自己总结:
一对一双向关联问题,比较复杂
Hibernate映射多对一关联,不能够双向关联,因为反过来变成多对一就产生了冗余。
数据库:SQL Server 2000
JDK版本:JDK 1.5
开发IDE:Ecclipse 3.2+MyEclipse 5.0
使用SQL Server2000数据库,新建数据库shirdrn,新建两张表teacher和student,分别如下所示:
教师表teacher:
学生表student:
从学生的角度看,与教师是多对一关联,即一个学生一定对应一个教师(而从教师角度看就是一对多,我们选定以学生为参照),所以在学生表的映射文件里面配置与教师的关联。其实,也就是我们的意图是想通过学生获取教师的信息,而不需要通过教师获取学生信息,这些是限定条件。所以必须能从Student的一方get到Teacher,因而就在学生里配置关联。
student表对应的映射文件和持久化类分别为:
映射文件Student.hbm.xml:
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<!--
Mapping file autogenerated by MyEclipse - Hibernate Tools
-->
<hibernate-mapping>
<class name="org.shirdrn.entity.Student" table="student">
<id name="id" type="java.lang.Integer">
<column name="id" />
<generator class="identity" />
</id>
<property name="sno" type="java.lang.String">
<column name="sno" length="10" not-null="true" />
</property>
<property name="sname" type="java.lang.String">
<column name="sname" length="50" not-null="true" />
</property>
<property name="dept" type="java.lang.String">
<column name="dept" length="50" />
</property>
<many-to-one name="teacher"
column="id"
class="org.shirdrn.entity.Teacher"
insert="false"
update="false"
cascade="save-update">
</many-to-one>
</class>
</hibernate-mapping>
持久化类Student.java:
package org.shirdrn.entity;
/**
* Student generated by MyEclipse - Hibernate Tools
*/
public class Student implements java.io.Serializable {
private Integer id;
private String sno;
private String sname;
private String dept;
private Teacher teacher;
/** default constructor */
public Student() {
}
/** minimal constructor */
public Student(String sno, String sname) {
this.sno = sno;
this.sname = sname;
}
/** full constructor */
public Student(String sno, String sname, String dept) {
this.sno = sno;
this.sname = sname;
this.dept = dept;
}
// Property accessors
public Integer getId() {
return this.id;
}
public void setId(Integer id) {
this.id = id;
}
public String getSno() {
return this.sno;
}
public void setSno(String sno) {
this.sno = sno;
}
public String getSname() {
return this.sname;
}
public void setSname(String sname) {
this.sname = sname;
}
public String getDept() {
return this.dept;
}
public void setDept(String dept) {
this.dept = dept;
}
public Teacher getTeacher() {
return teacher;
}
public void setTeacher(Teacher teacher) {
this.teacher = teacher;
}
}
teacher表对应的映射文件和持久化类分别为:
映射文件Teacher.hbm.xml:
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<!--
Mapping file autogenerated by MyEclipse - Hibernate Tools
-->
<hibernate-mapping>
<class name="org.shirdrn.entity.Teacher" table="teacher" schema="dbo" catalog="hibernate">
<id name="id" type="java.lang.Integer">
<column name="id" />
<generator class="identity" />
</id>
<property name="tname" type="java.lang.String">
<column name="tname" length="50" not-null="true" />
</property>
</class>
</hibernate-mapping>
持久化类Teacher.java:
package org.shirdrn.entity;
/**
* Teacher generated by MyEclipse - Hibernate Tools
*/
public class Teacher implements java.io.Serializable {
// Fields
private Integer id;
private String tname;
// Constructors
/** default constructor */
public Teacher() {
}
/** full constructor */
public Teacher(String tname) {
this.tname = tname;
}
// Property accessors
public Integer getId() {
return this.id;
}
public void setId(Integer id) {
this.id = id;
}
public String getTname() {
return this.tname;
}
public void setTname(String tname) {
this.tname = tname;
}
}
建立测试类为MyTest.java如下:
package org.shirdrn.test;
import org.hibernate.Session;
import org.hibernate.Transaction;
import org.shirdrn.HibernateSessionFactory;
import org.shirdrn.entity.Student;
import org.shirdrn.entity.Teacher;
public class MyTest {
public static void main(String[] args){
Session session = HibernateSessionFactory.getSession();
Transaction tx = null;
try{
tx = session.beginTransaction();
Student stu = new Student();
stu.setSno("2008002");
stu.setSname("shirdrn");
Teacher t = new Teacher();
t.setTname("王老师");
stu.setTeacher(t);
session.save(stu);
System.out.println("success");
tx.commit();
}
catch(Exception e){
tx.rollback();
e.printStackTrace();
}
finally{
HibernateSessionFactory.closeSession();
}
}
}
在many-to-one里面配置了 cascade="save-update",级联存储更新。在向student表插入记录的同时,也在对应的teacher表里面插入一条记录,因为一个学生一定对应一个教师,而且配置了关联,而且又是级联保存,在控制台上可以看到:
Hibernate: insert into hibernate.dbo.teacher (tname) values (?) select scope_identity()
Hibernate: insert into student (sno, sname, dept) values (?, ?, ?) select scope_identity()
可以看到,是先向teacher表插入记录,当Hibernate映射文件加载的时候,检测到cascade="save-update"配置,在程序执行到stu.setTeacher(t);的时候,就已经确定要对teacher表执行insert操作。
如果 不设置 cascade="save-update",则只执行了一条插入语句,只向学生表插入了一条记录:
Hibernate: insert into student (sno, sname, dept) values (?, ?, ?) select scope_identity()
但是如果insert="false" update="false"没有指定,则会创建SessionFactory失败,出现下面的异常:
log4j:WARN No appenders could be found for logger (org.hibernate.cfg.Environment).
log4j:WARN Please initialize the log4j system properly.
%%%% Error Creating SessionFactory %%%%
org.hibernate.MappingException: Repeated column in mapping for entity: org.shirdrn.entity.Student column: id (should be mapped with insert="false" update="false")
at org.hibernate.mapping.PersistentClass.checkColumnDuplication(PersistentClass.java:504)
at org.hibernate.mapping.PersistentClass.checkPropertyColumnDuplication(PersistentClass.java:526)
at org.hibernate.mapping.PersistentClass.checkColumnDuplication(PersistentClass.java:544)
at org.hibernate.mapping.PersistentClass.validate(PersistentClass.java:335)
at org.hibernate.mapping.RootClass.validate(RootClass.java:188)
at org.hibernate.cfg.Configuration.validate(Configuration.java:839)
at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1000)
at org.shirdrn.HibernateSessionFactory.rebuildSessionFactory(HibernateSessionFactory.java:60)
at org.shirdrn.HibernateSessionFactory.getSession(HibernateSessionFactory.java:43)
at org.shirdrn.test.MyTest.main(MyTest.java:11)
Exception in thread "main" java.lang.NullPointerException
at org.shirdrn.test.MyTest.main(MyTest.java:26)
我认为异常的原因是重复配置导致的。因为insert和update的默认值都是true,表明Student随时可执行插入更新操作。而我们使用cascade="save-update"又配置了一次,虽然是在关联中配置的,但是Student也是关联的一方,因此异常提示Repeated column。
然而,并不是说,如果在Student.hbm.xml的many-to-one中配置insert="false" update="false"而且也不配置cascade="save-update",执行测试程序就不会执行插入操作了,象下面这样:
<many-to-one name="teacher"
column="id"
class="org.shirdrn.entity.Teacher"
insert="false"
update="false"
>
</many-to-one>
仍然可以向student表插入记录,因为那个配置insert="false" update="false"是对于关联来说的,但看Student本身,它的insert还是true,所以执行插入操作。
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