Hibernate框架-多对多关联

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

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：

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>