Hibernate框架-多对多关联

最新推荐文章于 2024-07-25 10:39:41 发布

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这篇博客主要探讨了Hibernate框架下多对多关联的实现，包括单向和双向两种情况。在单向N-N关联中，关联通过@ManyToMany和@JoinTable注解实现。而在双向N-N关联中，两端都需要Set集合和对应的Getter、Setter方法，且@ManyToMany注解需在两端使用，连接表的配置需保持一致。通过示例代码展示了如何使用Annotation和XML方式进行配置，详细解释了如何管理和控制关联关系。

这里写图片描述
首先明确一点：单双与否不影响数据库表的设计，单双向是体现在持久化类的设计上

单向N-N关联

单向的N-N关联和1-N关联的持久化化类完全相同，控制关系的一端需要增加一个Set集合，Set集合中存储的是被关联的持久化实例对象

Annotation方式

N-N关联必须使用连接表，因此需要@JoinTable来映射连接表，需要使用@ManyToMany注解来修饰代表关联实体的集合属性：
@ManyToMany注解有以下属性：

属性	是否必需	说明
cascade	否	指定Hibernate对关联实体采用的级联策略
fetch	否	指定抓取关联实体时的抓取策略，该属性值有FetchTyoe.LAZY和FetchType.EAGER(@ManyToOne注解的默认值)，LAZY是指真正用到关联实体才会去数据库获取
mappedBy	否	该属性指定关联实体的哪一个属性用于维护关联关系
targetEntity	否	该属性指定关联实体的类名，默认情况下，Hibernate将通过反射来判断关联实体的类名

由Teacher控制关系：

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

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;
import javax.persistence.Table;

@Entity
@Table(name="teacher")
public class Teacher {

    private int id;
    private String name;
    private String description;
    private Set<Student> students = new HashSet<Student>();

    @Id
    @GeneratedValue
    public int getId() {
        return id;
    }

    @Column(length=50)
    public String getName() {
        return name;
    }

    public String getDescription() {
        return description;
    }

    @ManyToMany(cascade=CascadeType.ALL)
    //由于是多对多，所以unique要置为falses
    @JoinTable(
            name="teacher_student",
            joinColumns={           
                    @JoinColumn(    
                            name="teacher_id",  
                            referencedColumnName="id",

                            unique=false    
                    )
            },
            inverseJoinColumns={    
                    @JoinColumn(
                            name="student_id",
                            referencedColumnName="id",
                            unique=false
                    )
            }
    )
    public Set<Student> getStudents() {
        return students;
    }

    public void setId(int id) {
        this.id = id;
    }

    public void setStudents(Set<Student> students) {
        this.students = students;
    }

    public void setName(String name) {
        this.name = name;
    }

    public void setDescription(String description) {
        this.description = description;
    }
}

Student持久化类：

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;

import javax.persistence.Table;

@Entity
@Table(name="student")
public class Student {

    private int id;
    private String name;

    @Id
    @GeneratedValue
    public int getId() {
        return id;
    }

    @Column(length=50)
    public String getName() {
        return name;
    }

    public void setId(int id) {
        this.id = id;
    }

    public void setName(String name) {
        this.name = name;
    }

}

控制台打印的建表语句
这里写图片描述

XML方式

teacher.hbm.xml文件

<?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="pers.msidolphin.hibernate.many2many">
    <class name="Teacher" table="teacher">
        <id name="id" column="id">
            <generator class="native"></generator>
        </id>
        <property name="name" column="name" length="50"/>
        <property name="description" column="description"/>
        <!-- table属性设置连接表的表名 -->
        <set name="students" table="teacher_student">
            <!-- key属性设置当前表在连接表中的外键字段 -->
            <key column="teacher_id" unique="false" ></key>
            <many-to-many class="pers.msidolphin.hibernate.many2many.Student">
                <column name="student_id" unique="false"></column>
            </many-to-many>
        </set>
    </class>
</hibernate-mapping>

student.hbm.xml文件

<?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="pers.msidolphin.hibernate.many2many">
    <class name="Student" table="student">
        <id name="id" column="id">
            <generator class="native"></generator>
        </id>
        <property name="name" column="name" length="50"/>
    </class>
</hibernate-mapping>

结果和注解相同

双向N-N关联

双向N-N关联需要两端都使用Set集合用来存储关联持久化实例对象，并且都要生成相应的Getter和Setter方法，增加对集合属性的访问

Annotation方式

双向N-N关联需要在两端分别使用@ManyToMany注解修饰集合属性，并在两端都使用@JoinTable注解映射连接表，并且两端指定的连接表的表名和外键列名应该相同

Teacher持久化类：

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

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;
import javax.persistence.Table;

@Entity
@Table(name="teacher")
public class Teacher {

    private int id;
    private String name;
    private String description;
    private Set<Student> students = new HashSet<Student>();

    @Id
    @GeneratedValue
    public int getId() {
        return id;
    }

    @Column(length=50)
    public String getName() {
        return name;
    }

    public String getDescription() {
        return description;
    }

    @ManyToMany(cascade=CascadeType.ALL)
    //由于是多对多，所以unique要置为falses
    @JoinTable(
            name="teacher_student",
            joinColumns={           
                    @JoinColumn(    
                            name="teacher_id",  
                            referencedColumnName="id",
                            unique=false    
                    )
            },
            inverseJoinColumns={    
                    @JoinColumn(
                            name="student_id",
                            referencedColumnName="id",
                            unique=false
                    )
            }
    )
    public Set<Student> getStudents() {
        return students;
    }

    public void setId(int id) {
        this.id = id;
    }

    public void setStudents(Set<Student> students) {
        this.students = students;
    }

    public void setName(String name) {
        this.name = name;
    }

    public void setDescription(String description) {
        this.description = description;
    }
}

Student持久化类

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

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;
import javax.persistence.Table;

@Entity
@Table(name="student")
public class Student {

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

    @Id
    @GeneratedValue
    public int getId() {
        return id;
    }

    @Column(length=50)
    public String getName() {
        return name;
    }


    @ManyToMany(cascade=CascadeType.ALL)
    @JoinTable(
            name="teacher_student",
            joinColumns={
                    @JoinColumn(
                            name="student_id",
                            referencedColumnName="id",
                            unique=false

                    )
            },
            inverseJoinColumns={
                    @JoinColumn(
                            name="teacher_id",
                            referencedColumnName="id",
                            unique=false
                    )
            }
    )
    public Set<Teacher> getTeachers() {
        return teachers;
    }


    public void setId(int id) {
        this.id = id;
    }

    public void setName(String name) {
        this.name = name;
    }


    public void setTeachers(Set<Teacher> teachers) {
        this.teachers = teachers;
    }

}

控制台打印的建表语句：
这里写图片描述
注：如果希望某一方放弃控制关联关系，则可以在这一端的@ManyToMany注解中指定mappedBy属性，那么这一端就不需要使用@JoinTable注解映射连接表了
如下所示：Teacher持久化类放弃控制关联关系

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

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;
import javax.persistence.Table;

@Entity
@Table(name="teacher")
public class Teacher {

    private int id;
    private String name;
    private String description;
    private Set<Student> students = new HashSet<Student>();

    @Id
    @GeneratedValue
    public int getId() {
        return id;
    }

    @Column(length=50)
    public String getName() {
        return name;
    }

    public String getDescription() {
        return description;
    }

    @ManyToMany(mappedBy="teachers", cascade=CascadeType.ALL)
    public Set<Student> getStudents() {
        return students;
    }

    public void setId(int id) {
        this.id = id;
    }

    public void setStudents(Set<Student> students) {
        this.students = students;
    }

    public void setName(String name) {
        this.name = name;
    }

    public void setDescription(String description) {
        this.description = description;
    }
}

XML方式

两端都要使用<set>标签，注意两端连接表名和外键字段名一致即可
teacher.hbm.xml

<?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="pers.msidolphin.hibernate.many2many">
    <class name="Teacher" table="teacher">
        <id name="id" column="id">
            <generator class="native"></generator>
        </id>
        <property name="name" column="name" length="50"/>
        <property name="description" column="description"/>
        <!-- table属性设置连接表的表名 -->
        <set name="students" table="teacher_student">
            <!-- key属性设置当前表在连接表中的外键字段 -->
            <key column="teacher_id" unique="false" ></key>
            <many-to-many class="pers.msidolphin.hibernate.many2many.Student">
                <column name="student_id" unique="false"></column>
            </many-to-many>
        </set>
    </class>
</hibernate-mapping>

student.hbm.xml

<?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="pers.msidolphin.hibernate.many2many">
    <class name="Student" table="student">
        <id name="id" column="id">
            <generator class="native"></generator>
        </id>
        <property name="name" column="name" length="50"/>
        <set name="teachers" table="teacher_student">
            <key column="student_id" unique="false"></key>
            <many-to-many class="pers.msidolphin.hibernate.many2many.Teacher">
                <column name="teacher_id" unique="false"></column>
            </many-to-many>
        </set>
    </class>
</hibernate-mapping>