总结在ssm整合中，Mybatis出现Mapped Statements collection already contains value for xxxxx的解决方案

先贴一段报错信息：

　　前面的都不是很重要，看最后灰色标注的那段。。。。

严重: 异常将上下文初始化事件发送到类的侦听器实例.[org.springframework.web.context.ContextLoaderListener]
org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'sqlSessionfactory' defined in class path resource [applicationContext-dao.xml]: Invocation of init method failed; nested exception is org.springframework.core.NestedIOException: Failed to parse mapping resource: 'file [D:\apache-tomcat-8.5.46\webapps\ssm_blog\WEB-INF\classes\org\blog\dao\FriendMapper.xml]'; nested exception is org.apache.ibatis.builder.BuilderException: Error parsing Mapper XML. Cause: java.lang.IllegalArgumentException: Mapped Statements collection already contains value for org.blog.dao.FriendMapper.searchFriend
 at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1566)
 at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:539)
 at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:476)
 at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:302)
 at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)
 at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:298)
 at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:193)
 at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:743)
 at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:757)
 at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:480)
 at org.springframework.web.context.ContextLoader.configureAndRefreshWebApplicationContext(ContextLoader.java:403)
 at org.springframework.web.context.ContextLoader.initWebApplicationContext(ContextLoader.java:306)
 at org.springframework.web.context.ContextLoaderListener.contextInitialized(ContextLoaderListener.java:106)
 at org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:4699)
 at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5165)
 at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150)
 at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1412)
 at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1402)
 at java.util.concurrent.FutureTask.run(FutureTask.java:266)
 at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
 at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
 at java.lang.Thread.run(Thread.java:745)
Caused by: org.springframework.core.NestedIOException: Failed to parse mapping resource: 'file [D:\apache-tomcat-8.5.46\webapps\ssm_blog\WEB-INF\classes\org\blog\dao\FriendMapper.xml]'; nested exception is org.apache.ibatis.builder.BuilderException: Error parsing Mapper XML. Cause: java.lang.IllegalArgumentException: Mapped Statements collection already contains value for org.blog.dao.FriendMapper.searchFriend
 at org.mybatis.spring.SqlSessionFactoryBean.buildSqlSessionFactory(SqlSessionFactoryBean.java:466)
 at org.mybatis.spring.SqlSessionFactoryBean.afterPropertiesSet(SqlSessionFactoryBean.java:340)
 at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeInitMethods(AbstractAutowireCapableBeanFactory.java:1625)
 at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1562)
 ... 21 more
Caused by: org.apache.ibatis.builder.BuilderException: Error parsing Mapper XML. Cause: java.lang.IllegalArgumentException: Mapped Statements collection already contains value for org.blog.dao.FriendMapper.searchFriend
 at org.apache.ibatis.builder.xml.XMLMapperBuilder.configurationElement(XMLMapperBuilder.java:120)
 at org.apache.ibatis.builder.xml.XMLMapperBuilder.parse(XMLMapperBuilder.java:92)
 at org.mybatis.spring.SqlSessionFactoryBean.buildSqlSessionFactory(SqlSessionFactoryBean.java:464)
 ... 24 more
Caused by: java.lang.IllegalArgumentException: Mapped Statements collection already contains value for org.blog.dao.FriendMapper.searchFriend
 at org.apache.ibatis.session.Configuration$StrictMap.put(Configuration.java:782)
 at org.apache.ibatis.session.Configuration$StrictMap.put(Configuration.java:754)
 at org.apache.ibatis.session.Configuration.addMappedStatement(Configuration.java:578)
 at org.apache.ibatis.builder.MapperBuilderAssistant.addMappedStatement(MapperBuilderAssistant.java:288)
 at org.apache.ibatis.builder.xml.XMLStatementBuilder.parseStatementNode(XMLStatementBuilder.java:107)
 at org.apache.ibatis.builder.xml.XMLMapperBuilder.buildStatementFromContext(XMLMapperBuilder.java:135)
 at org.apache.ibatis.builder.xml.XMLMapperBuilder.buildStatementFromContext(XMLMapperBuilder.java:128)
 at org.apache.ibatis.builder.xml.XMLMapperBuilder.configurationElement(XMLMapperBuilder.java:118)
 ... 26 more

当出现Mapped Statements collection already contains value for。。。。。（意思大概：xxx 在statements 执行语句的集合中已经存在了。。。（塑料英语，大佬轻拍））

这种报错时

错误原因有：

1.mapper中存在id重复的值

2.mapper中的parameterType或resultType为空。

例如：<select id="findProductById" parameterType=" "> </select>

 

3. 在使用@Select等注解的情况下，方法名即为mapper的id，重载的方法会报这个错。

4. mapper复制  忘了改namespace指向的类，所以两个mapper指向同一个mapper，所以报了这个错。

5.sqlMapperConfig.xml与application-dao.xml（文件名不一定相同，我在这个文件中主要配置的是，sqlSessionFactory放入IOC容器和扫描mapper包下的文件，并将mapper接口的实现类放入容器）中

例如：

 <mappers>
  　　<package name="org.blog.dao"/>
 </mappers>

和

<bean class="org.mybatis.spring.mapper.MapperScannerConfigurer">
  <property name="basePackage" value="org.blog.dao"></property>
 </bean>

重复了，删掉一个就可以了。。。。