由于mybatis简单易学，比起Hibername来，更容易上手，代码也能自动生成。这几天研究了下代码自动生成的，参考：
http://0609xiaohua.iteye.com/blog/1453570
但是把代码复制进来，运行了下，却跑不起来，报以下错误：
Exception in thread "main" Java.lang.ExceptionInInitializerError
at com.test.Test.main(Test.java:12)
Caused by: org.apache.ibatis.exceptions.PersistenceException:
### Error building SqlSession.
### The error may exist in com/mapper/PetMapper.xml
### The error occurred while processing mapper_resultMap[BaseResultMap]
### Cause: org.apache.ibatis.builder.BuilderException: Error parsing SQL Mapper Configuration. Cause: java.lang.RuntimeException: Error parsing Mapper XML. Cause: java.lang.IllegalArgumentException: Result Maps collection already contains value for com.mapper.PetMapper.BaseResultMap
at org.apache.ibatis.exceptions.ExceptionFactory.wrapException(ExceptionFactory.java:8)
at org.apache.ibatis.session.SqlSessionFactoryBuilder.build(SqlSessionFactoryBuilder.java:32)
at org.apache.ibatis.session.SqlSessionFactoryBuilder.build(SqlSessionFactoryBuilder.java:16)
at com.util.SessionFactory.<clinit>(SessionFactory.java:21)
... 1 more
Caused by: org.apache.ibatis.builder.BuilderException: Error parsing SQL Mapper Configuration. Cause: java.lang.RuntimeException: Error parsing Mapper XML. Cause: java.lang.IllegalArgumentException: Result Maps collection already contains value for com.mapper.PetMapper.BaseResultMap
at org.apache.ibatis.builder.xml.XMLConfigBuilder.parseConfiguration(XMLConfigBuilder.java:85)
at org.apache.ibatis.builder.xml.XMLConfigBuilder.parse(XMLConfigBuilder.java:69)
at org.apache.ibatis.session.SqlSessionFactoryBuilder.build(SqlSessionFactoryBuilder.java:30)
... 3 more
Caused by: java.lang.RuntimeException: Error parsing Mapper XML. Cause: java.lang.IllegalArgumentException: Result Maps collection already contains value for com.mapper.PetMapper.BaseResultMap
at org.apache.ibatis.builder.xml.XMLMapperBuilder.configurationElement(XMLMapperBuilder.java:97)
at org.apache.ibatis.builder.xml.XMLMapperBuilder.parse(XMLMapperBuilder.java:73)
at org.apache.ibatis.builder.xml.XMLConfigBuilder.mapperElement(XMLConfigBuilder.java:255)
at org.apache.ibatis.builder.xml.XMLConfigBuilder.parseConfiguration(XMLConfigBuilder.java:83)
... 5 more
Caused by: java.lang.IllegalArgumentException: Result Maps collection already contains value for com.mapper.PetMapper.BaseResultMap
at org.apache.ibatis.session.Configuration$StrictMap.put(Configuration.java:578)
at org.apache.ibatis.session.Configuration$StrictMap.put(Configuration.java:551)
at org.apache.ibatis.session.Configuration.addResultMap(Configuration.java:356)
at org.apache.ibatis.builder.MapperBuilderAssistant.addResultMap(MapperBuilderAssistant.java:138)
at org.apache.ibatis.builder.xml.XMLMapperBuilder.resultMapElement(XMLMapperBuilder.java:231)
at org.apache.ibatis.builder.xml.XMLMapperBuilder.resultMapElement(XMLMapperBuilder.java:201)
at org.apache.ibatis.builder.xml.XMLMapperBuilder.resultMapElements(XMLMapperBuilder.java:196)
at org.apache.ibatis.builder.xml.XMLMapperBuilder.configurationElement(XMLMapperBuilder.java:93)
... 8 more
后来在CSDN上下载了个可以直接运行的eclipse 项目，再与之前的项目对比，好像也没什么不同。最后使绝招：代码对比工具，发现生成的PetMapper.xml多了几百行！！！
因为我在使用generator自动生成工具之后，又修改了generator，本以为修改命令里面的-overwrite参数能自动覆盖，但是事实上不可以，所以之前的和现在的两个混合在一起导致了错误。解决办法就是删掉之前生成的mapper和pojo包，重新生成，然后复制到工程下面。问题就解决了！！
原来是我在运行时没有把之前已经生成的PetMapper.xml删除掉，再次生成代码时，又附加了上去！所以在运行代码生成之前，要把以前已经生成的xml文件清掉，以妨出错。