第一个MyBatis程序

MyBatis 是一款优秀的持久层框架，它支持自定义 SQL、存储过程以及高级映射。

MyBatis 免除了几乎所有的 JDBC 代码以及设置参数和获取结果集的工作。

MyBatis 可以通过简单的 XML 或注解来配置和映射原始类型、接口和 Java POJO（Plain Old Java Objects，普通老式 Java 对象）为数据库中的记录。

项目结构目录

搭建程序 

1. 搭建 Maven 父子关系项

 2. 父项目 pom.xml 中导入项目依赖

<dependencies>
	<dependency>
		<groupId>junit</groupId>
		<artifactId>junit</artifactId>
		<version>4.11</version>
	</dependency>
	<dependency>
		<groupId>mysql</groupId>
		<artifactId>mysql-connector-java</artifactId>
		<version>8.0.29</version>
	</dependency>
	<dependency>
		<groupId>org.mybatis</groupId>
		<artifactId>mybatis</artifactId>
		<version>3.5.2</version>
	</dependency>
</dependencies>

3. 子Module内创建 package，分别为 dao、pojo、utils

 4.  在POJO中创建对应的实体类，如 User

public class User {
	private Integer id;
	private String username;
	
	.......Getter And Setter.....
		
	@Override
	public String toString() {
		return "User{" +
			"id=" + id +
			", username='" + username + '\'' +
			'}';
	}
}

5. 在resources目录中创建MyBatis配置文件 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">
<!--核心配置文件-->
<configuration>
	<!--  多环境配置  -->
	<environments default="development">
		<!--开发环境-->
		<environment id="development">
			<transactionManager type="JDBC"/>
			<dataSource type="POOLED">
				<property name="driver" value="com.mysql.cj.jdbc.Driver"/>
				<!--& 符号用 &amp;替代-->
				<property name="url" value="jdbc:mysql://127.0.0.1:3306/mybaits?useSSL=true&amp;useUnicode=true&amp;characterEncoding=UTF8"/>
				<property name="username" value="root"/>
				<property name="password" value="123456"/>
			</dataSource>
		</environment>
	</environments>
	<!-- 每一个mapper.xml都需要在核心配置文件中注册 -->
	<mappers>
		<mapper resource="com/free/dao/UserMapper.xml"/>
	</mappers>
</configuration>

6. 在 utils目录内通过 mybatis-config.xml 构建 SqlSessionFactory

package com.free.utils;

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 DB {
    private static SqlSessionFactory sqlSessionFactory;
    static {
        String resource = "mybatis-config.xml";
        InputStream inputStream = null;
        try {
            inputStream = Resources.getResourceAsStream(resource);
        } catch (IOException e) {
            e.printStackTrace();
        }
        sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
    }

    //从 SqlSessionFactory 中获取 SqlSession
    public static SqlSession getConnect(){
        return  sqlSessionFactory.openSession();
    }
}

7. 在dao目录内创建UserMapper接口以及UserMapper.xml

package com.free.dao;

import com.free.pojo.User;

import java.util.List;

public interface UserMapper {
    public List<User> getUserList();
}

<?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">

<!--namespace 绑定一个对应的DAO/Mapper接口-->
<mapper namespace="com.free.dao.UserMapper">
    <!-- id：接口内的方法  resultType：返回的实体类-->
    <select id="getUserList" resultType="com.free.pojo.User">
        select * from user limit 10
    </select>
</mapper>

8.编写测试类

 

package com.free.dao;

import com.free.pojo.User;
import com.free.utils.DB;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;

import java.util.List;

public class UserMapperTest {

    @Test
    public void test(){
        SqlSession connect = DB.getConnect();
        //方式一 getMapper
        UserMapper user = connect.getMapper(UserMapper.class);
        List<User> userList = user.getUserList();

        for(User item: userList){
            System.out.println(item);
        }
    }
}

可能遇到的问题与解决方案

1. org.apache.ibatis.binding.BindingException: Type interface com.free.dao.UserDao is not known to the MapperRegistry.

解决：

因为 每一个mapper.xml都需要在核心配置文件中注册，所以在 mybatis-config.xml 文件中添加

<mappers>
    <mapper resource="com/free/dao/UserMapper.xml"/>
</mappers>

2. Cause: org.apache.ibatis.builder.BuilderException: Error parsing SQL Mapper Configuration. Cause: java.io.IOException: Could not find resource com/free/dao/UserMapper.xml

解决：

因为maven默认只导出resource目录中xml，所以需要再 pom.xml 添加配置

添加完配置后，一定要刷新下 maven，防止有缓存

<!--在build中配置resources,来防止我们资源导出失败的问题 -->
<build>
    <resources>
        <resource>
            <directory>src/main/resources</directory>
            <includes>
                <include>**/*.properties</include>
                <include>**/*.xml</include>
            </includes>
            <filtering>true</filtering>
        </resource>
        <resource>
            <directory>src/main/java</directory>
            <includes>
                <include>**/*.properties</include>
                <include>**/*.xml</include>
            </includes>
            <filtering>true</filtering>
        </resource>
    </resources>
</build>