MyBatis学习总结（14）——Mybatis使用技巧总结

刺骨的言语ヽ痛彻心扉 2022-01-13 05:31 224阅读 0赞

[2019独角兽企业重金招聘Python工程师标准>>> ][2019_Python_] ![hot3.png][]

# 1、 区分 \#\{\} 和 $\{\}的不同应用场景 #

**1）\#\{\} 会生成预编译SQL，会正确的处理数据的类型，而$\{\}仅仅是文本替换。  
对于SQL： select \* from student where xCode = ‘S123456’；  
如果使用\#\{\}  
那么生成的SQL为：  
select \* from student where xCode = ? 传的值为’S123456’；  
如果使用$\{\}  
那么生成的SQL为：select \* from student where xCode = S123456  
如果xCode的数据类型为varchar,那么使用$\{\}就会报错。  
2）$\{\}一般用在order by, limit, group by等场所。  
假设我们使用\#\{\} 来指定order by字段，比如  
select \* from student order by \#\{xCode\},  
那么产生的SQL为  
select \* from student order by ?, 替换值后为  
select \* from student order by ‘xCode’  
Mybatis对xCode加了引号导致排序失败**

# 2、Spring环境用Mybatis-Spring的接口而不是Mybatis的原生接口 #

**在spring 环境使用mybatis-spring的好处有：  
1）我们可以使用Sping的声明式事务处理模型(@Transactional),而不用手动回归事务。  
2）mybatis-spring会优雅的关闭SqlSession，而不用手动关闭  
3）可以将数据库连接池交给spring管理，当程序停止的时候，spring会合适的关闭连接**

# 3、返回Map<ID, Entity>而不是List便于查找 #

**有时太多的表连接（join）性能太差，我们会将该SQL拆为多个SQL，然后在代码中组装起来。比如学生表和班级表，需要查询的结果为”学号，班级，姓名”，我们可以先查询“学号，班级ID,姓名”以及“班级ID，班级名次”，我们可以在查询班级表的时候返回Map<班级ID, 班级>, 然后迭代学生表的结果集，用班级ID到Map<班级ID, 班级>中查找对应的班级信息，然后用班级名称替换班级ID。  
接口声明为SqlSession.selectMap(String statement, String mapKey)**

# 4、使用Map封装查询的结果 #

**有时我们厌倦了为每个查询写一个Entity类，这时Map开始发挥它的功效。  
对于要返回“学号，班级，姓名”结果的查询，可以这样写Mapper:**

**如下声明我们的dao方法：**

**如果要将该查询结果转为JSON字符串返回，那么我们就可以直接将List<Map<String, Object>转为JSON，逻辑层不需要任何代码。  
如果返回的结果集需要按select中的字段顺序返回，那么将resultType=”java.util.HashMap” 换为resultType=”java.util.LinkedHashMap”**

# 5、使用Map封装查询结果时注意数据的类型映射 #

**对于如下的Mapper**

**Mybatis会傻傻的将sName的数据类型映射为byte\[\], 因为我们没有提供entity,mybatis也不知道我们想要什么类型，而sName是计算出来的值，mybatis也没有办法从数据库中获取字段的值，所以它就将其封装为byte\[\],解决办法很简单，加一个cast 函数**

# 6、正确的配置Mybatis 的Log #

**1）一个应用一般会使用很多的jar,各个jar依赖的log 实现不一样，Mybatis查找Log的顺序为（SLF4J,Apache Commons Logging,Log4j 2,Log4j,JDK logging），如果classpath中有slf4j记得添加相应的桥接jar,比如slf4j-log4j。许多web 服务器的classpath 会含有Apache Commons Logging，因此如果要使用Log4j,要么使用SLF4J桥接Log4j，要么在配置中强制指定使用Log4J。**

# 7、警惕Mybatis的Foreach的的副作用 #

**对于如下SQL：  
假设有如下的mapper:**

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      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>8</strong></span> 
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      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>9</strong></span> 
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      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>10</strong></span> 
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      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>11</strong></span> 
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      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>12</strong></span> 
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      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>13</strong></span> 
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      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>14</strong></span> 
     </div> 
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      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>15</strong></span> 
     </div> 
    </div> </td> 
   <td style="border:0px;width:672px;vertical-align:top;background:0px 50%;"> 
    <div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&lt;select id=”testForeach”&nbsp;&nbsp;parameterType=”map”&nbsp;&nbsp;resultType=”Student”&gt;</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;Select * from student</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;&lt;where&gt; </strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;&lt;if test=”ID != null and ID != ‘’ ”&gt;</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;ID = #{ ID}</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;&lt;/if&gt;</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;&lt;if test=” IDArr != null and IDArr.size()&gt;0”&gt;</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;And ID IN</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&lt;foreach collection="IDArr" open="("</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;separator="," close=")" item="ID"&gt;</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;${ ID}</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&lt;/foreach&gt;</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&lt;/if&gt;</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&lt;/where&gt;</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&lt;/select&gt;</strong></span> 
     </div> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

**当我们传入的IDArr时，最后产生的SQL为：  
Select \* from student where ID = ‘998’ AND ID IN ( ‘123’, ’234’,…..,’998’)  
解决办法：  
解决办法有  
1） 将红色的ID 换成别的名称，比如“item”。  
2） 这两个if 是对同一个字段判断，改为choose… when 结构**

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      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>9</strong></span> 
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      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>10</strong></span> 
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      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>11</strong></span> 
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      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>12</strong></span> 
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      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>13</strong></span> 
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      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>14</strong></span> 
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      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>15</strong></span> 
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      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>16</strong></span> 
     </div> 
    </div></td> 
   <td style="border:0px;width:672px;vertical-align:top;background:0px 50%;"> 
    <div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&lt;select id=”testForeach”&nbsp;&nbsp;parameterType=”map”&nbsp;&nbsp;resultType=”Student”&gt;</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;Select * from student</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;&lt;where&gt; </strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;&lt;choose&gt;</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&lt;when test=”ID != null and ID != ‘’ ”&gt;</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;ID = #{ ID}</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&lt;/when&gt;</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&lt;when test=” IDArr != null and IDArr.size()&gt;0”&gt;</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;And ID IN</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&lt;foreach collection="IDArr" open="("</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;separator="," close=")" item="ID"&gt;</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;${ ID}</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&lt;/foreach&gt;</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&lt;/when&gt;</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&lt;/choose&gt;</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&lt;/where&gt;</strong></span> 
     </div> 
    </div></td> 
  </tr> 
 </tbody> 
</table>

# 8、使用原生的SQL操作数据以提高效率 #

**对于一次插入多条数据，将其组装成 insert into xxx values (), () ()格式一次插入多行数据往往能极大的提高性能。**

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      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>1</strong></span> 
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      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>2</strong></span> 
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      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>3</strong></span> 
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      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>4</strong></span> 
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      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>5</strong></span> 
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      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>6</strong></span> 
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      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>7</strong></span> 
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      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>8</strong></span> 
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      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>9</strong></span> 
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      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>10</strong></span> 
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      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>11</strong></span> 
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      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>12</strong></span> 
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      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>13</strong></span> 
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      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>14</strong></span> 
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      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>15</strong></span> 
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      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>16</strong></span> 
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      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>17</strong></span> 
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      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>18</strong></span> 
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      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>19</strong></span> 
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      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>20</strong></span> 
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      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>21</strong></span> 
     </div> 
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      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>22</strong></span> 
     </div> 
    </div> </td> 
   <td style="border:0px;width:672px;vertical-align:top;background:0px 50%;"> 
    <div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>public int execute(String sql) { </strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;int affectedCount = 0;</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;Connection conn = null;</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;Statement stm = null;</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;try { </strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;conn = getSqlSession().getConnection();</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;stm = conn.createStatement();</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;stm.execute(sql);</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;affectedCount = stm.getUpdateCount();</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;} catch (SQLException e) { </strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;throw new RuntimeException(“execute[" + sql + "] failed”);</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;} finally { </strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;try{ </strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if(stm != null &amp;amp;&amp;amp; !stm.isClosed()) { </strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;stm.close();</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//conn will be released by mybatis framework</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;} catch(SQLException e) { </strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;}</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>&nbsp;&nbsp;&nbsp;&nbsp;return affectedCount;</strong></span> 
     </div> 
     <div> 
      <span style="font-family:'Courier New';font-size:18px;color:#ff0000;"><strong>}</strong></span> 
     </div> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

# 9、警惕MyBatis封装数据时性能损耗 #

**对于如下的mapper**

**在一个批处理程序中循环的调用了该方法250次，每次返回大概1w条记录，发现这个程序运行的很慢，用jrofiler 查看各个方法耗费的时间，居然80%的时间花在了student的setter上了，在这个过程中大概产生了250w个对象，而mybatis是利用发射封装Entity，代码大致如下：**

**解决的办法是让返回的行数和返回的字段尽量的少。**

转载于:https://my.oschina.net/zhanghaiyang/blog/606033

[2019_Python_]: https://my.oschina.net/u/2663968/blog/3061697
[hot3.png]: /images/20220113/c0c30d5f0cac45068d373013078d375d.png