JPA 和 Hibernate 中的 @Id 注释

Question

If I don't want to automatically generate the primary Key, instead, I want to supply the ID with the first column of the table as primary key.

2 A
4 B
7 D
13 E

...

I want the first column 2,4, 7, 13 to be the primary key of the table. Should I just use @Id to do the annotation?

@Entity
public class Code {
   @Id
   @Column(unique=true)
   private int id;

   ...
}

Or, if @Id is used, the primary key will be always automatically generated, instead of using the first column, in this case?

Answer 1

@Id will only declare the primary key. it will not insert generated value. if you use @GeneratedValue then it will generate the value of the field.

Answer 2

An object id (OID) is something that uniquely identifies an object . Within a VM this is typically the object's pointer. In a relational database table , a row is uniquely identified in its table by its primary key .

When persisting objects to a database you need a unique identifier for the objects , this allows you to query the object , define relationships to the object , and update and delete the object . In JPA the object id is defined through the @Id annotation and should correspond to the primary key of the object's table.

An object id can either be a natural id or a generated id. A natural id is one that occurs in the object and has some meaning in the application. Examples of natural ids include email addresses, phone numbers, and social insurance numbers. A generated id (also known as a surrogate id) is one that is generated by the system.

In JPA an @Id can be easily assigned a generated sequence number through the @GeneratedValue annotation.

Answer 3

如果您总是手动为您的实体提供主键值，那么@Id注释就足够了。