(單向關聯)
一 思路分析
1. 多對多(Teacher – Student)的單向關聯
首先考慮一個老師可以有多個學生的問題
2. 多對多可以看成兩個多對一,從多對多的分析上來看,如果從多個老師中查找多個學生,
不管是通過學生的id還是通過的是老師的id都會有冗餘,所以最好有個中間表來關聯這兩張表,中間表中有兩列,分別是老師的id和學生的id
二 代碼分析
1.Teacher類,在單向關聯中多對多中是一的一方
package com.hbsi.domain;
import java.util.Set;
//單向一對多 Teacher 一 對 多 Student
public class Teacher {
private int id;
private String name;
private Set<Student> students;//一個老師可以有多個學生
public Teacher() {
super();
// TODO Auto-generated constructor stub
}
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 Set<Student> getStudents() {
return students;
}
public void setStudents(Set<Student> students) {
this.students = students;
}
}
2.Studenet類 相對於一的一方多的一方
package com.hbsi.domain;
import java.util.Set;
public class Student {
private int id;
private String name;
public Student() {
super();
// TODO Auto-generated constructor stub
}
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;
}
}
3. 在Teacher中的映射文件 Teacher.hbm.xml文件
此映射文件因爲是多對一,所以建表的時候在這裏建一箇中間表用來關聯這兩個表
<?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="com.hbsi.domain">
<class name="Teacher" table="teacher">
<id name="id" column="id">
<generator class="native" />
</id>
<property name="name" column="name" />
<!-- 集合屬性的體現 多對多 table屬性指定的是中間表的名字 -->
<set name="students" table="teacher_student">
<!--查老師的id的 和中間表聯繫-->
<key column="teacher_id"/>
<!-- 查學生的id和中間表的關係 -->
<many-to-many class="Student" column="student_id"></many-to-many>
</set>
</class>
</hibernate-mapping>
4. 在Student中的映射文件 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="com.hbsi.domain">
<class name="Student" table="student">
<id name="id" column="id">
<generator class="native" />
</id>
<property name="name" column="name" />
</class>
</hibernate-mapping>
5. 測試
public class Many2Many {
public static void main(String[] args) {
add();
query(1);
}
//保存
public static void add() {
Session session = null;
Transaction transaction = null;
try {
session = HibernateUtil.getSession();
transaction = session.beginTransaction();
// 添加
Teacher t1 = new Teacher();
t1.setName("王老師");
Teacher t2 = new Teacher();
t2.setName("李老師");
Student s1 = new Student();
s1.setName("王同學");
Student s2 = new Student();
s2.setName("李同學");
Set<Student> ss = new HashSet<Student>();
ss.add(s1);
ss.add(s2);
t1.setStudents(ss);// 告訴Hibernate老師教了那兩個學生
t2.setStudents(ss);
session.save(t1);
session.save(t2);
session.save(s1);
session.save(s2);
transaction.commit();
} finally {
if (session != null) {
session.close();
}
}
}
//查找
public static void query(int id) {
Session session = null;
try {
session = HibernateUtil.getSession();
Teacher teacher = (Teacher) session.get(Teacher.class, id);
System.out.println("教室的姓名:" + teacher.getName());
System.out.println("教室的學生列表:");
Iterator<Student> iterator = teacher.getStudents().iterator();
while (iterator.hasNext()) {
Student student = iterator.next();
System.out.println(student.getName() + " ");
}
System.out.println();
} finally {
if (session != null) {
session.close();
}
}
}
}
注:在建表的時候很可能會出錯的,所以在映射文件的時候必須注意
在Teacher.hbm.xml中最核心的,在整個多對多中對核心的映射語句爲:
<set name="students" table="teacher_student">
<key column="teacher_id"/>
<many-to-many class="Student" column="student_id"></many-to-many>
</set>
說明:此標籤是在相對於多的一方,一的一方的映射文件,用set標籤,因爲映射的時候必須要有一箇中間表來關聯,所以在set標籤創建一個表,表中的字段就是Teacher中的id和Student中的id。通過中間表可以查找到Teacher中的id,通過Teacher中的id可以得到Student中的id,來查找到Student中的id,這樣,這兩個表通過中間表而關聯起來
(雙向關聯)
一 概念分析
1.多對多的經典實例(Teacher – Student)
二 代碼分析
1.Teacher類
package com.hbsi.domain;
import java.util.Set;
//單向一對多 Teacher 一 對 多 Student
public class Teacher {
private int id;
private String name;
private Set<Student> students;
public Teacher() {
super();
// TODO Auto-generated constructor stub
}
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 Set<Student> getStudents() {
return students;
}
public void setStudents(Set<Student> students) {
this.students = students;
}
}
2.Stduent類
package com.hbsi.domain;
import java.util.Set;
public class Student {
private int id;
private String name;
private Set<Teacher> teachers;
public Student() {
super();
// TODO Auto-generated constructor stub
}
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 Set<Teacher> getTeachers() {
return teachers;
}
public void setTeachers(Set<Teacher> teachers) {
this.teachers = teachers;
}
}
3.Teacher的映射文件 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="com.hbsi.domain">
<class name="Teacher" table="teacher">
<id name="id" column="id">
<generator class="native" />
</id>
<property name="name" column="name" />
<!-- 集合屬性的體現 多對多 table屬性指定的是中間表的名字 -->
<set name="students" table="teacher_student">
<!--查中間表的外鍵 和老師的id的關聯 老師的id-->
<key column="teacher_id"/>
<!-- 查學生的id和中間表的關係 -->
<many-to-many class="Student" column="student_id"></many-to-many>
</set>
</class>
</hibernate-mapping>
4.Student的映射文件 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="com.hbsi.domain">
<class name="Student" table="student">
<id name="id" column="id">
<generator class="native" />
</id>
<property name="name" column="name" />
<set name="teachers" table="teacher_student">
<key column="student_id"></key><!-- 查找這個學生被那個老師教 -->
<many-to-many class="Teacher" column="teacher_id"></many-to-many>
</set>
</class>
</hibernate-mapping>
5.測試類
public class Many2Many {
public static void main(String[] args) {
// add();
query(2);
}
//保存
public static void add() {
Session session = null;
Transaction transaction = null;
try {
session = HibernateUtil.getSession();
transaction = session.beginTransaction();
// 添加
Teacher t1 = new Teacher();
t1.setName("王老師");
Teacher t2 = new Teacher();
t2.setName("李老師");
Student s1 = new Student();
s1.setName("王同學");
Student s2 = new Student();
s2.setName("李同學");
Set<Student> ss = new HashSet<Student>();
ss.add(s1);
ss.add(s2);
t1.setStudents(ss);// 告訴Hibernate老師教了那兩個學生
t2.setStudents(ss);
session.save(t1);
session.save(t2);
session.save(s1);
session.save(s2);
transaction.commit();
} finally {
if (session != null) {
session.close();
}
}
}
//查詢
public static void query(int id) {
Session session = null;
try {
session = HibernateUtil.getSession();
Teacher teacher = (Teacher) session.get(Teacher.class, id);
System.out.println("教室的姓名:" + teacher.getName());
System.out.println("教室的學生列表:");
Iterator<Student> iterator = teacher.getStudents().iterator();
while (iterator.hasNext()) {
Student student = iterator.next();
System.out.println(student.getName() + " ");
}
System.out.println( );
Student student = (Student) session.get(Student.class, id);
System.out.println("學生的姓名:"+student.getName());
System.out.println("學生被教的老師:");
Iterator<Teacher> iterator2 = student.getTeachers().iterator();
while(iterator2.hasNext()){
Teacher teacher2 = iterator2.next();
System.out.println(teacher2.getName()+" ");
}
System.out.println();
} finally {
if (session != null) {
session.close();
}
}
}
}
注:
(1)Set<Teacher> tt = new HashSet<Teacher>();
tt.add(t1);
tt.add(t2);
s1.setTeachers(tt);
s2.setTeachers(tt);
如果在測試保存的時候添加會報錯,因爲前邊已經添加了,已經更新數據庫了,再更新會報
org.hibernate.exception.ConstraintViolationException錯誤,所以要注意
(2)映射文件中的信息一定要寫對
<set name="students" table="teacher_student">
<key column="teacher_id"/>
<many-to-many class="Student" column="student_id"></many-to-many>
</set>