
}
}輸出結果
Hibernate:
select
teacher0_.id as id1_1_,
teacher0_.name as name2_1_,
teacher0_.age as age3_1_
from
me.teacher teacher0_
Hibernate:
select
students0_.t_id as t_id3_0_0_,
students0_.id as id1_0_0_,
students0_.id as id1_0_1_,
students0_.name as name2_0_1_,
students0_.t_Id as t_Id3_0_1_
from
me.student students0_
where
students0_.t_id=?
Teacher [id=1, name=白起, age=27, students=[Student [id=2, name=蘇烈], Student [id=3, name=劉邦], Student [id=1, name=亞瑟]]]
Hibernate:
select
students0_.t_id as t_id3_0_0_,
students0_.id as id1_0_0_,
students0_.id as id1_0_1_,
students0_.name as name2_0_1_,
students0_.t_Id as t_Id3_0_1_
from
me.student students0_
where
students0_.t_id=?
Teacher [id=2, name=宮本武藏, age=25, students=[Student [id=6, name=蘭陵王], Student [id=4, name=孫悟空], Student [id=5, name=荊軻]]]
Hibernate:
select
students0_.t_id as t_id3_0_0_,
students0_.id as id1_0_0_,
students0_.id as id1_0_1_,
students0_.name as name2_0_1_,
students0_.t_Id as t_Id3_0_1_
from
me.student students0_
where
students0_.t_id=?
Teacher [id=3, name=后羿, age=55, students=[Student [id=7, name=百里守約], Student [id=9, name=公孫離], Student [id=8, name=馬可波羅]]]
可以看出，查詢執行了多次，多次生成了sql語句，效率低，所以接下來採用了表的“迫切左外連線”。具體程式碼：
package com.bright.test;
import java.util.List;
import org.hibernate.Session;
import com.bright.po.Student;
import com.bright.po.Teacher;
import com.bright.utils.SessionUtils;
public class TestTeacher {
public static void main(String[] args) {
Session session = SessionUtils.getSession();
List<Teacher> list = session.createQuery("select t from Teacher t left outer join fetch t.students").list();
for(Teacher t:list){
System.out.println(t);
}
SessionUtils.closeSession(session);
}
}
查詢結果：
Hibernate:
select
teacher0_.id as id1_1_0_,
students1_.id as id1_0_1_,
teacher0_.name as name2_1_0_,
teacher0_.age as age3_1_0_,
students1_.name as name2_0_1_,
students1_.t_Id as t_Id3_0_1_,
students1_.t_id as t_id3_0_0__,
students1_.id as id1_0_0__
from
me.teacher teacher0_
left outer join
me.student students1_
on teacher0_.id=students1_.t_id
Teacher [id=1, name=白起, age=27, students=[Student [id=3, name=劉邦], Student [id=1, name=亞瑟], Student [id=2, name=蘇烈]]]
Teacher [id=1, name=白起, age=27, students=[Student [id=3, name=劉邦], Student [id=1, name=亞瑟], Student [id=2, name=蘇烈]]]
Teacher [id=1, name=白起, age=27, students=[Student [id=3, name=劉邦], Student [id=1, name=亞瑟], Student [id=2, name=蘇烈]]]
Teacher [id=2, name=宮本武藏, age=25, students=[Student [id=4, name=孫悟空], Student [id=5, name=荊軻], Student [id=6, name=蘭陵王]]]
Teacher [id=2, name=宮本武藏, age=25, students=[Student [id=4, name=孫悟空], Student [id=5, name=荊軻], Student [id=6, name=蘭陵王]]]
Teacher [id=2, name=宮本武藏, age=25, students=[Student [id=4, name=孫悟空], Student [id=5, name=荊軻], Student [id=6, name=蘭陵王]]]
Teacher [id=3, name=后羿, age=55, students=[Student [id=8, name=馬可波羅], Student [id=7, name=百里守約], Student [id=9, name=公孫離]]]
Teacher [id=3, name=后羿, age=55, students=[Student [id=8, name=馬可波羅], Student [id=7, name=百里守約], Student [id=9, name=公孫離]]]
Teacher [id=3, name=后羿, age=55, students=[Student [id=8, name=馬可波羅], Student [id=7, name=百里守約], Student [id=9, name=公孫離]]]結構圖
