JPA Learning Notes (4)-using the table policy to generate primary keys __JPA

In the previous blog, mention was made of a note @generatedvalue, which is used to annotate a policy that generates a primary key, one of which is the table.

Note: This method is not commonly used and is only used when encountering special business requirements.

For annotations, refer to the JPA Learning notes (3)--JPA note How to use the table policy to generate primary keys

With this strategy, you need to create a table

Table Name: Id_generator

Field name	Description
Id	ID of this record
Pk_name	Primary Key Name
Pk_value	Primary key value

See here, it's going to be unclear, so. I'll explain later.

Insert a few data into this chart

ID	Pk_name	Pk_value
1	customer_id	1
2	student_id	10
3	order_id	100

Pk_name , customer_id represents the customer this table generates ID, its value is pk_value=1, this value does not represent the customer ID is 1, but an algorithm to automatically generate the ID, And this algorithm is related to this pk_value.

The same, student_id represents the STUDENT table generation ID, and the value is ten then how does JPA know to generate IDs for the other three tables based on this table?

Locate the Pk_value according to the horizontal ordinate.

Use annotations to tell JPA horizontal ordinate

Package Com.jpa.helloworld;
Import Javax.persistence.Column;
Import javax.persistence.Entity;
Import Javax.persistence.GeneratedValue;
Import Javax.persistence.GenerationType;
Import Javax.persistence.Id;
Import Javax.persistence.JoinColumn;
Import Javax.persistence.ManyToOne;
Import javax.persistence.Table;

Import Javax.persistence.TableGenerator; @Table (name= "T_order") @Entity the public class Order {@Column (name= "ID") @TableGenerator (name= "Id_generator",//Build
                    The name table= "Id_generator",//The table pkcolumnname= "Pk_name" used by the generator,//the corresponding field name in the table Pkcolumnvalue= "order_id",///The value of the above field valuecolumnname= "Pk_value",///value on Described three properties, you can navigate to the value of the pk_value in the table, such as: 1,10,100 allocationsize=10)//indicates that the primary key increases by one @GeneratedValue (Strategy=gener

    Ationtype.table, generator= "Id_generator")//The generator here corresponds to the name of the generator above @Id private Integer ID; @Column (name= "Order_name") Private STring Ordername;


    @JoinColumn (name= "user_id") @ManyToOne private user user;
    Public Integer GetId () {return id;
    The public void SetId (Integer id) {this.id = ID;
    Public String Getordername () {return ordername;
    } public void Setordername (String ordername) {this.ordername = Ordername;
    Public user GetUser () {return user;
    public void SetUser (user user) {this.user = user;
                @Override public String toString () {return "order [id=" + ID + ", ordername=" + Ordername + ", user="
    + user + "]";
 }


}

Each time the order table adds a record, the primary key increases by 10, so that the corresponding Pk_value value in the Id_generator table is increased by 1. But we don't need to ignore this value.

At this stage, it's all done.