Mapped classes must declare the primary key column of the database table. Most classes will also have a JavaBeans-style property holding the unique identifier of an instance. The <id> element defines the mapping from that property to the primary key column.

<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>

name (optional): The name of the identifier property.

type (optional): A name that indicates the Hibernate type.

column (optional - defaults to the property name): The name of the primary key column.

unsaved-value (optional - defaults to a "sensible" value): An identifier property value that indicates that an instance is newly instantiated (unsaved), distinguishing it from detached instances that were saved or loaded in a previous session.

access (optional - defaults to property): The strategy Hibernate should use for accessing the property value.

If the name attribute is missing, it is assumed that the class has no identifier property.

The unsaved-value attribute is almost never needed in Hibernate3.

There is an alternative <composite-id> declaration to allow access to legacy data with composite keys. We strongly discourage its use for anything else.