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. 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.
• <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.
The <generator> is the optional element under <id> element. The <generator> element is used to specify the class name to be used to generate the primary key for new record while saving a new record.
• <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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