#{}：是以预编译的映射，将参数设置到sql语句中，和jdbc的preraredStatement一样，使用占位符，防止sql注入。

${}：取出的值会直接拼装在sql中，会有安全问题。

大多数情况下的参数取值，我们都要用#{}的方式取值。

但是原生jdbc不支持占位符的地方，例如：分表，排序等等。。。我们可以使用${}

分表：比如按照年份的分表查询员工绩效等等如下：

select * from ${year}_table a where 1=1 order by  a.age  ${desc}

package com.yunqing.mybatis.dao;

import com.yunqing.mybatis.bean.User;
import org.apache.ibatis.annotations.Param;
import org.apache.ibatis.annotations.Select;

import java.util.List;
import java.util.Map;

public interface UserMapper {

//主要体现在此处，利用${tableName}获取了jdbc无法使用占位符？替代的数据库，其次用${orderName}获取了jdbc无法使用占位符？代替的排序desc倒序
    @Select("select * from ${tableName} order by id ${orderName}")
    List<User> getAllUser(Map<String,Object> map);

    User getUserByIdAndName(@Param("id")Integer id, @Param("name")String name);

    User getUserByMap(Map<String,Object> map);

    //User getUserByPoJo(User user);

    void insertUser(User user);

    void updateUser(User user);

    void deleteUserById(Integer id);


}

@Test
    public void getAllUser() throws IOException {
        //从xml中获取sqlSessionFactory
        String resource = "conf/mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);

        //获取sqlSession
        SqlSession sqlSession = sqlSessionFactory.openSession();
        //现在的mybatis接口式编程写法
        //相当于接口的实现类
        UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
        Map<String,Object> map = new HashMap<>();
//此处传入数据库名和排序的方式。
        map.put("tableName","t_user");
        map.put("orderName","desc");
        List<User> list = userMapper.getAllUser(map);
        System.out.println(list);
        sqlSession.close();
    }

注意：当插入一个null值的时候，#{email}在mysql中可以插入成功。

在oracle中会插入失败，报错。原因是jdbcType会在插入null值时转化成OTHER类型，oracle不支持OTHER类型。

两种解决办法：

1.#{email,jdbcType=NULL}

2.在全局设置中

<settings>
        <setting name="jdbcTypeForNull" value="NULL"/>
    </settings>

返回map形式的结果：

dao层接口：

　　 Map<String,Object> getUserByIdReturnMap(Integer id);
//告诉mybatis封装的时候哪个属性作为map的key
    @MapKey("name")
    Map<String,User> getAllUserReturnMap();

sql  xml

    <select id="getUserByIdReturnMap" resultType="map">
        SELECT * FROM t_user WHERE id = #{id}
    </select>
    <select id="getAllUserReturnMap" resultType="com.yunqing.mybatis.bean.User">
        SELECT * FROM t_user
    </select>

test测试类

/**
     * 结果返回map
     * @throws IOException
     */
    @Test
    public void getUserByIdReturnMap() throws IOException {
        String r = "mybatis-config.xml";
        InputStream is = Resources.getResourceAsStream(r);
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(is);
        SqlSession sqlSession = sqlSessionFactory.openSession(true);
        UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
        Map<String,Object> map = userMapper.getUserByIdReturnMap(1);
        System.out.println(map);
    }

    /**
     * 返回此种形式的map ---> Map<String,User> ---> pdd=User{id=9, name='pdd', age=33}
     * @throws IOException
     */
    @Test
    public void getAllUserReturnMap() throws IOException {
        String r = "mybatis-config.xml";
        InputStream is = Resources.getResourceAsStream(r);
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(is);
        SqlSession sqlSession = sqlSessionFactory.openSession(true);
        UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
        Map<String,User> map = userMapper.getAllUserReturnMap();
        System.out.println(map);
    }

测试结果：

接口1：{name=yx, id=1, age=12}

接口2：{pdd=User{id=9, name=’pdd’, age=33}, uzi=User{id=7, name=’uzi’, age=20}, yang=User{id=2, name=’yang’, age=23}, yangxu=User{id=3, name=’yangxu’, age=34}, kang=User{id=5, name=’kang’, age=18}, yx=User{id=1, name=’yx’, age=12}}