Hibernate

When we are using the hibernate the we also know about that in the mapping class must declare the primary key column of the database table. In the Most classes always having the JavaBeans-style property holding the unique identifier of an instance. The element defines the mapping from that property to the primay key column.

Id generator is used to auto generate id of objects.Different database support different types of generator. most commonly used generator that are supported by oracle are:

1. increment
2. sequential

increment

Increment generator uses highest value of primary key increases to generator id value.

<id
        name="propertyName"                                          
        type="typename"                                              
        column="column_name"                                         
        unsaved-value="null|any|none|undefined|id_value"             
        access="field|property|ClassName">                           
        node="element-name|@attribute-name|element/@attribute|."

        <generator class="generatorClass"/>
</id>

If the name attribute is missing, that assumed that the class has no identifier property. and the unsaved-value attribute is almost never needed in hibernate3. There is an altern declaration that allows access to legacy data with composite key. Its use is strongly discouraged for anything else.