Mybatis三分钟入门

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>
    <!--  为实体取个别名 -->
    <typeAliases>
        <typeAlias type="类的完全限定名" alias="别名"/>
        <!-- 也可以通过配置包的方式让mybatis自动为该包下的每个类以冠以该类的名称-->
        <package name="com.qfedu.mybatis.entity"/>
    </typeAliases>
    <environments default="development">
        <environment id="development">
            <!-- type可以为MANAGED交给web容器进行管理 -->
            <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>
    <!-- 包级别的路径 -->
    <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">
    <!-- 结果映射，如果名字和数据库是意义对应的话可以不用写 id，result等等，但要写的话必须每个字段都要意义对应，且全部都要写 -->
    <resultMap id="UserResult" type="User" >
        <!-- id映射 -->
        <id property="id" column="userId"/>
        <!-- 以下为属性映射 -->
        <result property="username" column="username"/>
        <result property="password" column="password"/>
        <!-- 容器映射 ofType:容器元素的类型,也可以引用另一个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语句 parameterType：参数类型， resultType:返回结果类型 #{}：使用和EL表达式差不多 
        例如#{username}相当于parameterType中的username属性
     -->
    <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.代码部分

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 {
        //获取配置文件内容
        InputStream inputStream = Resources.getResourceAsStream("com/qfedu/mybatis/mybatis-config.xml");
        //通过配置文件创建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();
        }
    }
}

总结：在学习mybatis的过程中，有一点特别注意，数据库的主键（一般是id）一定不要相同，加上前缀加以区分，否则mybatis查询时数据会遗漏。