不管是双向还是单向关联，数据库表的设计是一样的，一般都是在N的一方增加外键(如果在1的一方增加外键势必会造成冗余)

单向N-1关联

以Group和User为例，一个Group可以有多个User，一个User只能对应一个Group，典型的一对多(多对一)的案列

Annotation方式

对于N-1关联(无论单向还是双向)，都需要在N的一方给关联属性加上@ManyToOne注解 @ManyToOne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optional否该属性指定关联关系是否可选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

User实体：

import javax.persistence.Column; import javax.persistence.Entity; import javax.persistence.GeneratedValue; import javax.persistence.Id; import javax.persistence.JoinColumn; import javax.persistence.ManyToOne; import javax.persistence.Table; @Entity @Table(name="t_user") public class User { private int id; private String name; private Group group; @Id @GeneratedValue public int getId() { return id; } @Column(length=50) public String getName() { return name; } //关联实体属性加上@ManyToOne注解 @ManyToOne //指定映射的外键字段 @JoinColumn( name="group_id", referencedColumnName="id", unique=false ) public Group getGroup() { return group; } public void setGroup(Group group) { this.group = group; } public void setId(int id) { this.id = id; } public void setName(String name) { this.name = name; } }

Group实体：

import javax.persistence.Column; import javax.persistence.Entity; import javax.persistence.GeneratedValue; import javax.persistence.Id; import javax.persistence.Table; @Entity @Table(name="t_group") public class Group { private int id; private String name; private String description; @Id @GeneratedValue public int getId() { return id; } @Column(length=50) public String getName() { return name; } public String getDescription() { return description; } public void setId(int id) { this.id = id; } public void setName(String name) { this.name = name; } public void setDescription(String description) { this.description = description; } }

控制台输出的建表语句:

XML方式

user.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.many2one"> <class name="User" table="t_user"> <id name="id" column="id"> <generator class="native"></generator> </id> <property name="name" column="name" length="50"/> <many-to-one name="group"> <column name="group_id" unique="false"></column> </many-to-one> </class> </hibernate-mapping>

group.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.many2one"> <class name="Group" table="t_group"> <id name="id" column="id"> <generator class="native"></generator> </id> <property name="name" column="name" length="50"/> <property name="description" column="description"/> </class> </hibernate-mapping>

结果同上

单向1-N关联

单向1-N关联的实体类设计不同与N-1关联，需要在1的一方增加集合属性，该集合用于存储关联实体.

Annotation方式

为了映射1-N关联，需要在集合属性上加上@OneToMany注解 @OneToMany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orphanRemoval否该属性设置某个实体所关联的父实体不存在(即该实体对应记录的外键为null)时是否删除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(外键)，对于双向一定要在1的一方设置mappedBy，否则双方都会建立外键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

Group实体：

import java.util.HashSet; import java.util.Set; import javax.persistence.Column; import javax.persistence.Entity; import javax.persistence.GeneratedValue; import javax.persistence.Id; import javax.persistence.JoinColumn; import javax.persistence.OneToMany; import javax.persistence.Table; @Entity @Table(name="t_group") public class Group { private int id; private String name; private String description; private Set<User> users = new HashSet<User>(); @Id @GeneratedValue public int getId() { return id; } @Column(length=50) public String getName() { return name; } public String getDescription() { return description; } //集合属性加上了@OneToMany注解 @OneToMany //显式指定映射的外键字段 @JoinColumn( name="group_id", referencedColumnName="id", unique=false ) public Set<User> getUsers() { return users; } public void setId(int id) { this.id = id; } public void setUsers(Set<User> users) { this.users = users; } public void setName(String name) { this.name = name; } public void setDescription(String description) { this.description = description; } }

User实体:

import javax.persistence.Column; import javax.persistence.Entity; import javax.persistence.GeneratedValue; import javax.persistence.Id; import javax.persistence.Table; @Entity @Table(name="t_user") public class User { 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; } }

控制台输出的建表语句: 可以看到和前面的多对一是一样的，因为外键总是处于多的一方的

XML方式

group.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.one2many"> <class name="Group" table="t_group"> <id name="id" column="id"> <generator class="native"></generator> </id> <property name="name" column="name" length="50"/> <property name="description" column="description"/> <!--映射集合属性--> <set name="users"> <key column="group_id" unique="false"></key> <!--关联属性完整类名--> <one-to-many class="pers.msidolphin.hibernate.one2many.User"/> </set> </class> </hibernate-mapping>

结果同上

双向N-1 or 1-N关联

关于双向的N-1或1-N关联实际上是一样的，双方都需要增加对关联属性的访问，N的一方增加关联实体的引用属性，1的一方增加集合属性，集合元素为关联实体

Annotation方式

对于双向关联，一定要设置mappedBy属性，而且不要让1的一方控制关联关联，而是N方控制关联关系(即外键总是建立在多的一方的)，并且在多的一方设置@ManyToOne注解，在1的一方设置@OneToMany注解 User实体:

import javax.persistence.Column; import javax.persistence.Entity; import javax.persistence.GeneratedValue; import javax.persistence.Id; import javax.persistence.JoinColumn; import javax.persistence.ManyToOne; import javax.persistence.Table; @Entity @Table(name="t_user") public class User { private int id; private String name; private Group group; @Id @GeneratedValue public int getId() { return id; } @Column(length=50) public String getName() { return name; } @ManyToOne(cascade={javax.persistence.CascadeType.ALL}) @JoinColumn( name="group_id", referencedColumnName="id", unique=false ) public Group getGroup() { return group; } public void setId(int id) { this.id = id; } public void setName(String name) { this.name = name; } public void setGroup(Group group) { this.group = group; } }

Group实体:

package pers.msidolphin.hibernate.one2many_many2one; import java.util.HashSet; import java.util.Set; import javax.persistence.Column; import javax.persistence.Entity; import javax.persistence.GeneratedValue; import javax.persistence.Id; import javax.persistence.OneToMany; import javax.persistence.Table; @Entity @Table(name="t_group") public class Group { private int id; private String name; private String description; private Set<User> users = new HashSet<User>(); @Id @GeneratedValue public int getId() { return id; } @Column(length=50) public String getName() { return name; } public String getDescription() { return description; } //一定要记得设置mappedBy @OneToMany(mappedBy="group") public Set<User> getUsers() { return users; } public void setId(int id) { this.id = id; } public void setUsers(Set<User> users) { this.users = users; } public void setName(String name) { this.name = name; } public void setDescription(String description) { this.description = description; } }

控制台输出的建表语句:

XML方式

group.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.one2many_many2one"> <class name="Group" table="t_group"> <id name="id" column="id"> <generator class="native"></generator> </id> <property name="name" column="name" length="50"/> <property name="description" column="description"/> <set name="users"> <!-- 注意：对于xml方式的多对一/一对多双向关联，指定的映射字段名称时两端务必保持一致，否则会创建两个外键 --> <key column="group_id" unique="false"></key> <one-to-many class="pers.msidolphin.hibernate.one2many_many2one.User"/> </set> </class> </hibernate-mapping>

user.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.one2many_many2one"> <class name="User" table="t_user"> <id name="id" column="id"> <generator class="native"></generator> </id> <property name="name" column="name" length="50"/> <many-to-one name="group"> <!-- 注意：对于xml方式的多对一/一对多双向关联，指定的映射字段名称时两端务必保持一致，否则会创建两个外键 --> <column name="group_id" unique="false"></column> </many-to-one> </class> </hibernate-mapping>