Spring combat (3) -- JPA

UG-91P39EG 2020-11-11 11:01:01
spring combat jpa


JAP yes spring Integrate Persistent components simplify sql The operation process , Here's how it works

First step :

 stay pom Import in file JPA Related dependence of
 <dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-data-jpa</artifactId>
</dependency>
 And configure the database connection
spring:
datasource:
driver-class-name: com.mysql.cj.jdbc.Driver
url: jdbc:mysql://localhost:3306/crm?characterEncoding=utf-8&serverTimezone=UTC
username: root
password: root
type: com.alibaba.druid.pool.DruidDataSource
jpa:
hibernate:
ddl-auto: update
show-sql: true

The second step

 Create a solid object ( The entity object corresponds to the database table )
package com.spring.entity;
import javax.persistence.*;
@Entity // This is an entity class 
@Table(name = "t_order") // Set the table name of the entity in the database 
public class Order {

@Id // Primary key must be set id Indicates that the field is id If it does not exist, an error will be created 
@GeneratedValue(strategy = GenerationType.AUTO) // Set primary key properties 
@Column(name = "oid")
private Integer oid;
@Column(name = "orderName")
private String orderName;
@Column(name = "number")
private Integer number;
@Column(name = "address")
private String address;
public Integer getOid() {

return oid;
}
public void setOid(Integer oid) {

this.oid = oid;
}
public String getOrderName() {

return orderName;
}
public void setOrderName(String orderName) {

this.orderName = orderName;
}
public Integer getNumber() {

return number;
}
public void setNumber(Integer number) {

this.number = number;
}
public String getAddress() {

return address;
}
public void setAddress(String address) {

this.address = address;
}
}

@Entity This is an entity class

@Table(name = “t_order”) Set the table name of the entity in the database

@Id Primary key must be set id Indicates that the field is id If it does not exist, an error will be created

@GeneratedValue annotation , stay JPA in ,@GeneratedValue The meaning of annotation is to generate a unique primary key for an entity (JPA Every entity is required to Entity, There must be and only one primary key ),@GeneratedValue Provides the primary key generation strategy .@GeneratedValue Annotations have two properties , Namely strategy and generator, among generator The value of the property is a string , The default is "", It declares the name of the primary key generator ( Corresponding to the primary key generator with the same name @SequenceGenerator and @TableGenerator).
Specific notes introduce @GeneratedValue Annotations,

@Column(name = “oid”) Set the field name corresponding to the table in the database

list establish One to many Many to many For one more One to one annotations use

  • ManyToOne( For one more ) ManyToOne( For one more ) A one-way : No middle table , But you can use @Joincolumn(name=" ") To specify the name of the generated foreign key , Foreign keys are generated in multiple party tables !
  • OneToMany( One to many ) OneToMany( One to many ) A one-way : Intermediate table will be generated , You can use @onetoMany @Joincolumn(name=" ") Avoid creating intermediate tables , And the name of the foreign key ( Do not look @joincolumn In one, it says , But it's in the multiple table )
  • OneToMany ,ManyToOne OneToMany ,ManyToOne two-way ( Two notes are used together ): If not @OneToMany To add mappedy Properties will produce intermediate tables , It's usually in @ManyToOne Note to I'll add a note to it @Joincolumn(name=" ") To specify the name of the foreign key ( explain : The more side is the relationship maintenance side , The relationship maintenance side is responsible for updating foreign key records , The maintainer has no right to update Foreign key records )!(@OneToMany(mappedBy=“ One to many , The attribute of one of many ”) appear mapby For the maintained end ||| The default is delayed loading )
    Use cases
@ManyToOne(fetch=FetchType.LAZY)
@JoinColumn(name="child_id")
private OrderChild orderChild;
@OneToMany(mappedBy="orderChild",fetch=FetchType.LAZY,cascade={
CascadeType.MERGE})
@NotFound(action=NotFoundAction.IGNORE) // The delegate can be empty , Allow for null
private List<OrderChildGoods> goodsList;

@ManyToOne The default is to load immediately ,@OneToMany The default is lazy loading
But if you add @NotFound After that fetch=FetchType.LAZY It doesn't work , That is, setting @NotFound After the change to load immediately eager

cascade: Cascade operation permissions

CascadeType.PERSIST: Cascading persistence , That's Cascade preservation . For example, a student has many grades , Save the students , Then it will cascade to save the score information of each subject

CascadeType.REMOVE: cascading deletion , Delete students , Then the students' grades are also removed from the database

CascadeType.MERGE: And , The student achievement information has been modified , Then the modified score information will be saved to the database , And above persist The difference is similar to put and post The difference between

CascadeType.DETACH: Cascade dissociation , To delete a student message , But the students id As a foreign key in the score sheet , Can't delete directly

CascadeType.REFRESH: Cascade refresh

CascadeType.ALL: All of the above combinations , Use with caution

fetch: Set the loading method of the associated object

FetchType.EAGER: Immediately load , For example, when loading Student object information , Load the student's grade information immediately

FetchType.LAZy: Delay loading , Load when needed

The third step

 stay springboot Add... To the startup class of @EnableJpaRepositories annotation This is something that has to be added
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