hibernate组件映射详解

在Hibernate中,component是某个实体的逻辑组成部分，它与实体的根本区别是没有oid（对象标识符），component是一个被包含的对象,它作为值类型被持久化，而非一个实体。
下面举两个例子说明其使用和配置：

1.以User和Address为例，一个User有id（Long类型）、name（String类型）、address地址(Address类型)，其中Address类中没有id属性（即无对象标识符），则address就是User的一个component组件。

User持久化类：

[java]
view plaincopyprint?

public class User { private Long id; private String name; private Address address; //省略set、get方法 }

public class User  {
private Long id;
private String name;
private Address address;
//省略set、get方法
}

组件类Address：

[java]
view plaincopyprint?

public class Address {//注意Address类中无id属性（Address并不会被映射为一张表）

private String addressName;
private String addressValue;
//省略set、get方法
}

public class Address {//注意Address类中无id属性（Address并不会被映射为一张表）
private String addressName;
private String addressValue;
//省略set、get方法
}

User.hbm.xml配置：

[java]
view plaincopyprint?

<hibernate-mapping> <class name="bean.User" table="users"> <id name="id" column="id" type="long"> <generator class="increment"></generator> </id> <property name="name" column="name" type="string"></property> <component name="address" class="bean.Address"> <property name="addressName" column="address_name"></property> <property name="addressValue" column="address_value"></property> </component> </class> </hibernate-mapping>

<hibernate-mapping>
<class name="bean.User" table="users">
<id name="id" column="id" type="long">
<generator class="increment"></generator>
</id>
<property name="name" column="name" type="string"></property>
<component name="address" class="bean.Address">
<property name="addressName" column="address_name"></property>
<property name="addressValue" column="address_value"></property>
</component>
</class>
</hibernate-mapping>

说明：

(1).User类映射的表为users,id为其主键，name为User的一般属性，被映射为name字段。

(2).<component>指定User类的address为User的组件，address并不是一个可以直接映射为表字段的属性而是一个类对象，class指定类的名称，<component>的子元素<property>指定组件类的属性与users表字段的映射关系。

(3)不必为Address写一个Address.hbm.xml映射文件，因为Address是作为User的一部分即值类型而不是实体被持久化的，所以Address的属性会与User的其它属性一样被映射为users表中的字段，而不会将Address映射为单独的一个表。

(4)User与Address的关系是"整体与部分"，也可以将他们表示为一对一关联关系，这样的话两者就是“平等"关系，那么Address就会被映射为单独的一张表（需要配置Address.hbm.xml文件）。

(5)users表的结构为：



保存User对象：

[java]
view plaincopyprint?

tx=session.beginTransaction();
User user=new User();
user.setName("zhangsan");
Address address=new Address();
address.setAddressName("HomeAddress");
address.setAddressValue("青岛");
user.setAddress(address);
session.save(user);
tx.commit();

tx=session.beginTransaction();
User user=new User();
user.setName("zhangsan");
Address address=new Address();
address.setAddressName("HomeAddress");
address.setAddressValue("青岛");
user.setAddress(address);
session.save(user);
tx.commit();

执行的SQL语句为：

[java]
view plaincopyprint?

Hibernate: insert into users (name, address_name, address_value, id) values (?, ?, ?, ?)

Hibernate: insert into users (name, address_name, address_value, id) values (?, ?, ?, ?)

2.若User可有多个地址，即User类中的address属性为Set<Address>类型时，配置如下：



持久化类User：

[java]
view plaincopyprint?

public class User { private Long id; private String name; private Set<Address> address; //省略set、get方法 }

public class User  {
private Long id;
private String name;
private Set<Address> address;
//省略set、get方法
}

Address类不变。

配置User.hbm.xml：

[java]
view plaincopyprint?

<class name="bean.User" table="users">
<id name="id" column="id" type="long">
<generator class="increment"></generator>
</id>
<property name="name" column="name" type="string"></property>
<!--注意<set>中并不是<one-to-many>而是<composite-element>-->
<set name="address" table="address_table">
<key column="user_id"></key>
<composite-element class="bean.Address">
<property name="addressName" column="address_name"></property>
<property name="addressValue" column="address_value"></property>
</composite-element>
</set>
</class>

<class name="bean.User" table="users">
<id name="id" column="id" type="long">
<generator class="increment"></generator>
</id>
<property name="name" column="name" type="string"></property>
<!--注意<set>中并不是<one-to-many>而是<composite-element>-->
<set name="address" table="address_table">
<key column="user_id"></key>
<composite-element class="bean.Address">
<property name="addressName" column="address_name"></property>
<property name="addressValue" column="address_value"></property>
</composite-element>
</set>
</class>

说明：

(1)与一对多关联关系不同，<set>中使用的不是<one-to-many>而是<composite-element>，Hibernate会将User对象address属性（Set类型）中的所有元素映射到另外一张表中（显然users中的一个记录不能表示Set中的多个元素），table属性指定另一张表的名称为address_table,该表存放Set中的元素，并且该表中还有一个参照users表id的外键字段user_id。

(2)users表与address_table表的结构为：（使用Hibernate自动建表功能创建）



address_table表的各字段说明：



可以发现address_table表中并没有主键，只有user_id参照users表中的主键id。

保存User对象：

[java]
view plaincopyprint?

tx=session.beginTransaction();
User user=new User();
user.setName("zhangsan");
Address address=new Address();
address.setAddressName("HomeAddress");
address.setAddressValue("青岛");
Address address2=new Address();
address2.setAddressName("SchoolAddress");
address2.setAddressValue("北京");
user.setAddress(new HashSet<Address>());
user.getAddress().add(address);
user.getAddress().add(address2);
session.save(user);
tx.commit();

tx=session.beginTransaction();
User user=new User();
user.setName("zhangsan");
Address address=new Address();
address.setAddressName("HomeAddress");
address.setAddressValue("青岛");
Address address2=new Address();
address2.setAddressName("SchoolAddress");
address2.setAddressValue("北京");
user.setAddress(new HashSet<Address>());
user.getAddress().add(address);
user.getAddress().add(address2);
session.save(user);
tx.commit();

输出的SQL语句为：

[java]
view plaincopyprint?

Hibernate: insert into users (name, id) values (?, ?) Hibernate: insert into address_table (user_id, address_name, address_value) values (?, ?, ?) Hibernate: insert into address_table (user_id, address_name, address_value) values (?, ?, ?)

Hibernate: insert into users (name, id) values (?, ?)
Hibernate: insert into address_table (user_id, address_name, address_value) values (?, ?, ?)
Hibernate: insert into address_table (user_id, address_name, address_value) values (?, ?, ?)

address_table的内容：



（因为address_table表没有主键，所以在MySQL WorkBench中无法通过图形化界面添加记录，但是可以操纵SQL语句添加记录，例：



要插入记录的user_id必须为users表中存在的id（因为user_id参照users表的id），可使用update语句更改使表中存在相同的记录，下面会查询id为1L的User对象的Set中的Address对象以检验Hibernate是否会自动排除重复的元素以符合Set中无重复元素的原则。

）

查询User对象：

[java]
view plaincopyprint?

User u=(User)session.get(User.class, 1L);
Set<Address> address=u.getAddress();
for (Iterator iterator = address.iterator(); iterator.hasNext();) {
Address a = (Address) iterator.next();
System.out.println(a.getAddressName()+":"+a.getAddressValue());}

User u=(User)session.get(User.class, 1L);
Set<Address> address=u.getAddress();
for (Iterator iterator = address.iterator(); iterator.hasNext();) {
Address a = (Address) iterator.next();
System.out.println(a.getAddressName()+":"+a.getAddressValue());}

控制台输出：

[java]
view plaincopyprint?

Hibernate: select user0_.id as id0_0_, user0_.name as name0_0_ from users user0_ where user0_.id=? Hibernate: select address0_.user_id as user1_0_, address0_.address_name as address2_0_, address0_.address_value as address3_0_ from address_table address0_ where address0_.user_id=? HomeAddress:青岛 HomeAddress:青岛 H:S Hibernate: delete from address_table where user_id=? and address_name=? and address_value=? Hibernate: delete from address_table where user_id=? and address_name=? and address_value=? Hibernate: delete from address_table where user_id=? and address_name=? and address_value=? Hibernate: insert into address_table (user_id, address_name, address_value) values (?, ?, ?) Hibernate: insert into address_table (user_id, address_name, address_value) values (?, ?, ?) Hibernate: insert into address_table (user_id, address_name, address_value) values (?, ?, ?)

Hibernate: select user0_.id as id0_0_, user0_.name as name0_0_ from users user0_ where user0_.id=?
Hibernate: select address0_.user_id as user1_0_, address0_.address_name as address2_0_, address0_.address_value as address3_0_ from address_table address0_ where address0_.user_id=?
HomeAddress:青岛
HomeAddress:青岛
H:S
Hibernate: delete from address_table where user_id=? and address_name=? and address_value=?
Hibernate: delete from address_table where user_id=? and address_name=? and address_value=?
Hibernate: delete from address_table where user_id=? and address_name=? and address_value=?
Hibernate: insert into address_table (user_id, address_name, address_value) values (?, ?, ?)
Hibernate: insert into address_table (user_id, address_name, address_value) values (?, ?, ?)
Hibernate: insert into address_table (user_id, address_name, address_value) values (?, ?, ?)

可知，Hibernate并没有过滤掉多余的元素，对于输出的3条delete和insert语句，尚不知其机制，但我认为Hibernate没有过滤掉重复的元素是因为Address类中没有实现hashCode()及equals()方法，使用MyEclipse在Address类中生成hashCode及equals方法后再执行上述查询语句：

控制台输出：

[java]
view plaincopyprint?

Hibernate: select user0_.id as id0_0_, user0_.name as name0_0_ from users user0_ where user0_.id=? Hibernate: select address0_.user_id as user1_0_, address0_.address_name as address2_0_, address0_.address_value as address3_0_ from address_table address0_ where address0_.user_id=? HomeAddress:青岛 H:S

Hibernate: select user0_.id as id0_0_, user0_.name as name0_0_ from users user0_ where user0_.id=?
Hibernate: select address0_.user_id as user1_0_, address0_.address_name as address2_0_, address0_.address_value as address3_0_ from address_table address0_ where address0_.user_id=?
HomeAddress:青岛
H:S

可知Hibernate已将多余的重复元素排除。