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:

"-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

Hibernate mapping documents are simple xml documents. Here are important elements of the mapping file:.

1. element
   The first or root element of hibernate mapping document is element. Between the <hibernate-mapping> tag class element(s) are present.
   
2. element
   The 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 element, several mappings are possible.
   
3. element
   The element in unique identifier to identify and object. In fact element map with the primary key of the table. In our code :
   
   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.
     
4. element
   The 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.
   
   
5. 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.