被 BindingException: Invalid bound statement (not found)困扰两天了

是一个 IDEA 中运行的 Springboot+mybatis 项目； 启动成功，但调用到 xml 中所有的方法时就报错方法不存在；

网上所说的方法都试遍了: 比如检查 xml 中<namespace>已与 mapper 相对应； xml 与接口 dao 中方法都存在； xml 中无中文； 去掉了空格； 没有把 IDEA 中的“ xx.xx.xx ”看成了一个文件夹； 方法的大小写对应了； jdbc.url 数据库连接无误...

比如登录方法里的 checkUsername，捣鼓两天还没解决，都要怀疑人生了。

2019-04-14 17:02:25.139 ERROR 45978 --- [nio-8086-exec-2] o.a.c.c.C.[.[.[.[dispatcherServlet] : Servlet.service() for servlet [dispatcherServlet] in context with path [/tocean] threw exception [Request processing failed; nested exception is org.apache.ibatis.binding.BindingException: Invalid bound statement (not found): com.example.boottest.dao.UserMapper.checkUsername] with root cause org.apache.ibatis.binding.BindingException: Invalid bound statement (not found): com.example.boottest.dao.UserMapper.checkUsername at org.apache.ibatis.binding.MapperMethod$SqlCommand.<init>(MapperMethod.java:225) ~[mybatis-3.4.4.jar:3.4.4] at org.apache.ibatis.binding.MapperMethod.<init>(MapperMethod.java:48) ~[mybatis-3.4.4.jar:3.4.4] at org.apache.ibatis.binding.MapperProxy.cachedMapperMethod(MapperProxy.java:65) ~[mybatis-3.4.4.jar:3.4.4] at org.apache.ibatis.binding.MapperProxy.invoke(MapperProxy.java:58) ~[mybatis-3.4.4.jar:3.4.4] at com.sun.proxy.$Proxy83.checkUsername(Unknown Source) ~[na:na] at com.example.boottest.service.serviceImpl.UserServiceImpl.checkValid(UserServiceImpl.java:80) ~[classes/:na] at com.example.boottest.service.serviceImpl.UserServiceImpl.register(UserServiceImpl.java:50) ~[classes/:na] at com.example.boottest.service.serviceImpl.UserServiceImpl$$FastClassBySpringCGLIB$$73dc063f.invoke(<generated>) ~[classes/:na] at org.springframework.cglib.proxy.MethodProxy.invoke(MethodProxy.java:218) ~[spring-core-5.1.6.RELEASE.jar:5.1.6.RELEASE] at org.springframework.aop.framework.CglibAopProxy$DynamicAdvisedInterceptor.intercept(CglibAopProxy.java:684) ~[spring-aop-5.1.6.RELEASE.jar:5.1.6.RELEASE] at com.example.boottest.service.serviceImpl.UserServiceImpl$$EnhancerBySpringCGLIB$$424584ae.register(<generated>) ~[classes/:na] at com.example.boottest.controller.UserController.register(UserController.java:44) ~[classes/:na] at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_131] at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_131] at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_131] at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_131] at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:189) ~[spring-web-5.1.6.RELEASE.jar:5.1.6.RELEASE] at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:138) ~[spring-web-5.1.6.RELEASE.jar:5.1.6.RELEASE] at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:102) ~[spring-webmvc-5.1.6.RELEASE.jar:5.1.6.RELEASE] at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:892) ~[spring-webmvc-5.1.6.RELEASE.jar:5.1.6.RELEASE] at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:797) ~[spring-webmvc-5.1.6.RELEASE.jar:5.1.6.RELEASE] at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87) ~[spring-webmvc-5.1.6.RELEASE.jar:5.1.6.RELEASE] at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1038) ~[spring-webmvc-5.1.6.RELEASE.jar:5.1.6.RELEASE] at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:942) ~[spring-webmvc-5.1.6.RELEASE.jar:5.1.6.RELEASE] at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1005) ~[spring-webmvc-5.1.6.RELEASE.jar:5.1.6.RELEASE] at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:897) ~[spring-webmvc-5.1.6.RELEASE.jar:5.1.6.RELEASE] at javax.servlet.http.HttpServlet.service(HttpServlet.java:634) ~[tomcat-embed-core-9.0.17.jar:9.0.17] at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:882) ~[spring-webmvc-5.1.6.RELEASE.jar:5.1.6.RELEASE] at javax.servlet.http.HttpServlet.service(HttpServlet.java:741) ~[tomcat-embed-core-9.0.17.jar:9.0.17] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) ~[tomcat-embed-core-9.0.17.jar:9.0.17] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.17.jar:9.0.17] at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53) ~[tomcat-embed-websocket-9.0.17.jar:9.0.17] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.17.jar:9.0.17] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.17.jar:9.0.17] at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:99) ~[spring-web-5.1.6.RELEASE.jar:5.1.6.RELEASE] at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-5.1.6.RELEASE.jar:5.1.6.RELEASE] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.17.jar:9.0.17] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.17.jar:9.0.17] at org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:92) ~[spring-web-5.1.6.RELEASE.jar:5.1.6.RELEASE] at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-5.1.6.RELEASE.jar:5.1.6.RELEASE] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.17.jar:9.0.17] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.17.jar:9.0.17] at org.springframework.web.filter.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:93) ~[spring-web-5.1.6.RELEASE.jar:5.1.6.RELEASE] at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-5.1.6.RELEASE.jar:5.1.6.RELEASE] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.17.jar:9.0.17] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.17.jar:9.0.17] at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:200) ~[spring-web-5.1.6.RELEASE.jar:5.1.6.RELEASE] at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-5.1.6.RELEASE.jar:5.1.6.RELEASE] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.17.jar:9.0.17] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.17.jar:9.0.17] at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:200) ~[tomcat-embed-core-9.0.17.jar:9.0.17] at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96) [tomcat-embed-core-9.0.17.jar:9.0.17] at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:490) [tomcat-embed-core-9.0.17.jar:9.0.17] at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:139) [tomcat-embed-core-9.0.17.jar:9.0.17] at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92) [tomcat-embed-core-9.0.17.jar:9.0.17] at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:74) [tomcat-embed-core-9.0.17.jar:9.0.17] at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343) [tomcat-embed-core-9.0.17.jar:9.0.17] at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:408) [tomcat-embed-core-9.0.17.jar:9.0.17] at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66) [tomcat-embed-core-9.0.17.jar:9.0.17] at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:834) [tomcat-embed-core-9.0.17.jar:9.0.17] at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1415) [tomcat-embed-core-9.0.17.jar:9.0.17] at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) [tomcat-embed-core-9.0.17.jar:9.0.17] at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) [na:1.8.0_131] at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) [na:1.8.0_131] at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-embed-core-9.0.17.jar:9.0.17] at java.lang.Thread.run(Thread.java:748) [na:1.8.0_131]