Hibernate, JBoss and auto-increment field

The element

This is the optional element under element. The element is used to specify the class name to be used to generate the primary key for new record while saving a new record. The element is used to pass the parameter (s) to the class. Here is the example of generator element from our first application:

In this case element do not generate the primary key and it is required to set the primary key value before calling save() method.

Here are the list of some commonly used generators in hibernate:

Generator	Description
increment	It generates identifiers of type long, short or int that are unique only when no other process is inserting data into the same table. It should not the used in the clustered environment.
identity	It supports identity columns in DB2, MySql, MS SQL Server, Sybase and HypersonicSQL. The returned identifier is of type long, short or int.
sequence	The sequence generator 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
hilo	The hilo generator 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. Do not use this generator with connections enlisted with JTA or with a user-supplied connection.
seqhilo	The seqhilo generator uses a hi/lo algorithm to efficiently generate identifiers of type long, short or int, given a named database sequence.
uuid	The uuid generator 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.
guid	It uses a database-generated GUID string on MS SQL Server and MySQL.
native	It picks identity, sequence or hilo depending upon the capabilities of the underlying database.
assigned	lets the application to assign an identifier to the object before save() is called. This is the default strategy if no element is specified.
select	retrieves a primary key assigned by a database trigger by selecting the row by some unique key and retrieving the primary key value.
foreign	uses the identifier of another associated object. Usually used in conjunction with a primary key association.

Therefore,
If your application is EJB, WS which create multiple threads, you should not use class=increment. Instead, you should use class=sequence or native. Or you will
encounter the problem: unique constrain violation.