第一个Mybatis程序

思路：搭建环境–>导入Mybatis–>编写代码–>测试

搭建环境

搭建数据库环境

create table mybatis;

use mybatis;

create table user (
id int(20) not null primary key,
username varchar(30) default null,
pwd varchar(30) default null

) engine=INNODB DEFAULT CHARSET=utf8;

insert into user(id,username,pwd) values
(1,'admin','123456'),
(2,'张三','123456'),
(3,'李四','123456');

新建项目

1、新建一个普通的maven项目
2、删除src目录
3、导入maven依赖

<dependencies>
        <!--mysql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>3.5.2</version>
        </dependency>
        <!--junit-->
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.12</version>
        </dependency>
    </dependencies>

创建一个模块

• 编写mybatis核心配置文件

<?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-->
<configuration>
    <!--环境-->
    <environments default="development">
        <environment id="development">
            <!--事务管理-->
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://localhost:3306/mybatis?useSSL=true&amp;useUnicode=true&amp;characterEncoding=UTF-8"/>
                <property name="username" value="root"/>
                <property name="password" value="root"/>
            </dataSource>
        </environment>
    </environments>

</configuration>

• 编写mybatis工具类

public class MybatisUtils {

    private static SqlSessionFactory sqlSessionFactory;

    static{
        try {
            // 使用Mybatis第一步：获取SqlSessionFactory对象
            String resource = "mybatis-config.xml";
            InputStream inputStream = Resources.getResourceAsStream(resource);
            sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    
    //既然有了 SqlSessionFactory，顾名思义，我们可以从中获得 SqlSession 的实例。
    // SqlSession 提供了在数据库执行 SQL 命令所需的所有方法。
    public static SqlSession getSqlSession() {
        return sqlSessionFactory.openSession();
    }
}

编写代码

• 实体类（实体类属性名要和数据库表中的属性名对应）

public class User {
    private int id;
    private String username;
    private String pwd;

    public User() {

    }

    public User(int id, String username, String pwd) {
        this.id = id;
        this.username = username;
        this.pwd = pwd;
    }

    public int getId() {
        return id;
    }

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

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getPwd() {
        return pwd;
    }

    public void setPwd(String pwd) {
        this.pwd = pwd;
    }

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", username='" + username + '\'' +
                ", pwd='" + pwd + '\'' +
                '}';
    }
}

• Dao接口

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

• 接口实现类有原来的UserDaoImpl转变为一个Mapper配置文件

<?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">
<!--namespace=绑定一个对应的Dao/Mapper接口-->
<mapper namespace="com.lyh.dao.UserDao">
    <!--id对应UserDao中的方法名-->
    <select id="getUserList" resultType="com.lyh.pojo.User">
        select * from mybatis.user
    </select>
</mapper>

测试

public class UserDaoTest {

    @Test
    public void test(){

        //第一步：获得sqlSession对象
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        //执行SQL
        //方式一：getMapper
        UserDao mapper = sqlSession.getMapper(UserDao.class);
        List<User> userList = mapper.getUserList();
        
        //方式二：不推荐使用
       // List<User> userList = sqlSession.selectList("com.lyh.dao.UserDao.getUserList");
        
        for (User user: userList){
            System.out.println(user);
        }
        // 关闭SqlSession
        sqlSession.close();
    }
}

注意可能会用到的问题：
错误一：配置文件没有注册
org.apache.ibatis.binding.BindingException: Type interface com.lyh.dao.UserDao is not known to the MapperRegistry.

 <!--每一个Mapper.xml都需要在Mybatis核心配置文件中注册-->
<mappers>
    <mapper resource="com/lyh/dao/UserMapper.xml/"></mapper>
</mappers>

错误二：Maven导出资源问题
Could not find resource com/lyh/dao/UserMapper.xml/
解决方案：

<!--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>