Java编程：MyBatis读取数据库数据

2021-09-11 05:08:53

1、文件结构

一共涉及3个文件夹8个文件，心累…

注意config文件夹和lib文件夹需要设置为资源文件夹，不然找不到文件

2、依赖下载

放入lib文件夹

mybatis： https://github.com/mybatis/mybatis-3/releases

mysql-connector: https://dev.mysql.com/downloads/connector/j/

3、数据准备

demo的数据库下有一个names的数据表，字段类型和数据如下

mysql> desc names;
+-------+--------------+------+-----+---------+----------------+
| Field | Type         | Null | Key | Default | Extra          |
+-------+--------------+------+-----+---------+----------------+
| id    | int(10)      | NO   | PRI | NULL    | auto_increment |
| name  | varchar(255) | YES  | UNI | NULL    |                |
| age   | int(10)      | YES  |     | NULL    |                |
+-------+--------------+------+-----+---------+----------------+

mysql> select * from names;
+----+--------+------+
| id | name   | age  |
+----+--------+------+
|  2 | 大红   |   27 |
|  3 | 大壮   |   24 |
|  4 | 秀英   |   25 |
+----+--------+------+

4、Person类编写

其实就是一个普通的java类

Person.class


public class Person {
    private Integer id;
    private  String name;
    private Integer age;

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String toString(){
        return "[Person] id: "+ id + " name: " + name + " age :"+ age;
    }
}

5、配置映射器

方式一文件配置映射器：

映射接口 PersonMapper.class

public interface PersonMapper {
    public Person getPersonById(Integer id);
}

映射文件 PersonMapper.xml

<?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">

<!--
1.PersonMapper为命名空间,为映射接口的全类名
2.id为唯一标识，与PersonMapper接口中的对应方法名相同
3.resultType为查询后的返回类型
4.#{id}指从传递过来的参数中取出id值
-->
<mapper namespace="PersonMapper">
    <select id="getPersonById" resultType="Person">
    select id,name, age from names where id = #{id}
  </select>
</mapper>

方式二注解配置映射器：

PersonMapperAnnoation.class

import org.apache.ibatis.annotations.Select;

public interface PersonMapperAnnoation {
        @Select("select id, name, age from names where id = #{id}")
        public Person getPersonById(Integer id);
}

这样看来，方式2更为简洁和清晰

6、数据库配置填入数据库的地址，账号，密码

配置映射器，可以使用两种方式resource 和 class

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN" "http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
    <environments default="development">
        <environment id="development">
            <!-- 配置事务管理器，采用的是JDBC的管理器方式 -->
            <transactionManager type="JDBC"/>
            <!--  POOLED代表采用MyBatis内部提供的连接池方式 -->
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://localhost:3306/demo"/>
                <property name="username" value="root"/>
                <property name="password" value="123456"/>
            </dataSource>
        </environment>
    </environments>
    <!--  引入映射器 包名+文件名-->
    <mappers>
        <mapper resource="PersonMapper.xml"/>
        <mapper class="PersonMapperAnnoation"/>
    </mappers>
</configuration>

7、代码测试

import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;

import java.io.IOException;
import java.io.InputStream;

public class mybatisDemo {
    public static void main(String[] args) throws IOException{

        //1.根据MyBatis的配置文件，即mybatis-config.xml创建SqlSessionFactory
        SqlSessionFactory sqlSessionFactory = getSqlSessionFactory();
        //2.获取session实例开启会话，其能直接执行*已经映射的SQL语句*
        SqlSession session = sqlSessionFactory.openSession();
        try {
            // 方式1 查询
            //3.获取接口的实现类对象
            PersonMapper personMapper = session.getMapper(PersonMapper.class);
            //4.执行查询操作
            Person person = personMapper.getPersonById(2);
            System.out.println(person);
            
            // 方式2 查询
            //3.获取接口的实现类对象
            PersonMapperAnnoation personMapperAnnoation = session.getMapper(PersonMapperAnnoation.class);
            //4.执行查询操作
            Person person1 = personMapperAnnoation.getPersonById(2);
            System.out.println(person1);

        } finally {
            //4.关闭会话session
            session.close();
        }
    }

    //根据MyBatis的配置文件，即mybatis-config.xml创建SqlSessionFactory
    public static SqlSessionFactory getSqlSessionFactory() throws IOException {
        String resource = "mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        return sqlSessionFactory;
    }

}

/*
两种方式都能查到数据
[Person] id: 2 name: 大红 age :27
[Person] id: 2 name: 大红 age :27
*/

参考

（二）MyBatis学习笔记-HelloWorld

码农公寓