本章主要是描述几种经典映射关系，顺带比较Hibernate4.x和Hibernate5.x之间的区别。

一、建立测试工程目录

有关实体类之间的相互映射关系，Hibernate官方文档其实描述的非常详细，这里只提供几种常见映射。（推荐4.3.11版本的 hibernate-release-4.3.11.Final\documentation\manual）

二、编写映射关系

（1）one2one单表内嵌映射：

packagemodel.family;importjavax.persistence.Embedded;importjavax.persistence.Entity;importjavax.persistence.GeneratedValue;importjavax.persistence.Id;@Entitypublic classHusband {private intid;privateString husbandName;privateWife wife;@Id@GeneratedValuepublic intgetId() {returnid;}public void setId(intid) {this.id =id;}publicString getHusbandName() {returnhusbandName;}public voidsetHusbandName(String husbandName) {this.husbandName =husbandName;}//两个实体对象共用一张数据表，提高查询速度
@EmbeddedpublicWife getWife() {returnwife;}public voidsetWife(Wife wife) {this.wife =wife;}}

packagemodel.family;//不用添加任何注解，持久化过程通过主表完成
public classWife {privateString wifeName;publicString getWifeName() {returnwifeName;}public voidsetWifeName(String wifeName) {this.wifeName =wifeName;}
}

（2）one2one外键映射：

packagemodel.userinfo;importjavax.persistence.CascadeType;importjavax.persistence.Entity;importjavax.persistence.FetchType;importjavax.persistence.GeneratedValue;importjavax.persistence.Id;importjavax.persistence.OneToOne;importjavax.persistence.Transient;@Entitypublic classUser {private intid;privateString username;privateString password;privateString confirm;privateInformation info;@Id@GeneratedValuepublic intgetId() {returnid;}public void setId(intid) {this.id =id;}publicString getUsername() {returnusername;}public voidsetUsername(String username) {this.username =username;}publicString getPassword() {returnpassword;}public voidsetPassword(String password) {this.password =password;}/** 延迟加载，级联操作。 * 删除开启了级联的一方，被级联的一方也会被删除* 注意：如果session的操作是通过hibernate控制，延迟加载不会出问题。如果是通过手工开启实物，操作不当延迟加载可能抛出懒加载异常*/@OneToOne(fetch= FetchType.LAZY, mappedBy = "user", cascade =CascadeType.ALL)publicInformation getInfo() {returninfo;}public voidsetInfo(Information info) {this.info =info;}//本字段不参与持久化过程
@TransientpublicString getConfirm() {returnconfirm;}public voidsetConfirm(String confirm) {this.confirm =confirm;}
}

packagemodel.userinfo;importjava.util.Date;importjavax.persistence.Entity;importjavax.persistence.GeneratedValue;importjavax.persistence.Id;importjavax.persistence.OneToOne;importjavax.persistence.Temporal;importjavax.persistence.TemporalType;//注解也可以直接配置在字段上，但是不推荐。据说原因是可能破坏oop封装。但是我觉得有时这样配置可以让代码显得更加整洁，特别是在Spring中。
@Entitypublic classInformation {@Id@GeneratedValueprivate intid;@OneToOneprivateUser user;@Temporal(TemporalType.DATE)privateDate resgisterDate;privateString address;public intgetId() {returnid;}public void setId(intid) {this.id =id;}publicUser getUser() {returnuser;}public voidsetUser(User user) {this.user =user;}publicDate getResgisterDate() {returnresgisterDate;}public voidsetResgisterDate(Date resgisterDate) {this.resgisterDate =resgisterDate;}publicString getAddress() {returnaddress;}public voidsetAddress(String address) {this.address =address;}}

（3）many2many多表映射：

场景描述：学校里有多个老师，每个老师教授多个学生，每个学生每一门课程会有一个得分。

 packagemodel.school;importjava.util.HashSet;importjava.util.Set;importjavax.persistence.Entity;importjavax.persistence.GeneratedValue;importjavax.persistence.Id;importjavax.persistence.ManyToMany;@Entitypublic classTeacher {private intid;privateString tchName;private Set<Student> students = new HashSet<Student>();@Id@GeneratedValuepublic intgetId() {returnid;}public void setId(intid) {this.id =id;}publicString getTchName() {returntchName;}public voidsetTchName(String tchName) {this.tchName =tchName;}//老师和学生的对应表由学生一方负责维护@ManyToMany(mappedBy = "teachers")public Set<Student>getStudents() {returnstudents;}public void setStudents(Set<Student>students) {this.students =students;}}

packagemodel.school;importjava.util.HashSet;importjava.util.Set;importjavax.persistence.Entity;importjavax.persistence.GeneratedValue;importjavax.persistence.Id;importjavax.persistence.JoinColumn;importjavax.persistence.JoinTable;importjavax.persistence.ManyToMany;importjavax.persistence.OneToMany;@Entitypublic classStudent {private intid;privateString stuName;private Set<Teacher> teachers = new HashSet<Teacher>();private Set<Score> scores = new HashSet<Score>();@Id@GeneratedValuepublic intgetId() {returnid;}public void setId(intid) {this.id =id;}publicString getStuName() {returnstuName;}public voidsetStuName(String stuName) {this.stuName =stuName;}/** many2many必须使用中间表，配置中间表的表明和列名*/@ManyToMany@JoinTable(name= "student_teacher", joinColumns = { @JoinColumn(name = "studentId") }, inverseJoinColumns ={@JoinColumn(name= "teacherId") })public Set<Teacher>getTeachers() {returnteachers;}public void setTeachers(Set<Teacher>teachers) {this.teachers =teachers;}//学生同分数之间的关系同样交给多的一方负责维护@OneToMany(mappedBy = "student")public Set<Score>getScores() {returnscores;}public void setScores(Set<Score>scores) {this.scores =scores;}}

packagemodel.school;importjavax.persistence.Entity;importjavax.persistence.GeneratedValue;importjavax.persistence.Id;importjavax.persistence.ManyToOne;@Entitypublic classScore {private intid;private intcourseScore;privateTeacher teacher;privateStudent student;@Id@GeneratedValuepublic intgetId() {returnid;}public void setId(intid) {this.id =id;}public intgetCourseScore() {returncourseScore;}public void setCourseScore(intcourseScore) {this.courseScore =courseScore;}@ManyToOnepublicTeacher getTeacher() {returnteacher;}public voidsetTeacher(Teacher teacher) {this.teacher =teacher;}@ManyToOnepublicStudent getStudent() {returnstudent;}public voidsetStudent(Student student) {this.student =student;}}

按照以上的映射关系生成数据表以后会注意到，其实老师和学生之间的关系表纯粹多余，分数表已经维护了双方的关系。重新优化他们之间的映射关系：

packagemodel.school;importjavax.persistence.Entity;importjavax.persistence.GeneratedValue;importjavax.persistence.Id;@Entitypublic classTeacher {private intid;privateString tchName;@Id@GeneratedValuepublic intgetId() {returnid;}public void setId(intid) {this.id =id;}publicString getTchName() {returntchName;}public voidsetTchName(String tchName) {this.tchName =tchName;}}

packagemodel.school;importjava.util.HashSet;importjava.util.Set;importjavax.persistence.Entity;importjavax.persistence.GeneratedValue;importjavax.persistence.Id;importjavax.persistence.OneToMany;@Entitypublic classStudent {private intid;privateString stuName;private Set<Score> scores = new HashSet<Score>();@Id@GeneratedValuepublic intgetId() {returnid;}public void setId(intid) {this.id =id;}publicString getStuName() {returnstuName;}public voidsetStuName(String stuName) {this.stuName =stuName;}@OneToMany(mappedBy= "student")public Set<Score>getScores() {returnscores;}public void setScores(Set<Score>scores) {this.scores =scores;}}

packagemodel.school;importjavax.persistence.Entity;importjavax.persistence.GeneratedValue;importjavax.persistence.Id;importjavax.persistence.ManyToOne;@Entitypublic classScore {private intid;private intcourseScore;privateTeacher teacher;privateStudent student;@Id@GeneratedValuepublic intgetId() {returnid;}public void setId(intid) {this.id =id;}public intgetCourseScore() {returncourseScore;}public void setCourseScore(intcourseScore) {this.courseScore =courseScore;}@ManyToOnepublicTeacher getTeacher() {returnteacher;}public voidsetTeacher(Teacher teacher) {this.teacher =teacher;}@ManyToOnepublicStudent getStudent() {returnstudent;}public voidsetStudent(Student student) {this.student =student;}}

由此可见，即使是一个相对复杂的映射关系也可以通过优化得到一个相对简单的数据模型。

（4）many2one和one2many单表树形映射：

场景描述：地图，一个国家包含多个省份，每个省份又包含多个城市...

packagemodel.tree;importjava.util.HashSet;importjava.util.Set;importjavax.persistence.CascadeType;importjavax.persistence.Column;importjavax.persistence.Entity;importjavax.persistence.GeneratedValue;importjavax.persistence.Id;importjavax.persistence.ManyToOne;importjavax.persistence.OneToMany;importjavax.persistence.Table;@Entity
@Table(name= "_tree")public classTree {private intid;privateString name;privateTree parent;private Set<Tree> children = new HashSet<Tree>();@Id@GeneratedValuepublic intgetId() {returnid;}public void setId(intid) {this.id =id;}@Column(name= "t_name", unique = true)publicString getName() {returnname;}public voidsetName(String name) {this.name =name;}@ManyToOnepublicTree getParent() {returnparent;}public voidsetParent(Tree parent) {this.parent =parent;}//删除根节点，与它相关的所有子节点全部删除@OneToMany(mappedBy = "parent", cascade =CascadeType.ALL)public Set<Tree>getChildren() {returnchildren;}public void setChildren(Set<Tree>children) {this.children =children;}
}

注意：以上4种映射关系在4.3.11版本中正常。但在5.0.6版本中id字段被系统强制指定为了@GeneratedValue(strategy=GenerationType.TABLE)的方式。我曾经尝试手工指定生成策略为auto或者identity均无效。如果是通过xml的方式配置是正常的，目前我还不清楚是什么原因导致的上述异常。这个问题造成了下面的映射关系目前只能在4.x版本中正常使用：

（5）one2one主键映射

packagemodel.personaddr;importjavax.persistence.Entity;importjavax.persistence.GeneratedValue;importjavax.persistence.Id;importjavax.persistence.OneToOne;importjavax.persistence.PrimaryKeyJoinColumn;@Entitypublic classPerson {private intid;privateString name;privateAddress address;@Id@GeneratedValuepublic intgetId() {returnid;}public void setId(intid) {this.id =id;}publicString getName() {returnname;}public voidsetName(String name) {this.name =name;}//两张表通过主键关联@OneToOne(optional = true)@PrimaryKeyJoinColumnpublicAddress getAddress() {returnaddress;}public voidsetAddress(Address address) {this.address =address;}
}

packagemodel.personaddr;importjavax.persistence.Entity;importjavax.persistence.GeneratedValue;importjavax.persistence.Id;importjavax.persistence.OneToOne;@Entitypublic classAddress {private intid;privateString local;privatePerson person;@Id@GeneratedValuepublic intgetId() {returnid;}public void setId(intid) {this.id =id;}publicString getLocal() {returnlocal;}public voidsetLocal(String local) {this.local =local;}@OneToOne(mappedBy= "address")publicPerson getPerson() {returnperson;}public voidsetPerson(Person person) {this.person =person;}}

ps：好在主键映射在实际使用中并不常见。

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最后按照惯例，提供整个项目的完整目录结构和IDE版本信息