初学MyBatis第一天跟着敲代码，一直报错，报错到崩溃，错误如下
org.apache.ibatis.exceptions.PersistenceException:
### Error querying database. Cause: java.lang.IllegalArgumentException: Mapped Statements collection does not contain value for com.atguigu.mybatis.EmployeeMapper.selectEmp
### Cause: java.lang.IllegalArgumentException: Mapped Statements collection does not contain value for com.atguigu.mybatis.EmployeeMapper.selectEmp
at org.apache.ibatis.exceptions.ExceptionFactory.wrapException(ExceptionFactory.java:30)
at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:150)
at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:141)
at org.apache.ibatis.session.defaults.DefaultSqlSession.selectOne(DefaultSqlSession.java:77)
at com.atguigu.mybatis.test.MyBatisTest.test(MyBatisTest.java:63)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.junit.platform.commons.util.ReflectionUtils.invokeMethod(ReflectionUtils.java:389)
at org.junit.jupiter.engine.execution.ExecutableInvoker.invoke(ExecutableInvoker.java:115)
at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.lambda$invokeTestMethod$6(TestMethodTestDescriptor.java:167)
at org.junit.jupiter.engine.execution.ThrowableCollector.execute(ThrowableCollector.java:40)
at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.invokeTestMethod(TestMethodTestDescriptor.java:163)
at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:110)
at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:57)
at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor.lambda$execute$3(HierarchicalTestExecutor.java:83)
at org.junit.platform.engine.support.hierarchical.SingleTestExecutor.executeSafely(SingleTestExecutor.java:66)
at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor.execute(HierarchicalTestExecutor.java:77)
at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor.lambda$null$2(HierarchicalTestExecutor.java:92)
at java.util.stream.ForEachOps$ForEachOp$OfRef.accept(Unknown Source)
at java.util.stream.ReferencePipeline$2$1.accept(Unknown Source)
at java.util.Iterator.forEachRemaining(Unknown Source)
at java.util.Spliterators$IteratorSpliterator.forEachRemaining(Unknown Source)
at java.util.stream.AbstractPipeline.copyInto(Unknown Source)
at java.util.stream.AbstractPipeline.wrapAndCopyInto(Unknown Source)
at java.util.stream.ForEachOps$ForEachOp.evaluateSequential(Unknown Source)
at java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(Unknown Source)
at java.util.stream.AbstractPipeline.evaluate(Unknown Source)
at java.util.stream.ReferencePipeline.forEach(Unknown Source)
at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor.lambda$execute$3(HierarchicalTestExecutor.java:92)
at org.junit.platform.engine.support.hierarchical.SingleTestExecutor.executeSafely(SingleTestExecutor.java:66)
at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor.execute(HierarchicalTestExecutor.java:77)
at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor.lambda$null$2(HierarchicalTestExecutor.java:92)
at java.util.stream.ForEachOps$ForEachOp$OfRef.accept(Unknown Source)
at java.util.stream.ReferencePipeline$2$1.accept(Unknown Source)
at java.util.Iterator.forEachRemaining(Unknown Source)
at java.util.Spliterators$IteratorSpliterator.forEachRemaining(Unknown Source)
at java.util.stream.AbstractPipeline.copyInto(Unknown Source)
at java.util.stream.AbstractPipeline.wrapAndCopyInto(Unknown Source)
at java.util.stream.ForEachOps$ForEachOp.evaluateSequential(Unknown Source)
at java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(Unknown Source)
at java.util.stream.AbstractPipeline.evaluate(Unknown Source)
at java.util.stream.ReferencePipeline.forEach(Unknown Source)
at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor.lambda$execute$3(HierarchicalTestExecutor.java:92)
at org.junit.platform.engine.support.hierarchical.SingleTestExecutor.executeSafely(SingleTestExecutor.java:66)
at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor.execute(HierarchicalTestExecutor.java:77)
at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor.execute(HierarchicalTestExecutor.java:51)
at org.junit.platform.engine.support.hierarchical.HierarchicalTestEngine.execute(HierarchicalTestEngine.java:43)
at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:170)
at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:154)
at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:90)
at org.eclipse.jdt.internal.junit5.runner.JUnit5TestReference.run(JUnit5TestReference.java:86)
at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:538)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:760)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:460)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:206)
Caused by: java.lang.IllegalArgumentException: Mapped Statements collection does not contain value for com.atguigu.mybatis.EmployeeMapper.selectEmp
at org.apache.ibatis.session.Configuration$StrictMap.get(Configuration.java:860)
at org.apache.ibatis.session.Configuration.getMappedStatement(Configuration.java:693)
at org.apache.ibatis.session.Configuration.getMappedStatement(Configuration.java:686)
at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:147)
... 56 more
查找网上资料，都说是sql语句写错了，或者id重复了，实在是前前后后找了很久，还是没找出任何问题。
找了很久，还重新打了一下文件，最后发现问题可能是出现在sql的唯一标识这个地方写错了，即以下代码的第一个参数，复制修改，终于成功
Employee employee = openSession.selectOne("com.atguigu.mybatis.dao.EmployeeMapper.selectEmp", 1);
控制台成功效果如下
DEBUG 10-23 15:30:13,109 ==> Preparing: select id,last_name lastName,email,gender from tb1_employee where id = ? (BaseJdbcLogger.java:145) DEBUG 10-23 15:30:13,141 ==> Parameters: 1(Integer) (BaseJdbcLogger.java:145) DEBUG 10-23 15:30:13,160 <== Total: 1 (BaseJdbcLogger.java:145) Employee [id=1, lastName=tom, email=tom@guigu.com, gender=0]
下次碰到此类问题，再进行总结