Mybatis-plus

MyBatis-Plus

用Mybatid-plus实现增删改查

1.将StudentMapper接口继承BaseMapper，将拥有了BaseMapper中的所有方法：

public interface StudentMapper extends BaseMapper<Student> {
}

2.编写实体类

@Data
@AllArgsConstructor
@NoArgsConstructor
@TableName("students")//表名
public class Student {
    private String stuNum;
    private String stuName;
    private String stuGender;
    private int stuAge;
}

3.编写StudentMapper对应的xml文件

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

<mapper namespace="com.wenping.mapper.StudentMapper">

</mapper>

4.在数据库xml中配置StudentMapper.xml

<mappers>
        <mapper resource="UserMapper.xml"/>
        <mapper resource="StudentMapper.xml"/>
    </mappers>

5.使用MP中的MybatisSqlSessionFactoryBuilder进程构建,编写测试类

public class TestMybatis_plus {
    @Test
    public void testStudent() throws IOException {
        InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
        SqlSessionFactory sessionFactory = new MybatisSqlSessionFactoryBuilder().build(inputStream);

        SqlSession sqlSession = sessionFactory.openSession();
        StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);

        List<Student> students = mapper.selectList(null);

        for (Student student:students
             ) {
            System.out.println(student);
        }
    }
}

6.测试结果
![](https://img2022.cnblogs.com/blog/2525580/202205/2525580-20220513214416259-530148364.png)

7.出现的问题
7.1 table不存在
解决：在实体类上注解表名@TableName("students")
7.2 Unknowncolumn字段名infieldlist
解决：实体类名与数据库字段名不匹配
7.3 运行成功，但查询返回null
解决：在数据库中，如果存在字段为xx_xx之类的，在实现映射成实例的时候就要求实例的属性为驼峰标识，比如：xxXx，
也可以添加注解@TableField(数据库字段名)。
例：stu_num在实体类中写为stuNum
8.测试增删改
8.1.1添加数据
```java
//添加数据
    @Test
    public void testInsertStudent() throws IOException {
        InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
        SqlSessionFactory sessionFactory = new MybatisSqlSessionFactoryBuilder().build(inputStream);

        SqlSession sqlSession = sessionFactory.openSession(true);//ture为默认提交事务
        StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);
        Student student = new Student("2021005", "文文", "女", 18);
        mapper.insert(student);

    }

8.1.2结果

8.2.1查询特定数据

//查询带有特定条件的数据
    @Test
    public void testPartStudent() throws IOException {
        InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
        SqlSessionFactory sessionFactory = new MybatisSqlSessionFactoryBuilder().build(inputStream);

        SqlSession sqlSession = sessionFactory.openSession();
        StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);
        QueryWrapper<Student> queryWrapper = new QueryWrapper<Student>();
        queryWrapper.like("stu_gender", "男");
        List<Student> student = mapper.selectList(queryWrapper);

        for (Student s:student
        ) {
            System.out.println(s);
        }
    }

8.2.2结果

8.3.1 删除数据

 //删除数据
    @Test
    public void testDelStudent() throws IOException {
        InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
        SqlSessionFactory sessionFactory = new MybatisSqlSessionFactoryBuilder().build(inputStream);

        SqlSession sqlSession = sessionFactory.openSession(true);
        StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);
        mapper.deleteById(2021005);

    }

8.3.2找不到数据库中的ID
解决：

 @TableId(type = IdType.AUTO)//告诉下面的字段为id
    private String stuNum;