代码如下：

这是不可以的，会报错：

2016-08-18 11:36:00,267 [main] ERROR [org.mybatis.spring.mapper.MapperFactoryBean] - Error while adding the mapper 'interface com.unix21.mapper.UserMapper' to configuration.
  java.lang.IllegalArgumentException: Mapped Statements collection already contains value for com.unix21.mapper.UserMapper.findById
	at org.apache.ibatis.session.Configuration$StrictMap.put(Configuration.java:837)
	at org.apache.ibatis.session.Configuration$StrictMap.put(Configuration.java:809)
	at org.apache.ibatis.session.Configuration.addMappedStatement(Configuration.java:633)
	at org.apache.ibatis.builder.MapperBuilderAssistant.addMappedStatement(MapperBuilderAssistant.java:302)
	at org.apache.ibatis.builder.annotation.MapperAnnotationBuilder.parseStatement(MapperAnnotationBuilder.java:325)
	at org.apache.ibatis.builder.annotation.MapperAnnotationBuilder.parse(MapperAnnotationBuilder.java:130)
	at org.apache.ibatis.binding.MapperRegistry.addMapper(MapperRegistry.java:72)
	at org.apache.ibatis.session.Configuration.addMapper(Configuration.java:706)
	at org.mybatis.spring.mapper.MapperFactoryBean.checkDaoConfig(MapperFactoryBean.java:81)

class MapperFactoryBean在mybatis-spring.jar

原因一步步调试出来

org.mybatis.spring.mapper.MapperFactoryBean.checkDaoConfig

class Configuration在mybatis.jar

org.apache.ibatis.session.Configuration.addMapper

org.apache.ibatis.binding.MapperRegistry.addMapper

knownMappers

MapperProxyFactory

knownMappers.put

org.apache.ibatis.builder.annotation.MapperAnnotationBuilder.parse

Method[] methods = type.getMethods();

org.apache.ibatis.builder.annotation.MapperAnnotationBuilder.parseStatement

resultMapId = parseResultMap(method);

parseResultMap

applyResultMap

applyResults

applyResultMap

addResultMap

回到applyResultMap

回到parseResultMap

回到parseStatement

org.apache.ibatis.builder.MapperBuilderAssistant.addMappedStatement

statementBuilder.parameterMap

org.apache.ibatis.mapping.MappedStatement.parameterMap

回到addMappedStatement

org.apache.ibatis.mapping.MappedStatement.Builder.build

回到addMappedStatement

重点来了

org.apache.ibatis.session.Configuration.addMappedStatement

mappedStatements定义

StrictMap定义继承自HashMap重写了put

多了一行判断

if (containsKey(key)) 

第一次当然是containsKey(key)=false

返回addMappedStatement，mappedStatements添加完成

一直退到parse

第二次for循环methods

一直下一步…到了第二次mappedStatements.put

containsKey(key)==true产生异常