Hibernate Framework : ORM Tool :: Hibernate Collection Mapping Example

• Collection Mapping Intro
• map/set/list/bag/array Example
• Download Examples

Understanding Collection Mapping

• As we know , collection framework that is used to store a group of objects into a single unit.
• This collection framework has the interfaces like Set, List, and Map that are used in Hibernate applications to persists objects of POJO class in database tables.
• The Hibernate O/R mapping framework provides the <set>, there is also <list>,
• <map>, <bag>, <array> mapping elements that allow you to work with various objects.

These elements are know as collection mapping elements.

<map | set | list | bag
name="propertyName" 
table="table_name" 
schema="schema_name" 
lazy="true|extra|false" 
inverse="true|false" 
cascade="all|none|save-update|delete|all-delete-orphan|delet e-orphan"
sort="unsorted|natural|comparatorClass" 
order-by="column_name asc|desc" 
where="arbitrary sql where condition" 
fetch="join|select|subselect" 
batch-size="N" 
access="field|property|ClassName" 
optimistic-lock="true|false" 
mutable="true|false" 
node="element-name|."
embed-xml="true|false">
<key .... />
<map-key .... />
<element .... />

• name: the collection property name
• table (optional – defaults to property name): the name of the collection table. It is not used for one-to-many associations.
• schema (optional): the name of a table schema to override the schema declared on the root element
• lazy (optional – defaults to true): disables lazy fetching and specifies that the association is always eagerly fetched. It can also be used to enable "extra-lazy" fetching where most operations do not initialize the collection. This is suitable for large collections.
• inverse (optional – defaults to false): marks this collection as the "inverse" end of a bidirectional association.
• cascade (optional – defaults to none): enables operations to cascade to child entities.
• sort (optional): specifies a sorted collection with natural sort order or a given comparator class.
• order-by (optional, JDK1.4 only): specifies a table column or columns that define the iteration order of the Map, Set or bag, together with an optional asc or desc.
• where (optional): specifies an arbitrary SQL WHERE condition that is used when retrieving or removing the collection. This is useful if the collection needs to contain only a subset of the available data.
• fetch (optional, defaults to select): chooses between outer-join fetching, fetching by sequential select, and fetching by sequential subselect.
• batch-size (optional, defaults to 1): specifies a "batch size" for lazily fetching instances of this collection.
• access (optional – defaults to property): the strategy Hibernate uses for accessing the collection property value.
• optimistic-lock (optional – defaults to true): specifies that changes to the state of the collection results in increments of the owning entity’s version. For one-to-many associations you may want to disable this setting.
• mutable (optional – defaults to true): a value of false specifies that the elements of the collection never change. This allows for minor performance optimization in some cases.

Here are some examples

1. <set>

A set element is used to store an unordered and unique collection of values. A set of strings:

<set name="names" table="person_names">
<key column="person_id"/>
<element column="person_name" type="string"/>
</set>

Or

<set name="emailaddress" table="EMP_email_Address">
<key column="emp_id"/>
<element column="email_address" type="string"/>
</set>

2. <bag>

The <bag> element is used to store unordered and duplicate collection of objects. A bag containing integers with an iteration order determined by the order-by attribute:

<bag name="sizes" table="item_sizes" order-by="size asc">
<key column="item_id"/>
<element column="size" type="integer"/>
</bag>

3. <array>

An array of entities, in this case, a many-to-many association:

<array name="addresses"
table="PersonAddress"
cascade="persist">
<key column="personId"/>
<list-index column="sortOrder"/>
<many-to-many column="addressId" class="Address"/>
</array>

4. <map>

A map element is used to store a collection of objects, values, or elements in the key/value pairs. A map from string indices to dates:

<map name="holidays" table="holidays" schema="dbo"
order-by="hol_name asc">
<key column="id"/>
<map-key column="hol_name" type="string"/>
<element column="hol_date" type="date"/>
</map>

5. <list>

A list element is used to store a collection of objects, values, or elements that can contains duplicate elements.

<list name="carComponents" table="CarComponents">
<key column="carId"/>
<list-index column="sortOrder"/>
<composite-element class="CarComponent">
<property name="price"/>
<property name="type"/>
<property name="serialNumber" column="serialNum"/>
</composite-element>
</list>

hibernate.cfg.xml

<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
<session-factory>
<property name="connection.username">root</property>
<property name="connection.url">jdbc:mysql://localhost:3306/javaskoolDB</property>
<property name="dialect">org.hibernate.dialect.MySQLDialect</property>
<property name="pfname">javaskoolMySQLDriver</property>
<property name="connection.password">admin</property>
<property name="connection.driver_class">com.mysql.jdbc.Driver</property>
<property name="hbm2ddl.auto">update</property>
<property name="show_sql">true</property>
<mapping resource="com/javaskool/item.hbm.xml" />
</session-factory>
</hibernate-configuration>

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• map/set/list/bag Example
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