Primary Key Generator In Java
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- Primary Key Generator In Java Software
This annotation defines a primary key generator that may be referenced by name when a generator element is specified for the GeneratedValue annotation. A table generator may be specified on the entity class or on the primary key field or property. The scope of the generator name is global to the persistence unit (across all generator types).
- Since:
- Java Persistence 1.0
); Statement st = conn.createStatement; st.executeUpdate('create table survey (id int,name varchar, PRIMARY KEY (id) );'); st.executeUpdate('insert into survey (id,name ) values (1,'nameValue')'); ResultSet rs = null; DatabaseMetaData meta = conn.getMetaData; // The Oracle database stores its table names as Upper-Case, // if you pass a. Public abstract java.lang.String pkColumnValue (Optional) The primary key value in the generator table that distinguishes this set of generated values from others that may be stored in the table. Defaults to a provider-chosen value to store in the primary key column of the generator table Default: '. Hibernate Tips is a series of posts in which I describe a quick and easy solution for common Hibernate questions. If you have a question for a future Hibernate Tip, please leave a comment below. Question: You explained that the GenerationType.SEQUENCE is the most efficient primary key generation strategy and that MySQL doesn’t support it. What shall I do if I need to support PostgreSQL.
| Required Element Summary | |
|---|---|
String | name(Required) A unique generator name that can be referenced by one or more classes to be the generator for id values. |
| Optional Element Summary | |
|---|---|
int | allocationSize(Optional) The amount to increment by when allocating id numbers from the generator. |
String | catalog(Optional) The catalog of the table. |
int | initialValue(Optional) The initial value to be used when allocating id numbers from the generator. |
String | pkColumnName(Optional) Name of the primary key column in the table. |
String | pkColumnValue(Optional) The primary key value in the generator table that distinguishes this set of generated values from others that may be stored in the table. |
String | schema(Optional) The schema of the table. |
String | table(Optional) Name of table that stores the generated id values. |
UniqueConstraint[] | uniqueConstraints(Optional) Unique constraints that are to be placed on the table. |
String | valueColumnName(Optional) Name of the column that stores the last value generated. |
| Element Detail |
|---|
name
- (Required) A unique generator name that can be referenced by one or more classes to be the generator for id values.
table
(Optional) Name of table that stores the generated id values.Defaults to a name chosen by persistence provider.
- Default:
- '
catalog
(Optional) The catalog of the table.Defaults to the default catalog.
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- Default:
- '
schema
(Optional) The schema of the table.Defaults to the default schema for user.
- Default:
- '
pkColumnName
(Optional) Name of the primary key column in the table.Defaults to a provider-chosen name.
- Default:
- '
valueColumnName
(Optional) Name of the column that stores the last value generated.Defaults to a provider-chosen name.
- Default:
- '
pkColumnValue
(Optional) The primary key value in the generator table that distinguishes this set of generated values from others that may be stored in the table.Defaults to a provider-chosen value to store in the primary key column of the generator table
- Default:
- '
initialValue
- (Optional) The initial value to be used when allocating id numbers from the generator.
- Default:
- 0
allocationSize
- (Optional) The amount to increment by when allocating id numbers from the generator.
- Default:
- 50
uniqueConstraints
(Optional) Unique constraints that are to be placed on the table. These are only used if table generation is in effect. These constraints apply in addition to primary key constraints.Defaults to no additional constraints.
- Default:
- {}
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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.
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.
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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. | '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 |
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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#sequencingKey Generator Download
Using 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
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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
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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.