Hibernate映射集合属性

　　Hibernate要求持久化集合属性字段必须声明为接口,实际的接口可以是java.util.Set,java.util.Collection,java.util.List,java.util.Map,java.util.SortedSet,java.util.SortedMap等,甚至是自己定义类型(只需要实现org.hibernate.usertype.UserCollectionType接口即可)。
　　Hibernate之所以要求用集合接口声明集合属性字段,是因为当程序持久某个实例时,Hibernate会自动把程序中的集合实现类替换成Hibernate自己的集合实现类,因此不要试图把Hibernate集合属性强制转换为集合实现类,如HashSet,HashMap等,但可以转换为Set,Map等集合,因为Hibernate自己的集合类也实现了Map,Set等接口。

集合映射的元素大致有如下这些:
list:用于映射List集合属性
set:用于映射Set集合属性
map:用于映射Map集合属性
array:用于映射数组集合属性
rimitive-array:专门用于映射基本数据类型的数组
bag:用于映射无须集合
idbag:用于映射无序集合,但为集合增加逻辑次序

List集合属性：Person.java和Person.hbm.xml

public class Person{
    private String id;
    private String name;
    private List<String> schools;
    ......    
}

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="com.test.domain">

    <class name="Person" table="person">
        <id name="id" column="id" type="integer">
            <generator class="native" />
        </id>
        <property name="name" type="string" length="255" />
        <list name="school" table="school">
            <key column="person_id" not-null="true" />
            <list-index column="list_order" />
            <element type="string" column="school_name" />
        </list>
    </class>
</hibernate-mapping>

数组属性：Person.java和Person.hbm.xml

public class Person{
    private String id;
    private String name;
    private String[] schools;
    ......    
}

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="com.test.domain">

    <class name="Person" table="person">
        <id name="id" column="id" type="integer">
            <generator class="native" />
        </id>
        <property name="name" type="string" length="255" />
        <array name="school" table="school">
            <key column="person_id" not-null="true" />
            <list-index column="list_order" />
            <element type="string" column="school_name" />
        </array>
    </class>
</hibernate-mapping>

set集合属性：Person.java和Person.hbm.xml

public class Person{
    private String id;
    private String name;
    private Set<String> schools;
    ......    
}

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="com.test.domain">

    <class name="Person" table="person">
        <id name="id" column="id" type="integer">
            <generator class="native" />
        </id>
        <property name="name" type="string" length="255" />
        <set name="schools" table="school">
            <key column="person_id" not-null="true" />            
            <element type="string" column="school_name"  not-null="true" />
        </set>
    </class>
</hibernate-mapping>

bag元素映射：Person.java和Person.hbm.xml

public class Person{
    private String id;
    private String name;
    private Collection<String> schools;
    ......    
}

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="com.test.domain">

    <class name="Person" table="person">
        <id name="id" column="id" type="integer">
            <generator class="native" />
        </id>
        <property name="name" type="string" length="255" />
        <bag name="schools" table="school">
            <key column="person_id" not-null="true" />            
            <element type="string" column="school_name"  not-null="true" />
        </bag>
    </class>
</hibernate-mapping>

Map集合属性：Person.java和Person.hbm.xml

public class Person{
    private String id;
    private String name;
    private Map<String,Float> schools;
    ......    
}

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="com.test.domain">

    <class name="Person" table="person">
        <id name="id" column="id" type="integer">
            <generator class="native" />
        </id>
        <property name="name" type="string" length="255" />
        <map name="schools" table="school">
            <key column="person_id" not-null="true" />    
            <map-key column="subject" type="string" />
            <element column="grade"  not-null="float" />
        </map>
    </class>
</hibernate-mapping>