MyBatis入门（idea环境）

以查询员工信息为例入门mybatis！

一、使用idea创建maven项目：

创建好后，项目结构如下

image.png

二、配置pom.xml

主要配置resources目录、mysql-connector、mybatis

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.mybatis.example</groupId>
    <artifactId>Mybatis</artifactId>
    <version>1.0-SNAPSHOT</version>
    <build>
        <resources>
            <!-- 配置resources目录在java目录下 -->
            <resource>
                <directory>src/main/java</directory>
                <includes>
                    <include>**/**</include>
                </includes>
            </resource>
        </resources>
    </build>
    <dependencies>
        <!-- mysql 连接驱动 -->
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>5.1.47</version>
        </dependency>

        <!-- mybatis -->
        <dependency>
            <groupId>org.mybatis</groupId>
            <artifactId>mybatis</artifactId>
            <version>3.4.6</version>
        </dependency>

    </dependencies>
</project>

注意：在eclipse中直接new Source Folder就行，但是在idea中需要配置resource的目录

image.png

三、新建包名，以及员工实体类

image.png

public class Employee {
    private Integer id;
    private String lastName;
    private Integer gender;
    private String email;

    public Employee() {
    }

    // getter and setter ...

    @Override
    public String toString() {
        return "Employee{" +
                "id=" + id +
                ", lastName='" + lastName + '\'' +
                ", gender=" + gender +
                ", email='" + email + '\'' +
                '}';
    }
}

四、在resource目录下新建：

mybatsis-config.xml 和 EmployeeMapper配置文件

image.png

mybatis-config.xml:

<?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">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://localhost:3306/mybatis"/>
                <property name="username" value="root"/>
                <property name="password" value="123456"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <mapper resource="EmployeeMapper.xml"/>
<!--        <package name="com.mybatis.example"/>-->
    </mappers>
</configuration>

EmployeeMapper.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">
<mapper namespace="com.mybatis.example.EmployeeMapper">
    <select id="selectEmployee" resultType="com.mybatis.example.bean.Employee">
        select * from employee where id = #{id}
    </select>
</mapper>

五、编写main方法测试

package com.mybatis.example;

import com.mybatis.example.bean.Employee;
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 Main {

    public static void main(String[] args) throws IOException {
        String resource = "mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession session = sqlSessionFactory.openSession();
        Employee employee = session.selectOne("com.mybatis.example.EmployeeMapper.selectEmployee", 1);
        System.out.println(employee);
        session.close();
    }
}

输出结果：

Employee{id=1, lastName='null', gender=1, email='tom@qq.com'}

lastName='null' 这个问题等下再说。

六、以上就是以Mapper.xml配置文件的方式，访问数据库。

若不想使用EmployeeMapper.xml文件，则可以使用下面的方式：

1：更改mybatis-config.xml文件中的mappers节点为：

 <mappers>
<!--        <mapper resource="EmployeeMapper.xml"/>-->
        <package name="com.mybatis.example"/>
    </mappers>

2.新建EmployeeMapper接口

image.png

package com.mybatis.example;

import com.mybatis.example.bean.Employee;
import org.apache.ibatis.annotations.Select;

public interface EmployeeMappger {

    @Select("select * from employee where id = #{id}")
    Employee selectEmployee(int id);

}

3.修改main方法

    public static void main(String[] args) throws IOException {
        String resource = "mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession session = sqlSessionFactory.openSession();
//        Employee employee = session.selectOne("com.mybatis.example.EmployeeMapper.selectEmployee", 1);
        //先获取mapper对象，调用mapper对象的selectEmployee方法
        EmployeeMappger mapper = session.getMapper(EmployeeMappger.class);
        Employee employee = mapper.selectEmployee(1);
        System.out.println(employee);
        session.close();
    }

打印结果：

Employee{id=1, lastName='null', gender=1, email='tom@qq.com'}

七、下面解决刚才遇到的问题，lastName=null

由于数据库中的字段为last_name而实体类中的字段为lastName，所以未能匹配上。
解决方法：修改EmployeeMapper或EmployeeMapper.xml中的sql语句为：

"select id, last_name lastName, gender, email from employee where id = #{id}"
或
"select id, last_name as lastName, gender, email from employee where id = #{id}"
as可以省略

即可。
即给last_name起个别名为lastName。

注意！！！！之前没有在pom.xml中配置

 <build>
        <resources>
            <!-- 配置resources目录在java目录下 -->
            <resource>
                <directory>src/main/java</directory>
                <includes>
                    <include>**/**</include>
                </includes>
            </resource>
        </resources>
    </build>

导致找不到mybatis-config.xml，等很多问题，耽误了不少时间。