1.首先就是配置文件

<?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">
<configuration>
    <!--  為實體取個別名 -->
    <typeAliases>
        <typeAlias type="類的完全限定名" alias="別名"/>
        <!-- 也可以通過配置包的方式讓mybatis自動為該包下的每個類以冠以該類的名稱-->
        <package name="com.qfedu.mybatis.entity"/>
    </typeAliases>
    <environments default="development">
        <environment id="development">
            <!-- type可以為MANAGED交給web容器進行管理 -->
            <transactionManager type="JDBC" />
            <!-- 
                也可以配置JNDI
            < dataSource  type ="JNDI" >
                  < property  name ="DataSource"  value ="java:comp/env/jdbc/myDataSource" />
            </ dataSource > 
          -->
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.jdbc.Driver" />
                <property name="url" value="jdbc:mysql://localhost:3306/test?useUnicode=true&characterEncoding=utf8" />
                <property name="username" value="root" />
                <property name="password" value="ily" />
            </dataSource>
        </environment>
    </environments>
    <!-- 配置映射文件 -->
    <mappers>
    <!-- 包級別的路徑 -->
    <mapper resource="com/qfedu/mybatis/entity/UserMapper.xml" />
    </mappers>
</configuration>

2.然后是Mapper

<?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.qfedu.mybatis.dao.UserDao">
    <!-- 結果映射，如果名字和數據庫是意義對應的話可以不用寫 id，result等等，但要寫的話必須每個字段都要意義對應，且全部都要寫 -->
    <resultMap id="UserResult" type="User" >
        <!-- id映射 -->
        <id property="id" column="userId"/>
        <!-- 以下為屬性映射 -->
        <result property="username" column="username"/>
        <result property="password" column="password"/>
        <!-- 容器映射 ofType:容器元素的類型,也可以引用另一個resultMap -->
        <collection property="orders" ofType="Order">
            <id property="id" column="orderId"/>
            <result property="time" column="time"/>
            <result property="money" column="money"/>
        </collection>
    </resultMap>
    <!-- 
        以下配置sql語句 parameterType：參數類型， resultType:返回結果類型 #{}：使用和EL表達式差不多 
        例如#{username}相當于parameterType中的username屬性
     -->
    <delete id="delete" parameterType="User">
        delete from tb_user where username = #{username}
    </delete>
    <select id="findAllUser" resultMap="UserResult"> 
        select u.*, o.* from tb_user as u left join tb_orders as o on u.userId = o.userId
    </select>

    <select id="findById" parameterType="java.lang.Integer" resultType="User">
        select u.*, o.* from tb_user as u, tb_orders as o where u.id = #{id} and u.userId = o.userId
    </select>
    
    <insert id="save" parameterType="User"> 
        insert into tb_user values(default, #{username}, #{password}) 
    </insert>
</mapper>

3.代碼部分

public class User implements Serializable {

    private static final long serialVersionUID = 1L;
    private Integer id;
    private String username;
    private String password;
    private List<Order> orders;

    //省略getter和setter
}

public class Order implements Serializable {

    private static final long serialVersionUID = 1L;
    private Integer id;
    private Date time;
    private BigDecimal money;

}

public class Test {

    public static void main(String[] args) throws IOException {
        //獲取配置文件內容
        InputStream inputStream = Resources.getResourceAsStream("com/qfedu/mybatis/mybatis-config.xml");
        //通過配置文件創建SqlSessionFacroty
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession session = sqlSessionFactory.openSession();
        try {
            UserDao userDao = session.getMapper(UserDao.class);
            List<User> user = userDao.findAllUser();
            session.commit();
        } finally {
            session.close();
        }
    }
}

總結：在學習mybatis的過程中，有一點特別注意，數據庫的主鍵（一般是id）一定不要相同，加上前綴加以區分，否則mybatis查詢時數據會遺漏。