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Mar 06, 2017 Hibernate Composite Primary Key using Annotations We will be adding JPA Annotation to our Employe class. We have used the below JPA Annotations in our Employee class @Entity – This annotation will mark our Employee class as an Entity Bean. Hello, I'm working with JPA and Hibernate, and project requirement is to have primary key (id) value in application, before entity is persisted. JPA and Hibernate have techniques for generating id's during persist, but I need those id's before, and I need them to be unique. Oct 28, 2018 Hibernate also allows defining primary-keys made up of @ManyToOne associations combined with @Id annotation. In this case, the entity class should also fulfill the conditions of a primary-key class. In this case, the entity class should also fulfill the conditions of a primary-key class. Apr 19, 2013 Hibernate Tutorial Part 7 - Primary key Auto - Generation in Hibernate ( Hands on). I would talk on how to generate the value of primary key automatically for a.
The <generator> class is a sub-element of id. It is used to generate the unique identifier for the objects of persistent class. There are many generator classes defined in the Hibernate Framework.
All the generator classes implements the org.hibernate.id.IdentifierGenerator interface. The application programmer may create one's own generator classes by implementing the IdentifierGenerator interface. Hibernate framework provides many built-in generator classes:
1) assigned
It is the default generator strategy if there is no <generator> element . In this case, application assigns the id. For example:
2) increment
It generates the unique id only if no other process is inserting data into this table. It generates short, int or long type identifier. If a table contains an identifier then the application considers its maximum value else the application consider that the first generated identifier is 1. For each attribute value, the hibernate increment the identifier by 1. Syntax:
3) sequence
It uses the sequence of the database. if there is no sequence defined, it creates a sequence automatically e.g. in case of Oracle database, it creates a sequence named HIBERNATE_SEQUENCE. In case of Oracle, DB2, SAP DB, Postgre SQL or McKoi, it uses sequence but it uses generator in interbase. Syntax:
For defining your own sequence, use the param subelement of generator.
4) hilo
It uses high and low algorithm to generate the id of type short, int and long. Syntax:
5) native
It uses identity, sequence or hilo depending on the database vendor. Syntax:
6) identity
It is used in Sybase, My SQL, MS SQL Server, DB2 and HypersonicSQL to support the id column. The returned id is of type short, int or long. It is responsibility of database to generate unique identifier.
7) seqhilo
It uses high and low algorithm on the specified sequence name. The returned id is of type short, int or long.
8) uuid
It uses 128-bit UUID algorithm to generate the id. The returned id is of type String, unique within a network (because IP is used). The UUID is represented in hexadecimal digits, 32 in length.
9) guid
10) select
11) foreign
12) sequence-identity
Next TopicDialects In Hibernate
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Every JPA entity is required to have a field which maps to primary key of the database table. Such field must be annotated with
@Id .
Simple vs Composite primary keys
A simple primary key consists of a single Java field which maps to a single table column.
A composite primary key consists of multiple Java fields which individually map to separate columns. Supported types for a primary key
A simple primary key field or one of the composite primary key field should be one of the following types:
In this tutorial we are going to focus on generation strategies of simple primary key.
@GeneratedValue Annotation
This annotation defines the types of primary key generation strategies. If this annotation is not used then application is responsible to populate and manage @Id field values itself.
The use of the GeneratedValue annotation is only required to be supported for simple primary keys.
GenerationType enum defines four strategies: Generation Type . TABLE , Generation Type. SEQUENCE , Generation Type. IDENTITY and Generation Type. AUTO . Let's understand them with examples.
GenerationType.SEQUENCE
With this strategy, underlying persistence provider must use a database sequence to get the next unique primary key for the entities.
We have created the following Util class to reuse the code for other examples.
Also, in the persistence.xml, we have created four persistence-unit, so that we can try four GenerationType independently. We are using Hibernate as persistence provider.
Let's create main class to try out Entity1 key generation.
Output
Above output shows one table MYENTITY1 and one sequence HIBERNATE_SEQUENCE are created.
GenerationType.TABLE
With this strategy, underlying persistence provider must use a database table to generate/keep the next unique primary key for the entities.
Output
This time no sequence is generated, instead an additional table named 'HIBERNATE_SEQUENCES' is created to maintain primary key sequence.
GenerationType.IDENTITY
This GenerationType indicates that the persistence provider must assign primary keys for the entity using a database identity column. IDENTITY column is typically used in SQL Server. This special type column is populated internally by the table itself without using a separate sequence. If underlying database doesn't support IDENTITY column or some similar variant then the persistence provider can choose an alternative appropriate strategy. In this examples we are using H2 database which doesn't support IDENTITY column.
Output
Above output shows that a sequence is used for primary keys.
GenerationType.AUTO
This GenerationType indicates that the persistence provider should automatically pick an appropriate strategy for the particular database. This is the default GenerationType, i.e. if we just use @GeneratedValue annotation then this value of GenerationType will be used.
Spring Hibernate AnnotationsOutput
Above output shows that a sequence is used for primary keys.
When @GeneratedValue not used
If we don't use @GeneratedValue annotation at all, then we have to populate the unique primary keys ourselves. In this example, we are simply assigning it to the value returned from
System.nanoTime()
![]() Output
Above output shows that a no sequence or extra table were generated.
Example Project
Dependencies and Technologies Used:
Hibernate Primary Key Generator
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