Hibernate mapping documents are simple xml documents.
Here are important elements of the mapping file:.
The first or root element of hibernate mapping document is
<hibernate-mapping> element. Between the <hibernate-mapping>
tag class element(s) are present.
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.
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
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
The <generator> methodis 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
* Assigned - Assigned method is used when application code generates
the primary key.
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.