hibernate学习五（关系映射多对一与一对多）

一、多对一

　　多对一（或者一对多）：在学生与老师的情况下，一个老师可以教多个学生，但一个学生只能被教一个老师教；

　　对于类：在多的那方拥有一的那方的一个实体

二、修改student.java和teacher.java类

　　复制03工程，并更名为Hibernate_04_ManyToOne；

　　在多的一方中添加@ManyToOne注解

 package com.model;

 import javax.persistence.Embedded;
 import javax.persistence.Entity;
 import javax.persistence.GeneratedValue;
 import javax.persistence.Id;
 import javax.persistence.ManyToOne;
 import javax.persistence.OneToOne;
 import javax.persistence.Table;

 @Entity
 @Table(name="student")
 public class Student {
     private int id;
     private String name;
     private int age;
     private Teacher teacher;

     @Id
     @GeneratedValue
     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 int getAge() {
         return age;
     }
     public void setAge(int age) {
         this.age = age;
     }
     @ManyToOne
     public Teacher getTeacher() {
         return teacher;
     }
     public void setTeacher(Teacher teacher) {
         this.teacher = teacher;
     }
 }

 package com.model;

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

 @Entity
 @Table(name="teacher")
 public class Teacher {
     private int id;
     private String teacherName;
     private int teacherAge;
     private String title;

     @Id
     @GeneratedValue
     public int getId() {
         return id;
     }
     public void setId(int id) {
         this.id = id;
     }
     public String getTeacherName() {
         return teacherName;
     }
     public void setTeacherName(String teacherName) {
         this.teacherName = teacherName;
     }
     public int getTeacherAge() {
         return teacherAge;
     }
     public void setTeacherAge(int teacherAge) {
         this.teacherAge = teacherAge;
     }
     public String getTitle() {
         return title;
     }
     public void setTitle(String title) {
         this.title = title;
     }
 }

修改xml，添加使用teacher.java类的注解的配置；

        <mapping class="com.model.Teacher"/>

三、运行测试；

　　

　　

　　结果：

　　在student表中生成了一个外键，依赖teacher表，teacher表中没有特别的改变；

四、一对多

　　一对多：对于类，在一的那方存在多的那方的集合

五、修改student和teahcer类

　　复制04工程，并更名为Hibernate_05_OneToMany；

　　在一的那方添加集合类（推荐set），并添加@OneToMany注解

 package com.model;

 import javax.persistence.Embedded;
 import javax.persistence.Entity;
 import javax.persistence.GeneratedValue;
 import javax.persistence.Id;
 import javax.persistence.ManyToOne;
 import javax.persistence.OneToOne;
 import javax.persistence.Table;

 @Entity
 @Table(name="student")
 public class Student {
     private int id;
     private String name;
     private int age;

     @Id
     @GeneratedValue
     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 int getAge() {
         return age;
     }
     public void setAge(int age) {
         this.age = age;
     }
 }

 package com.model;

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

 import javax.persistence.Entity;
 import javax.persistence.GeneratedValue;
 import javax.persistence.Id;
 import javax.persistence.OneToMany;
 import javax.persistence.Table;

 @Entity
 @Table(name="teacher")
 public class Teacher {
     private int id;
     private String teacherName;
     private int teacherAge;
     private String title;
     private Set<Student> students = new HashSet<Student>();

     @Id
     @GeneratedValue
     public int getId() {
         return id;
     }
     public void setId(int id) {
         this.id = id;
     }
     public String getTeacherName() {
         return teacherName;
     }
     public void setTeacherName(String teacherName) {
         this.teacherName = teacherName;
     }
     public int getTeacherAge() {
         return teacherAge;
     }
     public void setTeacherAge(int teacherAge) {
         this.teacherAge = teacherAge;
     }
     public String getTitle() {
         return title;
     }
     public void setTitle(String title) {
         this.title = title;
     }
     @OneToMany
     public Set<Student> getStudents() {
         return students;
     }
     public void setStudents(Set<Student> students) {
         this.students = students;
     }
 }

六、测试运行

　　

　　结果生成了一张中间表teacher_student

　　

PS：关于双向关联，可以在studenjava中添加字段Teacher teacher，并使用注解@ManyToOne；

PSS：有双向关联的情况，加上mappingBy；

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