班級和學生一對多雙向對映

Class.java

package com.java1234.model;
import java.util.HashSet;
import java.util.Set;
public class Class {
private long id;
private String name;
private Set<Student> students=new HashSet<Student>();
public long getId() {
return id;
}
public void setId(long 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;
}

}

Student.java

package com.java1234.model;
public class Student {
private long id;
private String name;
private Class c;
public long getId() {
return id;
}
public void setId(long id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Class getC() {
return c;
}
public void setC(Class c) {
this.c = c;
}
@Override
public String toString() {
return "Student [id=" + id + ", name=" + name + "]";
}

}

Class.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.java1234.model">
<class name="Class" table="t_class">
<id name="id" column="classId">
<generator class="native"></generator>
</id>
<property name="name" column="className"></property>
<set name="students" cascade="delete" inverse="true">
<key column="classId"></key>
<one-to-many class="com.java1234.model.Student"/>
</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="com.java1234.model">
<class name="Student" table="t_student">
<id name="id" column="stuId">
<generator class="native"></generator>
</id>
<property name="name" column="stuName"></property>
<many-to-one name="c" column="classId" class="com.java1234.model.Class" cascade="save-update">                     </many-to-one>

</class>

</hibernate-mapping>

StudentTest.java

package com.java1234.service;


import java.util.Iterator;
import java.util.Set;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;
import com.java1234.model.Class;
import com.java1234.model.Student;
import com.java1234.util.HibernateUtil;
public class StudentTest {
private SessionFactory sessionFactory=HibernateUtil.getSessionFactory();
private Session session;
@Before
public void setUp() throws Exception {
session=sessionFactory.openSession(); // 生成一個session
    session.beginTransaction(); // 開啟事務
}
@After
public void tearDown() throws Exception {
session.getTransaction().commit(); // 提交事務
session.close(); // 關閉session
}
@Test
public void testSaveClassAndStudent() {
Class c=new Class();
    c.setName("08計本");
    Student s1=new Student();
    s1.setName("張三");
    Student s2=new Student();
    s2.setName("李四");
    c.getStudents().add(s1);
    c.getStudents().add(s2);    
    session.save(c);
    
}
@Test
public void getStudentsByClass(){
Class c=(Class)session.get(Class.class, Long.valueOf(2));
Set<Student> students=c.getStudents();
Iterator it=students.iterator();
while(it.hasNext()){
Student s=(Student)it.next();
System.out.println(s);
}
}

@Test
public void testAdd(){
Class c=new Class();
    c.setName("09計本");   
    Student s1=new Student();
    s1.setName("王五");
    session.save(c);
    session.save(s1);
}

@Test
public void testInverse(){
Class c=(Class)session.get(Class.class, Long.valueOf(1));
Student s=(Student)session.get(Student.class, Long.valueOf(1));
s.setC(c);
c.getStudents().add(s);
}

@Test
public void testDeleteClassCascade(){
Class c=(Class)session.get(Class.class, Long.valueOf(1));
session.delete(c);
}

}

解析

1.testSaveClassAndStudent()中只把c物件放在session中，配置對映檔案Class.hbm.xml時，也要加上cascade="save-update"

2.inverse屬性解析

先執行testAdd()方法，得到資料庫如下圖：

接著執行testInverse()，這兩句話執行的sql如下，資料庫自動補全student表裡的classId為1

                 s.setC(c);

c.getStudents().add(s);

當在Class.hbm.xml對映檔案中設定inverse="true"屬性是，只用寫s.setC(c);就行了，sql語句就只有前面那句了。

3.級聯刪除解析

在Class.hbm.xml中配置cascade="delete"，可以實現方法testDeleteClassCascade()中，只要刪除c，資料庫中也會自動刪除對應的student資訊。