Hibernate关联关系映射-----单向一对多配置

这里举了皇帝和大臣的例子，天子只有一个，而诸侯多的不胜计数。

实体：

package uni.one2many;

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

public class Emperor {
private int id;
private String name;
private Set<Minister> ministers = new HashSet<Minister>();

public Emperor() {

}

public Emperor(int id, String name, Set<Minister> ministers) {
super();
this.id = id;
this.name = name;
this.ministers = ministers;
}

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<Minister> getMinisters() {
return ministers;
}

public void setMinisters(Set<Minister> ministers) {
this.ministers = ministers;
}

}

package uni.one2many;

public class Minister {
private int id;
private String name;

public Minister() {

}

public Minister(int id, String name) {
super();
this.id = id;
this.name = name;
}

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;
}

}

　　

hbm配置文件：

<hibernate-mapping>
<class name="uni.one2many.Emperor">
<id name="id" column="did">
<generator class="native" />
</id>
<property name="name" type="string" column="name"></property>

<!-- confirure the uniredirection association -->
<set name="ministers" >
<key column="id" not-null="true"></key>
<one-to-many class="uni.one2many.Minister"/>
</set>
</class>

</hibernate-mapping>
<hibernate-mapping>
<class name="uni.one2many.Minister">
<id name="id" column="did">
<generator class="native" />
</id>
<property name="name" type="string" column="name"></property>
</class>

</hibernate-mapping>

　　

测试文件：

public class Test {

@org.junit.Test
public void testAdd() {
SessionFactory sf = HibernateUtil.getSessionFactory();
Session session = sf.getCurrentSession();
session.beginTransaction();

Minister m1 = new Minister();
m1.setName("廉颇");

Minister m2 = new Minister();
m2.setName("蔺相如");

Emperor emperor = new Emperor();
emperor.setName("齐威王");
emperor.getMinisters().add(m1);
emperor.getMinisters().add(m2);

session.save(emperor);
session.save(m1);
session.save(m2);
session.beginTransaction().commit();
}
}

　　

测试结果：

ibernate: insert into Emperor (name) values (?)
Hibernate: insert into Minister (name, id) values (?, ?)
Hibernate: insert into Minister (name, id) values (?, ?)
Hibernate: update Minister set id=? where did=?
Hibernate: update Minister set id=? where did=?

　　

这种映射关系在保存的过程中需要先将一的（主控方）一方先加以保存，否则会出现异常。我们可以对这种异常加以分析，从这种关系生成的表结构可以看出，在Minister的表中有一个指向Emperor的外键，由此可以推测出，如果这个外键有了非空约束，那么再存储Minister的过程中必然会出现引用为空的异常。