Question

我有2个实体，学生和电话，以及他们之间的一对多关系。

@Entity 
@Table(name = "STUDENT") 
public class Student { 

    private long studentId; 
    private String studentName; 
    private Set<Phone> studentPhoneNumbers = new HashSet<Phone>(0); 

    public Student() { 
    } 

    public Student(String studentName, Set<Phone> studentPhoneNumbers) { 
        this.studentName = studentName; 
        this.studentPhoneNumbers = studentPhoneNumbers; 
    } 

    @Id 
    @GeneratedValue 
    @Column(name = "STUDENT_ID") 
    public long getStudentId() { 
        return this.studentId; 
    } 

    public void setStudentId(long studentId) { 
        this.studentId = studentId; 
    } 

    @Column(name = "STUDENT_NAME", nullable = false, length = 100) 
    public String getStudentName() { 
        return this.studentName; 
    } 

    public void setStudentName(String studentName) { 
        this.studentName = studentName; 
    } 

    @OneToMany (mappedBy="student", cascade = CascadeType.ALL)       

    public Set<Phone> getStudentPhoneNumbers() { 
        return this.studentPhoneNumbers; 
    } 

    public void setStudentPhoneNumbers(Set<Phone> studentPhoneNumbers) { 
        this.studentPhoneNumbers = studentPhoneNumbers; 
    } 

} 


@Entity 
@Table(name = "PHONE") 
public class Phone { 

    private long phoneId; 
    private String phoneType; 
    private String phoneNumber; 

    private Student student; 
    private Set<Student> students = new HashSet<Student>(0); 

    public Phone() { 
    } 

    public Phone(String phoneType, String phoneNumber) { 
        this.phoneType = phoneType; 
        this.phoneNumber = phoneNumber; 
    } 

    @Id 
    @GeneratedValue 
    @Column(name = "PHONE_ID") 
    public long getPhoneId() { 
        return this.phoneId; 
    } 

    public void setPhoneId(long phoneId) { 
        this.phoneId = phoneId; 
    } 

    @Column(name = "PHONE_TYPE", nullable = false, length=10) 
    public String getPhoneType() { 
        return this.phoneType; 
    } 

    public void setPhoneType(String phoneType) { 
        this.phoneType = phoneType; 
    } 

    @Column(name = "PHONE_NUMBER", nullable = false, length=15) 
    public String getPhoneNumber() { 
        return this.phoneNumber; 
    } 

    public void setPhoneNumber(String phoneNumber) { 
        this.phoneNumber = phoneNumber; 
    } 

    @ManyToOne(cascade = CascadeType.ALL) 
    @JoinColumn (name="STUDENT_ID") 
    public Student getStudent() { 
        return this.student; 
    } 

    public void setStudent(Student student) { 
        this.student = student; 
    }

这是applicationContext.xml的代码：

  ... 
 <bean id="myDataSource" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close">   
        <property name="driverClassName" value="com.mysql.jdbc.Driver"/>   
        <property name="url" value="jdbc:mysql://localhost/Project"/>   
        <property name="username" value="root"/>   
        <property name="password" value="root"/>   
    </bean>   

    <bean id="mySessionFactory" class="org.springframework.orm.hibernate3.annotation.AnnotationSessionFactoryBean">   
        <property name="dataSource" ref="myDataSource" />   
        <property name="annotatedClasses">   
            <list>   
                <value>com.domain.Student</value>                         
                <value>com.domain.Phone</value>                           
            </list>   
        </property>   
        <property name="hibernateProperties">   
            <props>   
                <prop key="hibernate.dialect">org.hibernate.dialect.HSQLDialect</prop>   
                <prop key="hibernate.show_sql">true</prop>   
                <prop key="hibernate.hbm2ddl.auto">create</prop>   
            </props>   
        </property>   
    </bean>   

 <bean id="myClassDAO" class="com.project.dao.ClassDAOImpl"> 
        <property name="sessionFactory" ref="mySessionFactory"/> 
 </bean>   

...

我的ClassDAOImpl看起来像：

    public class ClassDAOImpl{ 

    private HibernateTemplate hibernateTemplate; 
    private Session session; 
    public void setSessionFactory(SessionFactory sessionFactory) { 
            this.hibernateTemplate = new HibernateTemplate(sessionFactory); 
            this.session = sessionFactory.openSession(); 
    }     

    public void updateStudent(){ 

            Transaction transaction = session.beginTransaction();           
            Student s = (Student)session.get(Student.class, new Long(1));   
            Set<Phone> phoneNumbers =s.getStudentPhoneNumbers();   
            phoneNumbers.add(new Phone ("house","12342342"));   
            s.setStudentPhoneNumbers(phoneNumbers);   
            session.update(s);   
            transaction.commit(); 

    } 

}

我只想给主键为“1”的学生添加电话。为此，首先我得到学生，然后通过添加新的手机来更新手机。最后我更新了实体Student。我查看了我的Phone表，新手机上有一个新行，但是外键（STUDENT_ID）出现“Null”值，因此没有注册关系。我怎么能解决它？

提前致谢

Answer 1

在JPA / Hibernate中，您始终必须自己维护双向关联的双方。关联的拥有方是Hibernate用来知道关联是否存在或已经改变的一方，它们是 no mappedBy属性的一方。

在您的情况下，Phone是拥有方。因此，如果你没有设置你创建的手机的student属性，Hibernate会认为手机没有链接到任何用户，即使用户将手机放在手机列表中也是如此。

附注：阅读有关声明式事务管理的Spring文档。你真的不应该做什么来获得一个开放的会话和管理交易。

Answer 2

试试这个：

public void updateStudent(){ 

        Transaction transaction = session.beginTransaction();           
        Student s = (Student)session.get(Student.class, new Long(1));   
        Set<Phone> phoneNumbers =s.getStudentPhoneNumbers();   
        Phone phone = new Phone("house", "12342342");
        phone.setStudent(s);
        phoneNumbers.add(phone);   
        s.setStudentPhoneNumbers(phoneNumbers);   
        session.update(s);   
        transaction.commit(); 

    }

或者更好的是，向学生添加一个实用工具方法，如下所示：

public void addPhone(Phone phone) {
    if (phone == null) throw new IllegalArgumentException("Cannot add a null phone to a Student!");
    phone.setStudent(this);
    getStudents().add(phone);
}

并将updateStudent方法更改为：

public void updateStudent(){ 

        Transaction transaction = session.beginTransaction();           
        Student s = (Student)session.get(Student.class, new Long(1));   
        Phone phone = new Phone("house", "12342342");
        s.addPhone(phone);
        session.update(s);   
        transaction.commit(); 

    }

另外，请确保已将级联选项设置为级联到手机。

Answer 3

在双向实体关系中，始终建议连接关系的两侧。尽管您确实拥有关系的拥有方（未标记为“mappedBy”的一方）。

例如，当您创建新关系时，请在学生实体上调用getStudentPhoneNumbers（）。addPhone（newPhone）以及在电话实体上调用setStudent（）。

更新实体（OneToMany关系）

3 个答案: