一對多（one-to-many）雙向關聯實例（Department- Employee）

這里的一對多雙向關聯是在域模型(實體對象模型)上的概念,在關系數據庫中,只存在外鍵參照關系,而且總是由"many"方參照"one"方,因為這樣才能消除冗余數據,因上關系數據庫實際上只支持多對一或一對一的單向關聯.在實體(類與類之間)各種各樣的關系中,數多對一的的單向關聯關系與數據庫中的外鍵參照關系最匹配了.

在我的前一篇文章多對一的基礎上進行修改。

１.修改Department實體類：

Java代碼 ?

package?com.reiyen.hibernate.domain??

public?class?Department?{??

??

????private?int?id;??

????private?String?name;??

????private?Set<Employee>?emps;???

??

　　　//setter和getter方法??

｝??

?２.修改Department.hbm.xml文件：

Xml代碼 ?

<?xml?version="1.0"?>??

<!DOCTYPE?hibernate-mapping?PUBLIC???

????"-//Hibernate/Hibernate?Mapping?DTD?3.0//EN"??

????"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">??

<hibernate-mapping?package="com.reiyen.hibernate.domain">??

????<class?name="Department"?>??

????????<id?name="id"?>??

????????????<generator?class="native"?/>??

????????</id>??

????????<property?name="name"?/>??

????????<set?name="emps">??

????????<key?column="depart_id"?/>??

????????<one-to-many?class="Employee"/>??

????????</set>??

????</class>??

</hibernate-mapping>??

<set>元素的屬性name:設定待映射的持久化類的屬性名,這里為Depatment類的emps屬性.

<set>元素的兩個子元素:

<key>:設定與所關聯的持久化類對應的表的外鍵,此處為Employee表的depart_id字段.

<one-to-many>:設定所關聯的持久化類,此處為Employee.

Hibernate根椐以上配置信息獲得如下信息:

<set>元素表明Department類的emps屬性為java.util.Set集合類型.

<class>子元素表明emps集合中存放的是一組Employee對象

<key>子元素表明employee表是通過外鍵depart_id參照department表的.

?

?３.　其余的保持原樣，測試類如下：

Java代碼 ?

public?class?Many2One?{??

??

????/**?

?????*?@param?args?

?????*/??

????public?static?void?main(String[]?args)?{??

????????Department?depart?=?add();??

????????Department?department?=?queryDepart(depart.getId());??

????????????????//department.getEmps();3??

//?System.out.println("emp?size:"?+?department.getEmps().size());4????

????}??

??

????static?Department?queryDepart(int?departId)?{??

????????Session?s?=?null;??

????????Transaction?tx?=?null;??

????????try?{??

????????????s?=?HibernateUtil.getSession();??

????????????tx?=?s.beginTransaction();??

????????????Department?depart?=?(Department)?s.get(Department.class,?departId);??

?????????????System.out.println("emp?size:"?+?depart.getEmps().size());//1????????

?????????????????????????//?System.out.println("emps?class:"?+?depart.getEmps().getClass());???//2???

?????????????????????????//?Hibernate.initialize(depart.getEmps());??

????????????tx.commit();??

????????????return?depart;??

????????}?finally?{??

????????????if?(s?!=?null)??

????????????????s.close();??

????????}??

????}??

??

????static?Department?add()?{??

????????Session?s?=?null;??

????????Transaction?tx?=?null;??

????????try?{??

????????????Department?depart?=?new?Department();??

????????????depart.setName("department?name");??

??????????????

????????????Employee?employee1?=?new?Employee();??

????????????employee1.setDepartment(depart);?//?對象模型：建立兩個對象的關聯???

????????????employee1.setName("employee1?name2");??

??????????????

????????????Employee?employee2?=?new?Employee();??

????????????employee2.setDepartment(depart);?//?對象模型：建立兩個對象的關聯???

????????????employee2.setName("employee2?name2");??

??

　　　　　　　//　Set<Employee>?set?=?new?HashSet<Employee>();??

????????????//????set.add(employee1);??

????????????//??set.add(employee2);??

????????????//??depart.setEmps(set);??

??????????????

????????????s?=?HibernateUtil.getSession();??

????????????tx?=?s.beginTransaction();??

????????????s.save(depart);??

????????????s.save(employee1);??

????????????s.save(employee2);??

????????????tx.commit();??

????????????return?depart;??

????????}?finally?{??

????????????if?(s?!=?null)??

????????????????s.close();??

????????}??

????}??

}??

?控制臺打印信息如下：

Hibernate: insert into Department (name) values (?)
Hibernate: insert into Employee (name, depart_id) values (?, ?)
Hibernate: insert into Employee (name, depart_id) values (?, ?)
Hibernate: select department0_.id as id1_0_, department0_.name as name1_0_ from Department department0_ where department0_.id=?
Hibernate: select emps0_.depart_id as depart3_1_, emps0_.id as id1_, emps0_.id as id2_0_, emps0_.name as name2_0_, emps0_.depart_id as depart3_2_0_ from Employee emps0_ where emps0_.depart_id=?

emp size:2

數據庫中記錄如下所示：

mysql> select * from employee;
+----+-----------------+-----------+
| id | name??????????? | depart_id |
+----+-----------------+-----------+
|? 1 | employee1 name2 |???????? 1 |
|? 2 | employee2 name2 |???????? 1 |
+----+-----------------+-----------+
2 rows in set (0.00 sec)

mysql> select * from department;
+----+-----------------+
| id | name??????????? |
+----+-----------------+
|? 1 | department name |
+----+-----------------+
1 row in set (0.00 sec)

?

雖然對象模型修改了，但數據庫關系模型并沒有改變。

?

如果把測試程序中注釋部分去掉，同時將注釋為“建立對象模型”這兩句程序注釋掉，雖然也能正確運行，但控制臺會多打印出兩條更新語句，說明數據庫多執行了兩次更新操作，效率上有影響，如下所示：

Hibernate: insert into Department (name) values (?)
Hibernate: insert into Employee (name, depart_id) values (?, ?)
Hibernate: insert into Employee (name, depart_id) values (?, ?)
Hibernate: update Employee set depart_id=? where id=?
Hibernate: update Employee set depart_id=? where id=?
Hibernate: select department0_.id as id1_0_, department0_.name as name1_0_ from Department department0_ where department0_.id=?
Hibernate: select emps0_.depart_id as depart3_1_, emps0_.id as id1_, emps0_.id as id2_0_, emps0_.name as name2_0_, emps0_.depart_id as depart3_2_0_ from Employee emps0_ where emps0_.depart_id=?
emp size:2

?

4.懶加載分析：

（1）如果將程序中標記為1的程序注釋掉，控制臺打印信息如下：

Hibernate: select department0_.id as id0_0_, department0_.name as name0_0_ from Department department0_ where department0_.id=?

此時只是將Department對象查詢出來了，而對應的Employee并沒有進行查詢。

(2）如果將程序中標記為1的程序注釋掉,同時將標記為2的前面的注釋去掉，運行程序，控制臺打印信息如下：

Hibernate: select department0_.id as id0_0_, department0_.name as name0_0_ from Department department0_ where department0_.id=?
emps class:class org.hibernate.collection.PersistentSet

此時可以看到其實Hibernate將hashset替換成了它自己寫的PersistentSet,所以才能實現懶加載功能。同時可以發現當調用department.getEmps();hibernate只是獲取了集合代理對象的引用，它并沒有去查詢數據庫來填充集合對象。你還可以進一步測試來驗證這個問題。如果你將程序中標記為1的程序注釋掉，同時去掉標記為3的語句前面的注釋，你會發現程序能正常運行，但如果去掉標記為4的語句前面的注釋，運行程序，則會拋出如下異常：

?org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: com.reiyen.hibernate.domain.Department.emps, no session or session was closed

(3)當進行department對象查詢時，從department的表結構中也不能判斷這個Department對象是否有對應的Employee，那為什么它不像一對一查詢時，也一次性把相關聯的對象查詢出來呢？？這是因為如果將關聯的對象查詢出來的話，因為數據較多，它會嚴重影響效率，而不像一對一時，反正從對象只有一個，一次性查詢出來影響也不是太大，所以hibernate就假定你有相對應的Employee，直接創建了一個集合代理對象的返回給你（把對象引用先給你），等你需要數據的時候再去查詢數據庫，以提高效率！

?

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