Primary Key Generator In Hibernate
The JPA specification supports 4 different primary key generation strategies that generate the primary key values programmatically or use database features, like auto-incremented columns or sequences. The only thing you have to do is to add the @GeneratedValue annotation to your primary key attribute and choose a generation strategy. I have a table which has no primary key. How to create the hibernate mapping xml without using the generator id. How are you accessing records in that table when not using Hibernate? When you use SQL directly. Aug 08, 2017 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. The table generator has a name, by which it is referenced in the @GeneratedValue annotation. The generator also lists the name of the specific database table, as well as the names of the key and value columns used to store identifier generators in the table.
Every JPA entity must have a primary key.
You can specify a primary key as a single primitive, or JDK object type entity field (see 'Configuring a JPA Entity Simple Primary Key Field').
You can specify a composite primary key made up of one or more primitive, or JDK object types using a separate composite primary key class (see 'Configuring a JPA Entity Composite Primary Key Class').
You can either assign primary key values yourself, or you can associate a primary key field with a primary key value generator (see 'Configuring JPA Entity Automatic Primary Key Generation').
Configuring a JPA Entity Simple Primary Key Field
The simplest primary key is one you specify as a single primitive or JDK object type entity field (see 'Using Annotations').
Note:
For a JPA entity primary key field code example, see:http://www.oracle.com/technology/tech/java/oc4j/ejb3/howtos-ejb3/howtoejb30mappingannotations/doc/how-to-ejb30-mapping-annotations.html#idUsing Annotations
Example 7-1 shows how to use the @Id annotation to specify an entity field as the primary key. In this example, primary key values are generated using a table generator (see 'Configuring JPA Entity Automatic Primary Key Generation').
Configuring a JPA Entity Composite Primary Key Class
A composite primary key is usually made up of two or more primitive or JDK object types. Composite primary keys typically arise when mapping from legacy databases when the database key is comprised of several columns. You can specify such a composite primary key with a separate composite primary key class (see 'Using Annotations')
A composite primary key class has the following characteristics:
It is a POJO class.
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It must be public and must have a public no-argument constructor.
If you use property-based access, the properties of the primary key class must be public or protected.
It must be serializable.
It must define
equalsandhashCodemethods.The semantics of value equality for these methods must be consistent with the database equality for the database types to which the key is mapped.
You can make the composite primary key class either an embedded class owned by the entity class, or a nonembedded class whose fields you map to multiple fields or properties of the entity class. In the latter case, the names of primary key fields or properties in the composite primary key class and those of the entity class must correspond and their types must be the same.
Using Annotations
Example 7-2 shows a typical embeddable composite primary key class. Example 7-3 shows how to configure a JPA entity with this embedded composite primary key class using the @EmbeddedId annotation.
Example 7-2 Embeddable Composite Primary Key Class
Example 7-3 JPA Entity With an Embedded Composite Primary Key Class
Example 7-5 shows a nonembedded composite primary key class. In this class, fields empName and birthDay must correspond in name and type to properties in the entity class. Example 7-5 shows how to configure a JPA entity with this nonembedded composite primary key class using the @IdClass annotation. Because entity class fields empName and birthDay are used in the primary key, you must also annotate them using the @Id annotation.
Example 7-4 Non-Embedded Composite Primary Key Class
Example 7-5 JPA Entity With a Mapped Composite Primary Key Class
Configuring JPA Entity Automatic Primary Key Generation
Typically, you associate a primary key field (see 'Configuring a JPA Entity Simple Primary Key Field') with a primary key value generator so that when an entity instance is created, a new, unique primary key value is assigned automatically.
Table 7-2 lists the types of primary key value generators that you can define.
Table 7-2 JPA Entity Primary Key Value Generators
| Type | Description | For more information, see .. |
|---|---|---|
Generated Id Table | A database table that the container uses to store generated primary key values for entities. Typically shared by multiple entity types that use table-based primary key generation. Each entity type will typically use its own row in the table to generate the primary key values for that entity class. Primary key values are positive integers. | 'Table Sequencing' in the Oracle TopLink Developer's Guide |
Table Generator | A primary key generator, which you can reference by name, defined at one of the package, class, method, or field level. The level at which you define it will depend upon the desired visibility and sharing of the generator. No scoping or visibility rules are actually enforced. Oracle recommends that you define the generator at the level for which it will be used. This generator is based on a database table. | Generate key csr on applications. 'Table Sequencing' in the Oracle TopLink Developer's Guide |
Sequence Generator | A primary key generator which you can reference by name, defined at one of the package, class, method, or field level. The level, at which you define it, will depend upon the desired visibility and sharing of the generator. No scoping or visibility rules are actually enforced. Oracle recommends that you define the generator at the level for which it will be used. This generator is based on a sequence object that the database server provides. | 'Native Sequencing With an Oracle Database Platform' in the Oracle TopLink Developer's Guide 'Native Sequencing With a Non-Oracle Database Platform' in the Oracle TopLink Developer's Guide |
Hibernate Primary Key Generator
Note:
For an EJB 3.0 automatic primary key generation code example, see:http://www.oracle.com/technology/tech/java/oc4j/ejb3/howtos-ejb3/howtoejb30mappingannotations/doc/how-to-ejb30-mapping-annotations.html#sequencingUsing Annotations
Example 7-6 shows how to use the @TableGenerator annotation to specify a primary key value generator based on a database table. The TopLink JPA persistence provider will attempt to create this table at deployment time: if it cannot, then you must follow your database documentation to ensure that this table exists before deployment. When a new instance of Address is created, a new value for entity field id is obtained from ADDRESS_GENERATOR_TABLE. In this case, you must set the @GeneratedValue annotation attribute strategy to TABLE and generator to ADDRESS_TABLE_GENERATOR.
Example 7-6 GeneratedValue Strategy Table: @TableGenerator
Example 7-7 shows how to use the @SequenceGenerator annotation to specify a primary key value generator based on a sequence object provided by the database. The TopLink JPA persistence provider will attempt to create this object at deployment time: if it cannot, then you must follow your database documentation to ensure that this sequence object exists before deployment. When a new instance of Address is created, a new value for entity field id is obtained from database sequence object ADDRESS_SEQ. In this case, you must set the @GeneratedValue annotation attribute strategy to SEQUENCE and generator to ADDRESS_SEQUENCE_GENERATOR.
Example 7-7 GeneratedValue Strategy Sequence: @SequenceGenerator
Example 7-8 shows how to use the @GeneratedValue annotation to specify a primary key value generator based on a primary key identity column (autonumber column). When a new instance of Address is persisted, the database assigns a value to the identity column. In this case, the TopLink JPA persistence provider re-reads the inserted row and updates the in-memory Address entity to set id to this value.
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An Object/relational mappings are usually defined in an XML document. This mapping file instructs Hibernate — how to map the defined class or classes to the database tables?
Though many Hibernate users choose to write the XML by hand, but a number of tools exist to generate the mapping document. These include XDoclet, Middlegen and AndroMDA for the advanced Hibernate users.
Let us consider our previously defined POJO class whose objects will persist in the table defined in next section.
There would be one table corresponding to each object you are willing to provide persistence. Consider above objects need to be stored and retrieved into the following RDBMS table −
Based on the two above entities, we can define following mapping file, which instructs Hibernate how to map the defined class or classes to the database tables.
You should save the mapping document in a file with the format <classname>.hbm.xml. We saved our mapping document in the file Employee.hbm.xml.
Let us see understand a little detail about the mapping elements used in the mapping file −
Primary Key Generation In Hibernate
The mapping document is an XML document having <hibernate-mapping> as the root element, which contains all the <class> elements.
The <class> elements are used to define specific mappings from a Java classes to the database tables. The Java class name is specified using the name attribute of the class element and the database table name is specified using the table attribute.
The <meta> element is optional element and can be used to create the class description.
The <id> element maps the unique ID attribute in class to the primary key of the database table. The name attribute of the id element refers to the property in the class and the column attribute refers to the column in the database table. The type attribute holds the hibernate mapping type, this mapping types will convert from Java to SQL data type.
The <generator> element within the id element is used to generate the primary key values automatically. The class attribute of the generator element is set to native to let hibernate pick up either identity, sequence, or hilo algorithm to create primary key depending upon the capabilities of the underlying database.
The <property> element is used to map a Java class property to a column in the database table. The name attribute of the element refers to the property in the class and the column attribute refers to the column in the database table. The type attribute holds the hibernate mapping type, this mapping types will convert from Java to SQL data type.
License Key Generator
There are other attributes and elements available, which will be used in a mapping document and I would try to cover as many as possible while discussing other Hibernate related topics.