Hibernate 一對一雙向關聯深入詳解

1、一對一相對比較簡單。hibernate主要是協助我們保持了兩張表的id一致！

先建兩張表

create table h_01.idcard (id integer not null, num integer, primary key (id));create table h_01.student (id integer not null, name varchar(255), primary key (id));

然後是POJO

package modle;public class IdCard {private int id;private int num;private Student student;public int getId() {return id;}public void setId(int id) {this.id = id;}public int getNum() {return num;}public void setNum(int num) {this.num = num;}public Student getStudent() {return student;}public void setStudent(Student student) {this.student = student;}}

package modle;public class Student {private int id;private String name;private IdCard idCard;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 IdCard getIdCard() {return idCard;}public void setIdCard(IdCard idCard) {this.idCard = idCard;}}

然後是對應檔：

重要的是這個地方：

<generator class="foreign">
                  <param name="property">student</param>
 </generator>

表示IdCard的id是根據student的id產生的。   generator就是定義id的產生方式。 param是定義有那個屬性，即student來確定id.

<hibernate-mapping>    <class name="modle.IdCard" table="idcard" catalog="h_01">  <id name="id" type="int">  <generator class="foreign">  <param name="property">student</param>  </generator>  </id>  <property name="num" type="int"></property>    <one-to-one name="student" class="modle.Student" fetch="select" ></one-to-one>    </class></hibernate-mapping>

再說下查詢：

1、一對一，hibernate預設的是用左外串連查詢。

比如，查詢一個student， hibernate使用做外串連直接查詢除了idcard.

Hibernate: select student0_.id as id3_2_, student0_.name as name3_2_, idcard1_.id as id4_0_, idcard1_.num as num4_0_, student2_.id as id3_1_, student2_.name as name3_1_ from h_01.student student0_ left outer join h_01.idcard idcard1_ on student0_.id=idcard1_.id left outer join h_01.student student2_ on idcard1_.id=student2_.id where student0_.id=?

有事我們並不需要idcard，為了速度  。 可以用消極式載入。
需要修改下查詢方式，即fetch屬性的配置，預設是join

<one-to-one name="student" class="modle.Student" fetch="select" ></one-to-one>

注意：兩個設定檔都要修改！

這下查詢就變成了：

Hibernate: select student0_.id as id3_0_, student0_.name as name3_0_ from h_01.student student0_ where student0_.id=?Hibernate: select idcard0_.id as id4_0_, idcard0_.num as num4_0_ from h_01.idcard idcard0_ where idcard0_.id=?

之所以有兩個查詢語句，因為我使用了card對象。所以又查詢了card

2、預設對student的儲存是不級聯的。不知道是不是和版本有關。

可以把student配置設定為級聯的。