项目中使用了mybatis-plus，使用Mapper文件动态代理生成简单的增删改查方法。在dao和service接口中定义需要操作数据库的方法。因此实际开发的流程应该是：serviceImpl ->daoImpl ->mapper.
在简单使用daoImpl接口实现selectByName操作的的时候。
org.apache.ibatis.binding.BindingException: Invalid bound statement (not found): com.example.dao.AdminsDao.selectByName at org.apache.ibatis.binding.MapperMethod$SqlCommand.(MapperMethod.java:235) ~[mybatis-3.5.10.jar:3.5.10] at com.baomidou.mybatisplus.core.override.MybatisMapperMethod. (MybatisMapperMethod.java:50) ~[mybatis-plus-core-3.5.2.jar:3.5.2] at com.baomidou.mybatisplus.core.override.MybatisMapperProxy.lambda$cachedInvoker$0(MybatisMapperProxy.java:111) ~[mybatis-plus-core-3.5.2.jar:3.5.2] at java.base/java.util.concurrent.ConcurrentHashMap.computeIfAbsent(ConcurrentHashMap.java:1708) ~[na:na] at com.baomidou.mybatisplus.core.toolkit.CollectionUtils.computeIfAbsent(CollectionUtils.java:115) ~[mybatis-plus-core-3.5.2.jar:3.5.2] at com.baomidou.mybatisplus.core.override.MybatisMapperProxy.cachedInvoker(MybatisMapperProxy.java:98) ~[mybatis-plus-core-3.5.2.jar:3.5.2] at com.baomidou.mybatisplus.core.override.MybatisMapperProxy.invoke(MybatisMapperProxy.java:89) ~[mybatis-plus-core-3.5.2.jar:3.5.2] at jdk.proxy2/jdk.proxy2.$Proxy63.selectByName(Unknown Source) ~[na:na] at com.example.controller.AdminsController.selectByName(AdminsController.java:24) ~[classes/:na] at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:104) ~[na:na] at java.base/java.lang.reflect.Method.invoke(Method.java:577) ~[na:na] at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:190) ~[spring-web-5.2.15.RELEASE.jar:5.2.15.RELEASE] at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:138) ~[spring-web-5.2.15.RELEASE.jar:5.2.15.RELEASE] at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:105) ~[spring-webmvc-5.2.15.RELEASE.jar:5.2.15.RELEASE] at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:878) ~[spring-webmvc-5.2.15.RELEASE.jar:5.2.15.RELEASE] at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:792) ~[spring-webmvc-5.2.15.RELEASE.jar:5.2.15.RELEASE] at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87) ~[spring-webmvc-5.2.15.RELEASE.jar:5.2.15.RELEASE] at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1040) ~[spring-webmvc-5.2.15.RELEASE.jar:5.2.15.RELEASE] at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:943) ~[spring-webmvc-5.2.15.RELEASE.jar:5.2.15.RELEASE] at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1006) ~[spring-webmvc-5.2.15.RELEASE.jar:5.2.15.RELEASE] at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:898) ~[spring-webmvc-5.2.15.RELEASE.jar:5.2.15.RELEASE] at javax.servlet.http.HttpServlet.service(HttpServlet.java:626) ~[tomcat-embed-core-9.0.46.jar:4.0.FR] at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:883) ~[spring-webmvc-5.2.15.RELEASE.jar:5.2.15.RELEASE] at javax.servlet.http.HttpServlet.service(HttpServlet.java:733) ~[tomcat-embed-core-9.0.46.jar:4.0.FR] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:227) ~[tomcat-embed-core-9.0.46.jar:9.0.46] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) ~[tomcat-embed-core-9.0.46.jar:9.0.46] at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53) ~[tomcat-embed-websocket-9.0.46.jar:9.0.46] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189) ~[tomcat-embed-core-9.0.46.jar:9.0.46] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) ~[tomcat-embed-core-9.0.46.jar:9.0.46] at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:100) ~[spring-web-5.2.15.RELEASE.jar:5.2.15.RELEASE] at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) ~[spring-web-5.2.15.RELEASE.jar:5.2.15.RELEASE] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189) ~[tomcat-embed-core-9.0.46.jar:9.0.46] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) ~[tomcat-embed-core-9.0.46.jar:9.0.46] at org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:93) ~[spring-web-5.2.15.RELEASE.jar:5.2.15.RELEASE] at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) ~[spring-web-5.2.15.RELEASE.jar:5.2.15.RELEASE] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189) ~[tomcat-embed-core-9.0.46.jar:9.0.46] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) ~[tomcat-embed-core-9.0.46.jar:9.0.46] at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:201) ~[spring-web-5.2.15.RELEASE.jar:5.2.15.RELEASE] at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) ~[spring-web-5.2.15.RELEASE.jar:5.2.15.RELEASE] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189) ~[tomcat-embed-core-9.0.46.jar:9.0.46] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) ~[tomcat-embed-core-9.0.46.jar:9.0.46] at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:202) ~[tomcat-embed-core-9.0.46.jar:9.0.46] at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:97) ~[tomcat-embed-core-9.0.46.jar:9.0.46] at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:542) ~[tomcat-embed-core-9.0.46.jar:9.0.46] at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:143) ~[tomcat-embed-core-9.0.46.jar:9.0.46] at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92) ~[tomcat-embed-core-9.0.46.jar:9.0.46] at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:78) ~[tomcat-embed-core-9.0.46.jar:9.0.46] at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:357) ~[tomcat-embed-core-9.0.46.jar:9.0.46] at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:374) ~[tomcat-embed-core-9.0.46.jar:9.0.46] at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65) ~[tomcat-embed-core-9.0.46.jar:9.0.46] at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:893) ~[tomcat-embed-core-9.0.46.jar:9.0.46] at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1707) ~[tomcat-embed-core-9.0.46.jar:9.0.46] at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) ~[tomcat-embed-core-9.0.46.jar:9.0.46] at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136) ~[na:na] at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635) ~[na:na] at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) ~[tomcat-embed-core-9.0.46.jar:9.0.46] at java.base/java.lang.Thread.run(Thread.java:833) ~[na:na]
大概意思理解就是找不到自定义sql的mapper文件？
解决思路 1.测试注入Mapper直接调用Mapper文件中的方法(1)检查yml文件中的mapper-locations地址是否能找到自定义mapper.xml文件
mybatis-plus:
configuration:
log-impl: org.apache.ibatis.logging.stdout.StdOutImpl
mapper-locations: classpath:mapper/*Mapper.xml
type-aliases-package: com.example.entity
（2）检测自定义mapper.xml文件
重点检查 namespace和xml开头格式
（3）直接注入mapper的selectByname方法测试
@Autowired
private AdminsMapper adminsMapper;
@RequestMapping("/test")
public Admins selectByName(){
String username=new String("admin");
Admins admins = adminsMapper.selectByName(username);
System.out.println(admins);
return admins;
}
测试结果：Mapper没有问题
2.切换DaoImpl中注入Mapper的方式为@Resource测试结果：不是注解问题
3.检查SpringBoot启动项的注解问题：@MapperScan
发现是注解@MapperScan的问题
@MapperScan的作用：
指定要变成实现类的接口所在的包，然后包下面的所有接口在编译之后都会生成相应的实现类
（使用这个注解后，Mapper接口就不需要加@Mapper注解了）
我在这里，对Mapper接口加上了@Mapper注解。导致了上述问题的出现。
这里如果不对Mapper接口加@Mapper注解还是会出现以上问题,同时去掉@MapperScan后还会出现DaoImpl无法找到mapper的bean。
因此问题的关键在于@MapperScan。
