MBP01 Import Mybatis-plus into Spring Boot

在Spring Boot项目内引入mybatis-plus

Install

Directly add maven dependencies in [pom.xml]. Note that the version of mysql-connector is no need to mention (see comment).

<!-- mybatis-plus-boot-starter -->
  <dependency>
    <groupId>com.baomidou</groupId>
    <artifactId>mybatis-plus-boot-starter</artifactId>
    <version>3.4.0</version>
  </dependency>

  <!-- mysql connector -->
  <dependency>
    <groupId>mysql</groupId>
    <artifactId>mysql-connector-java</artifactId>
    <!-- The managed version is 8.0.21 The artifact is managed in org.springframework.boot:spring-boot-dependencies:2.3.4.RELEASE -->
  </dependency>

注意我们是在spring boot中使用mybatis-plus，所以引入的包必须是mybatis-plus-boot-starter。

Configure

Write configurations in the [application.properties] under the resources folder:

spring.datasource.driver-class-name=com.mysql.cj.jdbc.Driver
spring.datasource.url=jdbc:mysql://localhost:3306/xxx?useSSL=true
spring.datasource.username=xxx
spring.datasource.password=xxxxxx

Entity class

Before creating entity classes, database tables should be designed and created.

In this example, I just create a simple user table and a objective table. And there is a one-to-many relationship.

create table users (
  id integer primary key not null auto_increment,
  username varchar(50) not null,
  password varchar(100) not null,
  question varchar(150),
  answer varchar(50)
);

create table objective (
  id integer primary key not null auto_increment,
  name varchar(50) not null,
  user_id integer not null,
  foreign key (user_id) REFERENCES users (id)
);

As for the entity class, I named it AppUser. We need to manually map the class to the table as they are different.

I recommend to use annotation which is clear and readable:

@TableName(value = "users")
public class AppUser {
    @TableId(value = "id", type = IdType.AUTO)
    private Integer id;
    @TableField(value = "username")
    private String username;
    @TableField(value = "password")
    private String password;
    @TableField(value = "question")
    private String question;
    @TableField(value = "answer")
    private String answer;
    @TableField(exit = false)
    private List<Objective> objectives;
}

If the column name in table is exactly the same with param name in class, then @TableField could be omitted.

@TableField(exit = false) indicates that there is no column in database that maps the property.

Primary key

We can use type property in @TableId to set the strategy in table's primary key.

Here are some options that will discussed in this article:

• ASSIGN_ID
• AUTO
• ASSIGN_UUID
• INPUT

ASSIGN_ID

The default strategy. Using algorithm (SnowFlake) to create a unique id. The type of primary key should be Long or String in Java, and BIGINT or VARCHAR in mysql.

AUTO

Using the auto increment strategy which is supported in database.

ASSIGN_UUID

Similar to ASSIGN_ID, but remove all '-' and the type must be String and VARCHAR.

INPUT

The id is input by users.

Mapper class

Just create an interface that extends BaseMapper. The interface has already intergrates basic CRUD methods.

public interface AppUserMapper extends BaseMapper<AppUser> {
    //public AppUser find
}

There are two ways to let Spring load the mapper classes:

1. Add @Mapper to every mapper classes, or
2. Add @MapperScan(basePackages="com...mapper") at the application entry class (shown below in the code)

@SpringBootApplication
@MapperScan(basePackages="com.myrrh.passbook.mapper")
public class PassbookApplication {

    public static void main(String[] args) {
        SpringApplication.run(PassbookApplication.class, args);
    }
}

Test

Just add some code in a Controller class (to make it simple, I skipped the service layer).

To run the code, just start the project and type the url in the browser.

@Autowired
AppUserMapper appUserMapper;

@RequestMapping("/testdb")
@ResponseBody
public String testdb() {
    // the id type is AUTO
    AppUser au = new AppUser("jisd", "89hf3", "My post-graduate best friend", "Hue");
    appUserMapper.insert(au);
    return "done";
}