针对hibernate 中多对多关系的查询时需要注意的点

如teacher表和student表是多对多关系，下面是他们的对象及Xml文件

Student.java

package com.landray.domain;
import java.util.HashSet;
import java.util.Set;
public class Student {

private int id;
private String name;
private Set<Teacher> teacher = new HashSet<Teacher>();

public Set<Teacher> getTeacher() {
return teacher;
}
public void setTeacher(Set<Teacher> teacher) {
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;
}

@Override
public String toString() {
return "Student [id=" + id + ", name=" + name + ", teacher="+teacher+"";
}

}

Teacher.java

package com.landray.domain;

import java.util.HashSet;
import java.util.Set;

public class Teacher {
private int id;
private String name;
private Set<Student> student = new HashSet<Student> ();
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Set<Student> getStudent() {
return student;
}
public void setStudent(Set<Student> student) {
this.student = student;
}
@Override
public String toString() {
return "Teacher [id=" + id + ", name=" + name + "]";
}

}

student.hbm.xml

<?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.landray.domain" >
<class name="Student" table="t_student" >
<id name="id" column="id" >
<generator class="native"></generator>
</id>
<property name="name" column="name" ></property>



<set name="teacher" table="t_student_course" fetch="join" inverse="false" cascade="all" >
<key column="sid" ></key>
<many-to-many class="Teacher" column="tid" ></many-to-many>
</set>

</class>

</hibernate-mapping>

teacher.hbm.xml

<?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.landray.domain" >
<class name="Teacher" table="t_teacher" >
<id name="id" column="id" >
<generator class="increment"></generator>
</id>
<property name="name" column="name" ></property>



<set name="student" table="t_student_course" inverse="true" cascade="save-update" >
<key column="tid" ></key>
<many-to-many class="Student" column="sid" ></many-to-many>
</set>

</class>
</hibernate-mapping>

hibernate.cfg.xml

<?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>


<property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>

<property name="hibernate.connection.username">root</property>

<property name="hibernate.connection.password">6677188</property>

<property name="hibernate.connection.url">jdbc:mysql://localhost:3306/beijingstud</property>


<property name="show_sql">true</property>

<property name="format_sql">true</property>

<property name="hbm2ddl.auto">update</property>


<property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>


<property name="hibernate.connection.autocommit">true</property>

<property name="hibernate.current_session_context_class">thread</property>

<mapping resource="com/landray/domain/Banji.hbm.xml"/>
<mapping resource="com/landray/domain/Student.hbm.xml"/>
<mapping resource="com/landray/domain/Teacher.hbm.xml"/>
</session-factory>

</hibernate-configuration>

测试方法

package com.landray.manytomany;
import java.util.List;
import org.hibernate.Criteria;
import org.hibernate.Query;
import org.hibernate.SQLQuery;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;
import org.hibernate.criterion.Restrictions;
import org.junit.Test;

import com.landray.domain.Banji;
import com.landray.domain.Student;
import com.landray.domain.Teacher;

public class Demo1 {

@Test
public void insert(){

//1 读取配置文件
Configuration conf = new Configuration().configure();
//2 根据配置创建Factory
SessionFactory sessionFactory = conf.buildSessionFactory();
//3 通过获得操作数据库的session对象
Session session = sessionFactory.openSession();
Transaction ts = session.beginTransaction();
//------------------------------------------------
Student student1 = new Student();

student1.setName("李华");

Student student2 = new Student();

student2.setName("李娜");

Teacher teacher1 = new Teacher();

teacher1.setName("王明");

Teacher teacher2 = new Teacher();

teacher2.setName("王亮");

student1.getTeacher().add(teacher1);
student1.getTeacher().add(teacher2);

student2.getTeacher().add(teacher1);
student2.getTeacher().add(teacher2);

session.save(student1);
session.save(student2);

ts.commit();
session.close();

}

@Test
public void select(){

//1 读取配置文件
Configuration conf = new Configuration().configure();
//2 根据配置创建Factory
SessionFactory sessionFactory = conf.buildSessionFactory();
//3 通过获得操作数据库的session对象
Session session = sessionFactory.openSession();
Transaction ts = session.beginTransaction();
//------------------------------------------------

Student student = (Student) session.get(Student.class, 1);
//Set sets = student.getTeacher();
System.out.println(student.toString());

//------------------------------------------------
ts.commit();
session.close();
}

注意的点：

1、多对多中，为了在select方法中检查方便，我们一般在对象中会重写toString方法，但是，在多对多的关系中，切记只能完整的重写一边的toString方法，一旦两边都完整的写了toString方法，会出现死循环，知道提示栈溢出。解决方法我想到两种，第一种，只完整的写一边的toString方法，另一边则把关联的那部分去掉。如在Teacher.java中我就把Student去掉了。public String toString() {
return "Teacher [id=" + id + ", name=" + name + "]";

}，只保留了id和姓名。第二种方法，不用toString方法来显示，直接通过get方法来把数据输出来。如上面的我们就可以直接把从select 方法中获得的student对象的set <Teacher> teacher 用get方法来显示出来。

2.一对多中，最好是在多的一方设置级联，主外键关联关系只是作为一种连接，方便查找并没有特定的裙带关系，你删除子表的数据是否会删除掉主表的数据这取决于你的业务需求，hibernate是支持级联的，如果你设置了级联删除那么就会连带主表的记录一并删除

针对hibernate 中多对多关系 的查询时需要注意的点

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