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！！！

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。