认识达内从这里开始

昆明IT培训的老师发现由于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：

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。