mybatis中 if test判断容易踩的坑

问题描述

起因是在某次mybatis的select语句中使用了

update *** set
...
<if test="user.count"> count = #{user.count},</if>
...

来执行更新user表各字段，然后发现count = 0的时候不会触发count字段的更新，导致了bug出现。

解决方法和原因

猜测可能是源码里面把0, '', null都当作null来处理，类似弱类型的判断，所以尝试修改代码：

<if test="user.count != null"> count = #{user.count},</if>

这次成功触发了更新，果然是上述问题。

查找相关资料后大概了解了原因，其实mybatis底层是用OGNL表达式来解析的，而这个表达式会把number类型的非0解析为true，把0解析为false，导致上述问题触发，OGNL官网的描述如下：

Interpreting Objects as Booleans
Any object can be used where a boolean is required. OGNL interprets objects as booleans like this:</br>

• If the object is a Boolean, its value is extracted and returned;</br>
• If the object is a Number, its double-precision floating-point value is compared with zero; non-zero is treated as true, zero as false;</br>
• If the object is a Character, its boolean value is true if and only if its char value is non-zero;</br>
• Otherwise, its boolean value is true if and only if it is non-null.

所以修改为 !=null即可解决这个类型转换的问题。

参考文章：

https://www.cnblogs.com/grasp/p/11268049.html