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link管理  ›  Hibernate - @GeneratedValue Annotation in JPA - GeeksforGeeks
python hibernate
https://www.geeksforgeeks.org/hibernate-generatedvalue-annotation-in-jpa/
道上混的火龙果
1 年前

@GeneratedValue annotation, the name itself suggests that it will generate something. This annotation is generally used in conjunction with @Id annotation to automatically generate unique values for primary key columns within our database tables. When creating an entity class we have to specify a primary key for that entity. For marking the field property as a primary key of the entity we use @Id annotation. When we apply @GeneratedValue annotation to our primary key field or property. It will instruct hibernate to automatically generate a unique value for that field during the process of persisting the entity into the database. The @GeneratedValue annotation provides us with different strategies for the generation of primary keys which are as follows :

  • GenerationType.IDENTITY : This strategy will help us to generate the primary key value by the database itself using the auto-increment column option. It relies on the database’s native support for generating unique values.
  • GenerationType.AUTO : This is a default strategy and the persistence provider which automatically selects an appropriate generation strategy based on the database usage.
  • GenerationType.TABLE : This strategy uses a separate database table to generate primary key values. The persistence provider manages this table and uses it to allocate unique values for primary keys.
  • GenerationType.SEQUENCE : This generation-type strategy uses a database sequence to generate primary key values. It requires the usage of database sequence objects, which varies depending on the database which is being used.

    • Example for @GeneratedValue Annotation

    Example 1:

    package com.example.java_test_application;
    // on the below line creating an entity class for the class
    // of Employee.
    @Entity
    public class Employee {
    // on the below line creating an employee id generated
    // value with the strategy for generation type.
    @GeneratedValue (strategy = GenerationType.IDENTITY)
    private Long empId;
    private String employeeName;
    private String employeeQualification;

    Explanation:

    In the above example, we are considering an Employee entity. Inside this entity, we are marking empID with @Id annotation to indicate it as a primary key. The @GeneratedValue annotation with GenerationType Identity indicates that the primary key will be generated automatically by the database. The @GenaratedValue annotation simplifies generating unique primary key values and allows persistence providers to handle this task automatically, reducing boilerplate code needed for managing the primary keys in the database.

    Example 2:

    @Entity
    public class Department {
    // on the below line creating a variable for department
    // id.
    @GeneratedValue (strategy = GenerationType.SEQUENCE)
    private Long departmentID;
    // on the below line creating a variable for department
    // name.
    private String departmentName;

    Explanation:

    In the above example, we are considering a Department entity. Inside this entity, we are marking departmentID with @Id annotation to indicate it as a primary key. The @GeneratedValue annotation with GenerationType SEQUENCE will generate primary key values which require the usage of the database sequence objects.

    Last Updated : 21 Jun, 2023 Like Article
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