多對(duì)一實(shí)例（Employee-Department）

1. E-R圖：

2. 實(shí)體類：

Department類：

Java代碼 ?

package?com.reiyen.hibernate.domain??

????public?class?Department?{??

??

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

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

??????????//setter和getter方法....??

????}??

?

?Employee類:

Java代碼 ?

??????package?com.reiyen.hibernate.domain??

public?class?Employee?{??

??

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

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

????private?Department?department;??

????//setter和getter方法??

}??

?

?3. 實(shí)體映射文件：

Department.hbm.xml如下：

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"?/>??

????????</class>??

????</hibernate-mapping>??

?

?Employee.hbm.xml如下：

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="Employee">??

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

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

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

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

????????????<!--?name="department"?這個(gè)名稱必須與Employee中的屬性名一致.?-->??

????????????<many-to-one?name="department"?column="depart_id"?class="Department"?/>??

????????</class>??

????</hibernate-mapping>??

?

?<many-to-one name=”department” column=”depart_id” />這種缺省情況,
hibernate會(huì)默認(rèn)去department對(duì)象中查找主鍵值,因?yàn)閔ibernate默認(rèn)的是外鍵對(duì)應(yīng)另一個(gè)表中的主鍵的,
如果想對(duì)應(yīng)department中的其它屬性,如”name”,則可以使用<many-to-one name=”department” column=”depart_id” property-ref=”name”/>也可以使用<many-to-one name=”department” />,這與 <many-to-one name=”depart” column=”depart_id” />的唯一區(qū)別就是它的column名也為department了,而不是depart_id.

<many-to-one >元素建立了department屬性和employee表的外鍵depart_id之間的映射.

name: 設(shè)定待映射的持久化類的屬性名,此外為employee類的department屬性.

column: 設(shè)定和持久化類的屬性對(duì)應(yīng)的表的外鍵,此外為employee表的外鍵depart_id.

class(可選):設(shè)定持久化類的屬性的類型,此處設(shè)定department的類型是Department類.

not-null(可選):如果為true,表示department屬性不能為null,該屬性的默認(rèn)值為false.當(dāng)為true時(shí),生成的employee表中depart_id外鍵會(huì)設(shè)置not-null約束,所以當(dāng)Hibernate保存Employee對(duì)象時(shí),會(huì)先檢查它的department屬性是否為null,如果為null,則會(huì)拋出異常.

3. 將兩個(gè)實(shí)體映射文件在hibernate.cfg.xml配置文件中注冊(cè)：（hibernate.cfg.xml在我一篇文章：第一個(gè)Hibernate實(shí)例中有）

Xml代碼 ?

<mapping?resource="com/reiyen/hibernate/domain/Department.hbm.xml"?/>??

<mapping?resource="com/reiyen/hibernate/domain/Employee.hbm.xml"?/>??

?4. 測(cè)試：

Java代碼 ?

public?class?Many2One?{??

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

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

????????System.out.println("depart?name:?"?+?depart.getName());??

????}??

static?Department?add()?{??

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

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

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

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

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

????????????Employee?employee?=?new?Employee();??

???????????????employee.setDepartment(depart);?//?對(duì)象模型：建立兩個(gè)對(duì)象的關(guān)聯(lián)??

???????????????????????employee.setName("employee?name");??

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

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

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

????????????????????s.save(employee);??

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

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

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

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

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

????????}??

????}??

}??

?運(yùn)行后，控制臺(tái)打印信息如下：

Hibernate: insert into Department (name) values (?)
Hibernate: insert into Employee (name, depart_id) values (?, ?)
depart name: department name

查看Employee數(shù)據(jù)庫表的創(chuàng)建語句如下：（注意紅色部分）

??? DROP TABLE IF EXISTS `test`.`employee`;
??? CREATE TABLE? `test`.`employee` (
??? ? `id` int(11) NOT NULL AUTO_INCREMENT,
??? ? `name` varchar(255) DEFAULT NULL,
??? ? `depart_id` int(11) DEFAULT NULL,
??? ? PRIMARY KEY (`id`),
??? ? KEY `FK4AFD4ACEA3E9AC0F` (`depart_id`),
??? ? CONSTRAINT `FK4AFD4ACEA3E9AC0F` FOREIGN KEY (`depart_id`) REFERENCES `department` (`id`)) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8;

?表中插入記錄如下：

Sql代碼 ?

mysql>?select?*?from?employee;??

+----+---------------+-----------+??

|?id?|?name??????????|?depart_id?|??

+----+---------------+-----------+??

|??1?|?employee?name?|?????????1?|??

+----+---------------+-----------+??

1?row?in?set?(0.00?sec)??

??

mysql>?select?*?from?department;??

+----+-----------------+??

|?id?|?name????????????|??

+----+-----------------+??

|??1?|?department?name?|??

+----+-----------------+??

1?row?in?set?(0.00?sec)??

?調(diào)換測(cè)試類兩句程序的順序,如下所示：

?

Java代碼 ?

s.save(employee);??

s.save(depart);??

則控制臺(tái)的信息如下所示：（多了一條更新語句）

Hibernate: insert into Employee (name, depart_id) values (?, ?)
Hibernate: insert into Department (name) values (?)
Hibernate: update Employee set name=?, depart_id=? where id=?
depart name: department name

Java代碼 ?

employee.setDepartment(depart);?//?對(duì)象模型：建立兩個(gè)對(duì)象的關(guān)聯(lián)??

這句話很重要，如果沒有這句話，則Department和Employee之間就沒有任何關(guān)系了。

如果你在Employee.hbm.xml中配置了

<many-to-one name="department" column="depart_id" not-null="true"/>

則必須先保存department再保存employee，否則會(huì)拋出：

org.hibernate.PropertyValueException: not-null property references a null or transient value異常！

查詢測(cè)試：

Java代碼 ?

public?class?Many2One?{??

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

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

????????Employee?employee?=?query(1);??

????????//?System.out.println("depart?name:"?+?employee.getDepartment().getName());?//?2??

????}??

??

????static?Employee?query(int?empId)?{??

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

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

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

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

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

????????????Employee?emp?=?(Employee)?s.get(Employee.class,?empId);??

????????????System.out.println("depart?name:"?+?emp.getDepartment().getName());?//?1??

??????????????//?System.out.println("depart?class:"?+?emp.getDepartment().getClass());?//?4???

?????????????//System.out.println("depart?id:"?+?emp.getDepartment().getId());??//5???????

????????????//?Hibernate.initialize(emp.getDepartment());?//?3??

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

????????????return?emp;??

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

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

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

????????}??

????}??

}??

?測(cè)試程序運(yùn)行后，控制臺(tái)打印出的sql如下:
Hibernate: select employee0_.id as id2_0_, employee0_.name as name2_0_, employee0_.depart_id as depart3_2_0_ from Employee employee0_ where employee0_.id=?
Hibernate: select department0_.id as id1_0_, department0_.name as name1_0_ from Department department0_ where department0_.id=?

depart? name:department name

當(dāng)相關(guān)聯(lián)的session沒有關(guān)閉時(shí)，訪問這些懶加載對(duì)象（代理對(duì)象）的屬性（getId和getClass除外）時(shí)，hibernate會(huì)初始化這些代理,所以將Employee對(duì)象和Department對(duì)象都查詢出來了。相當(dāng)于在數(shù)據(jù)庫中執(zhí)行兩次查詢:
首先執(zhí)行select * from employee where employee.id = ?
然后再執(zhí)行select * from department where department.id = ?

?

如果將測(cè)試程序中標(biāo)記為2的語句前的注釋去掉，同時(shí)把標(biāo)記為1的語句注釋掉，再運(yùn)行剛會(huì)出現(xiàn)如下所示的異常：

org.hibernate.LazyInitializationException: could not initialize proxy - no Session

解決辦法是把標(biāo)記為3的語句前的注釋去掉，即初始化懶加載的代理對(duì)象。（可以給query方法再傳遞一個(gè)參數(shù)，如boolean isInit,如果是true的話就初始化代理對(duì)象，即調(diào)用

Java代碼 ?

Hibernate.initialize(emp.getDepartment())??

否則，就不初始化代理對(duì)象）。

?

5.懶加載說明：

many-to-one（元素）懶加載：1).lazy!=false 2).fetch=select

能夠懶加載的對(duì)象都是被改寫過的代理對(duì)象，當(dāng)相關(guān)聯(lián)的session沒有關(guān)閉時(shí)，訪問這些懶加載對(duì)象（代理對(duì)象）的屬性（getId和getClass除外）時(shí)，hibernate會(huì)初始化這些代理，或用Hibernate.initialize(proxy)來初始化代理對(duì)象；
當(dāng)相關(guān)聯(lián)的session關(guān)閉后，再訪問懶加載的對(duì)象將會(huì)出現(xiàn)異常。

（1）把標(biāo)記為1的語句注釋掉，同時(shí)去掉標(biāo)記為4的語句前面的注釋，進(jìn)行測(cè)試，控制臺(tái)打印信息如下所示：

Hibernate: select employee0_.id as id1_0_, employee0_.name as name1_0_, employee0_.depart_id as depart3_1_0_ from Employee employee0_ where employee0_.id=?
depart class:class com.itcast.hibernate.domain.Department$$EnhancerByCGLIB$$b412e7fa

說明實(shí)現(xiàn)了懶加載，它只查詢出Employee對(duì)象，而沒有去查詢相應(yīng)的Department對(duì)象；同時(shí)可以看出，返回的是改寫過的代理對(duì)象，而不是實(shí)體類Department.

（2）把標(biāo)記為1的語句注釋掉，同時(shí)去掉標(biāo)記為5的語句前面的注釋，進(jìn)行測(cè)試，控制臺(tái)打印信息如下所示：

Hibernate: select employee0_.id as id1_0_, employee0_.name as name1_0_, employee0_.depart_id as depart3_1_0_ from Employee employee0_ where employee0_.id=?
depart id:1

當(dāng)相關(guān)聯(lián)的session沒有關(guān)閉時(shí)，訪問這些懶加載對(duì)象（代理對(duì)象）getId和getClass屬性，hibernate不會(huì)初始化這些代理。

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