1. Introduce mybatis plus

Mybatis plus is only an enhancement tool of mybatis. Based on mybatis, it is only enhanced without change. It is developed for simplification and efficiency improvement. Mybatis plus is only applicable to single tables, and multiple tables must use mybatis

2. Characteristics of mybatis plus

3. Use of mybatis plus

3.1 in POM Introducing dependency into XML

<dependency> 
<groupId>com.baomidou</groupId>
 <artifactId>mybatis-plus-boot-starter</artifactId>
 <version>3.4.1</version>
 </dependency>

3.2 configuration file application Properties (connect to database)

spring.datasource.username=root
spring.datasource.password=
spring.datasource.url=jdbc:mysql://localhost:3306/mybatisplus?serverTimezone=Asia/Shanghai
spring.datasource.driver-class-name=com.mysql.cj.jdbc.Driver

3.3 create an entity class (the parameter type shall be written as the packaging class of eight basic data types as far as possible)

The reasons for writing packaging are as follows:

1. Prevent null pointer exception

2. If the wrapper class cannot receive the parameter, it will prompt null, and the basic data type will not prompt

package com.thm.mybatisplus.mybatisplus.entry;

import com.baomidou.mybatisplus.annotation.*;
import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

import java.util.Date;

@Data//Get get set method
@AllArgsConstructor//Parametric structure
@NoArgsConstructor//Nonparametric structure
public class User {
   /* @TableId(type= IdType.ASSIGN_ID)*///Snowflakes automatically generate primary keys
    @TableId(type= IdType.AUTO)//The primary key in the database must be auto incremented*/
    private Long id;
    /*Used when the column name is inconsistent with the attribute name
    * @TableFiled(value="u-name")
    * Annotate the field names in the declaration database
    * */
    private String name;
    private Integer age;
    private String email;

    @TableLogic//Delete comments logically
    private Integer deleted;

    public User(Long id, String name, Integer age, String email, Integer deleted) {
        this.id = id;
        this.name = name;
        this.age = age;
        this.email = email;
        this.deleted = deleted;
    }

    public User(String name, Integer age, String email, Integer deleted) {
        this.name = name;
        this.age = age;
        this.email = email;
        this.deleted = deleted;
    }

    @TableField(fill = FieldFill.INSERT)//Auto fill in time when adding
    private Date createTime;

    @TableField(fill = FieldFill.INSERT_UPDATE)//Auto fill time when modifying
    private Date updateTime;
}

4. Interface mapper

Create an interface and inherit BaseMapper

5. Add package scanning annotation in active class

6. Write in the test class, query according to ID and query all

7. Add insert (written in test class)

  /*newly added*/
   @Test
    public void testinsert(){
      /*  User user=new User(null,"Wang Wuwu, "15,"234@qq.com",0);*///snowflake
      User user=new User("Zhang San",12,"234@qq.com",0);//No, the primary key in the database must be incremented automatically*/
        int i=userMapper.insert(user);
       System.out.println(i);
   }

8. Delete (logical deletion is based on the ID. after deletion, it cannot be queried, but the records still exist in the database)

First, add a field in the database

Add the following annotations and fields to the entity class

The code written in the test class is as follows

/*
   * Logical deletion*/
   @Test
    public void testdelete(){
        int i=userMapper.deleteById(1);
       System.out.println(i);
   }

9. Modify update

To automatically fill in time, first create a configuration class with the following code

Add two fields to the database

Add two attributes to the entity class

@TableField(fill = FieldFill.INSERT)//Auto fill in time when adding
    private Date createTime;

    @TableField(fill = FieldFill.INSERT_UPDATE)//Auto fill time when modifying
    private Date updateTime;

The code in the test class is as follows

/*modify*/
    @Test
     public  void testupdate(){
      User user=new User(140688314676011008L,"Zhang San",16,"1111@qq.com",0);
      int i=userMapper.updateById(user);
        System.out.println(i);
     }

10. Query - query by criteria

  /*
     * Condition query
     * */
     @Test
    public  void  testSelectByCondication(){
        // Wrapper: conditional wrapper class
         QueryWrapper<User> wrapper=new QueryWrapper<>();
         wrapper.between("age",10,19);
         wrapper.or();
         wrapper.like("name","Small");
         List<User> list=userMapper.selectList(wrapper);
         System.out.println(list);
     }

11. Paging query

The paging plug-in needs to be introduced into the configuration class

 @Bean
    public MybatisPlusInterceptor mybatisPlusInterceptor() {
        MybatisPlusInterceptor interceptor = new MybatisPlusInterceptor();
        interceptor.addInnerInterceptor(new PaginationInnerInterceptor(DbType.MYSQL));
        return interceptor;
    }

Test class

     /*
     The first step is to configure the paging plug-in
     * Query pagination displays the current page number and the current number of pages
     * */
     @Test
    public void testSelectByPage(){
         Page<User> page=new Page<>(2,4);
         Page<User> page1=userMapper.selectPage(page,null);
         System.out.println("Current total page number:"+page1.getPages());
         System.out.println("Total number of articles:"+page1.getTotal());
         System.out.println("Record of current page:"+page1.getRecords());
     }