The difference between ${ } and #{ } in Java Mybatis
好了,真正做开发也差不多一年了。一直都是看别人的博客,自己懒得写,而且也不会写博客,今天就开始慢慢的练习一下写博客吧。前段时间刚好在公司遇到这样的问题。
一、举例说明
select * from user where name = "dato"; select * from user where name = #{name}; select * from user where name = '${name}';
一般情况下,我们都不会注意到这里面有什么不一样的地方。因为这些sql都可以达到我们的目的,去查询名字叫dato的用户。
二、区别
动态 SQL 是 mybatis 的强大特性之一,也是它优于其他 ORM 框架的一个重要原因。mybatis 在对 sql 语句进行预编译之前,会对 sql 进行动态解析,解析为一个 BoundSql 对象,也是在此处对动态 SQL 进行处理的。在动态 SQL 解析阶段, #{ } 和 ${ } 会有不同的表现
select * from user where name = #{name};
#{} 在动态解析的时候, 会解析成一个参数标记符。就是解析之后的语句是:
select * from user where name = ?;
那么我们使用 ${}的时候
select * from user where name = '${name}';
${}在动态解析的时候,会将我们传入的参数当做String字符串填充到我们的语句中,就会变成下面的语句
select * from user where name = "dato";
预编译之前的 SQL 语句已经不包含变量了,完全已经是常量数据了。相当于我们普通没有变量的sql了。
综上所得, ${ } 变量的替换阶段是在动态 SQL 解析阶段,而 #{ }变量的替换是在 DBMS 中。
这是 #{} 和 ${} 我们能看到的主要的区别,除此之外,还有以下区别:
方式能够很大程度防止sql注入。
$方式无法防止Sql注入。
$方式一般用于传入数据库对象,例如传入表名.
一般能用#的就别用$.
所以我们在使用mybatis的时候,尽量的使用#方式!!!这是大家要注意的地方
推荐教程:《Java教程》
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