Hibernate mapping files detail

Mapping Declaration

Object/relational mappings are usually defined in XML document. The mapping language is Java-centric, meaning that mappings are constructed around persistent class declarations, not table declarations.
Mapping Document

The element maps the domain object with corresponding entity in the database. hibernate-mapping element allows you to nest several persistent mappings, as shown above. It is however good practice to map only a single persistent class in one mapping file and name it after the persistent superclass, e.g. Employee.hbm.xml,

name (optional): The fully qualified Java class name of the persistent class (or interface). If this attribute is missing, it is assumed that the mapping is for a non-POJO entity.
table (optional – defaults to the unqualified class name): The name of its database table.
discriminator-value (optional – defaults to the class name): A value that distiguishes individual subclasses, used for polymorphic behaviour. Acceptable values include null and not null.
mutable (optional, defaults to true): Specifies that instances of the class are (not) mutable.
schema (optional): Override the schema name specified by the root <hibernate-mapping> element.
catalog (optional): Override the catalog name specified by the root <hibernate-mapping>  element.

<generator> element

The optional <generator> child element names a Java class used to generate unique identifiers for instances of the persistent class. If any parameters are required to configure or initialize the generator instance, they are passed using the element.

All generators implement the interface org.hibernate.id.IdentifierGenerator. This is a very simple interface; some applications may choose to provide their own specialized implementations. However, Hibernate provides a range of built-in implementations. There are shortcut names for the built-in generators:

generates identifiers of type long, short or int that are unique only when no other process is inserting data into the same table. Do not use in a cluster. .
supports identity columns in DB2, MySQL, MS SQL Server, Sybase and HypersonicSQL. The returned identifier is of type long, short or int.
uses a sequence in DB2, PostgreSQL, Oracle, SAP DB, McKoi or a generator in Interbase. The returned identifier is of type long, short or int
uses a hi/lo algorithm to efficiently generate identifiers of type long, short or int, given a table and column (by default hibernate_unique_key and next_hi respectively) as a source of hi values. The hi/lo algorithm generates identifiers that are unique only for a particular database.
uses a hi/lo algorithm to efficiently generate identifiers of type long, short or int, given a named database sequence.
uses a 128-bit UUID algorithm to generate identifiers of type string, unique within a network (the IP address is used). The UUID is encoded as a string of hexadecimal digits of length 32.
uses a database-generated GUID string on MS SQL Server and MySQL.
picks identity, sequence or hilo depending upon the capabilities of the underlying database.
lets the application to assign an identifier to the object before save() is called. This is the default strategy if no element is specified.
retrieves a primary key assigned by a database trigger by selecting the row by some unique key and retrieving the primary key value
uses the identifier of another associated object. Usually used in conjunction with a primary key association.

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