Mybatis的Mapper中的方法为什么不能重载

前言

今天就写一篇文章从源码角度为大家解惑为什么Mybatis中的方法不能重载？

环境配置

本篇文章讲的一切内容都是基于Mybatis3.5和SpringBoot-2.3.3.RELEASE。

错误示范

举个栗子：假设现在有两个需求，一个是根据用户的id筛选用户，一个是根据用户的性别筛选，此时在Mapper中重载的方法如下：

public interface UserMapper {
 List selectList(@Param("userIds") List userIds);

 List selectList(Integer gender);
 }

这个并没有什么错误，但是启动项目，报出如下的错误：

Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'sqlSessionFactory' defined in class path resource [org/mybatis/spring/boot/autoconfigure/MybatisAutoConfiguration.class]: Bean instantiation via factory method failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [org.apache.ibatis.session.SqlSessionFactory]: Factory method 'sqlSessionFactory' threw exception; nested exception is org.springframework.core.NestedIOException: Failed to parse mapping resource: 'file [H:\work_project\demo\target\classes\mapper\UserInfoMapper.xml]'; nested exception is org.apache.ibatis.builder.BuilderException: Error parsing Mapper XML. The XML location is 'file [H:\work_project\demo\target\classes\mapper\UserInfoMapper.xml]'. Cause: java.lang.IllegalArgumentException: Mapped Statements collection already contains value for cn.cb.demo.dao.UserMapper.selectList. please check file [H:\work_project\demo\target\classes\mapper\UserInfoMapper.xml] and file [H:\work_project\demo\target\classes\mapper\UserInfoMapper.xml]
at org.springframework.beans.factory.support.ConstructorResolver.instantiate(ConstructorResolver.java:655)
at org.springframework.beans.factory.support.ConstructorResolver.instantiateUsingFactoryMethod(ConstructorResolver.java:635)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateUsingFactoryMethod(AbstractAutowireCapableBeanFactory.java:1336)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1176)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:556)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:516)
at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:324)
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:226)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:322)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202)
at org.springframework.beans.factory.config.DependencyDescriptor.resolveCandidate(DependencyDescriptor.java:276)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1307)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1227)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireByType(AbstractAutowireCapableBeanFactory.java:1509)
... 81 more

这么一大串什么意思？懵逼了~

大致的意思：cn.cb.demo.dao.UserMapper.selectList这个id已经存在了，导致创建sqlSessionFactory失败。

为什么不能重载？

• 通过上面的异常提示可以知道创建sqlSessionFactory失败了，这个想必已经不陌生吧，顾名思义，就是创建SqlSession的工厂。
• Springboot与Mybatis会有一个启动器的自动配置类MybatisAutoConfiguration，其中有一段代码就是创建sqlSessionFactory，如下图：

既然是创建失败，那么肯定是这里出现异常了，这里的「大致思路」就是：

解析XML文件和Mapper接口，将Mapper中的方法与XML文件中、等标签一一对应，那么Mapper中的方法如何与XML中这些标签对应了，当然是唯一的id对应了，具体如何这个id的值是什么，如何对应？下面一一讲解。

protected final Map mappedStatements = new StrictMap("Mapped Statements collection")
 .conflictMessageProducer((savedValue, targetValue) ->
  ". please check " + savedValue.getResource() + " and " + targetValue.getResource());

public void addMappedStatement(MappedStatement ms) {
 //key 是id 
 mappedStatements.put(ms.getId(), ms);
 }

List userInfos = userMapper.selectList(Arrays.asList("192","198"));

public MappedStatement getMappedStatement(String id) {
 return this.getMappedStatement(id, true);
 }