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Understanding Hibernate O/R Mapping
In the last example we created contact.hbm.xml to map
Contact Object to the Contact table in the database. Now let's understand the
each component of the mapping file.
To recall here is the content of contact.hbm.xml:
<?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>
<class
name="roseindia.tutorial.hibernate.Contact" table="CONTACT">
<id name="id" type="long" column="ID"
>
<generator class="assigned"/>
</id>
<property name="firstName">
<column name="FIRSTNAME"
/>
</property>
<property name="lastName">
<column
name="LASTNAME"/>
</property>
<property name="email">
<column name="EMAIL"/>
</property>
</class>
</hibernate-mapping>
Hibernate mapping documents are simple xml documents. Here are important
elements of the mapping file:.
-
<hibernate-mapping> element
The first or root element of hibernate mapping document is
<hibernate-mapping> element. Between the
<hibernate-mapping> tag class element(s) are present.
-
<class> element
The <Class> element maps the class object with corresponding entity in
the database. It also tells what table in the database has to access and
what column in that table it should use. Within one
<hibernate-mapping> element, several <class> mappings are
possible.
-
<id> element
The <id> element in unique identifier to identify and object. In fact
<id> element map with the primary key of the table. In our code :
<id name="id" type="long" column="ID" >
primary key maps to the ID field of the
table CONTACT. The attributes of the id element are:
-
name: The property name used by the
persistent class.
-
column: The column used to store the primary
key value.
-
type: The Java data type used.
-
unsaved-value: This is the value used to
determine if a class has been made persistent. If the value of the id
attribute is null, then it means that this object has not been
persisted.
-
<generator> element
The <generator> method is used to generate the primary
key for the new record. Here is some of the commonly used generators :
* Increment - This is used to generate primary keys of type
long, short or int that are unique only. It should not be used in the
clustered deployment environment.
* Sequence - Hibernate can also use the sequences to generate
the primary key. It can be used with DB2, PostgreSQL, Oracle, SAP DB
databases.
* Assigned - Assigned method is used when application code generates
the primary key.
-
<property> element
The property elements define standard Java attributes and their mapping into
database schema. The property element supports the column child element to
specify additional properties, such as the index name on a column or a
specific column type.
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