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这篇文章分享一下Springboot整合Elasticsearch时遇到的一个问题，项目正常启动，但是查询数据库的时候发生了一个异常java.sql.SQLException: com.mysql.cj.jdbc.Driver

java.sql.SQLException: com.mysql.cj.jdbc.Driverat com.alibaba.druid.util.JdbcUtils.createDriver(JdbcUtils.java:625) ~[druid-1.1.21.jar:1.1.21]at com.alibaba.druid.pool.DruidDataSource.init(DruidDataSource.java:897) [druid-1.1.21.jar:1.1.21]at com.alibaba.druid.pool.DruidDataSource.getConnection(DruidDataSource.java:1376) [druid-1.1.21.jar:1.1.21]at com.alibaba.druid.pool.DruidDataSource.getConnection(DruidDataSource.java:1372) [druid-1.1.21.jar:1.1.21]at com.alibaba.druid.pool.DruidDataSource.getConnection(DruidDataSource.java:109) [druid-1.1.21.jar:1.1.21]at org.springframework.jdbc.datasource.DataSourceUtils.fetchConnection(DataSourceUtils.java:158) [spring-jdbc-5.2.9.RELEASE.jar:5.2.9.RELEASE]at org.springframework.jdbc.datasource.DataSourceUtils.doGetConnection(DataSourceUtils.java:116) [spring-jdbc-5.2.9.RELEASE.jar:5.2.9.RELEASE]at org.springframework.jdbc.datasource.DataSourceUtils.getConnection(DataSourceUtils.java:79) [spring-jdbc-5.2.9.RELEASE.jar:5.2.9.RELEASE]at org.mybatis.spring.transaction.SpringManagedTransaction.openConnection(SpringManagedTransaction.java:80) [mybatis-spring-2.0.7.jar:2.0.7]at org.mybatis.spring.transaction.SpringManagedTransaction.getConnection(SpringManagedTransaction.java:67) [mybatis-spring-2.0.7.jar:2.0.7]at org.apache.ibatis.executor.BaseExecutor.getConnection(BaseExecutor.java:337) [mybatis-3.5.9.jar:3.5.9]at org.apache.ibatis.executor.SimpleExecutor.prepareStatement(SimpleExecutor.java:86) [mybatis-3.5.9.jar:3.5.9]at org.apache.ibatis.executor.SimpleExecutor.doQuery(SimpleExecutor.java:62) [mybatis-3.5.9.jar:3.5.9]at org.apache.ibatis.executor.BaseExecutor.queryFromDatabase(BaseExecutor.java:325) [mybatis-3.5.9.jar:3.5.9]at org.apache.ibatis.executor.BaseExecutor.query(BaseExecutor.java:156) [mybatis-3.5.9.jar:3.5.9]at org.apache.ibatis.executor.CachingExecutor.query(CachingExecutor.java:109) [mybatis-3.5.9.jar:3.5.9]at org.apache.ibatis.executor.CachingExecutor.query(CachingExecutor.java:89) [mybatis-3.5.9.jar:3.5.9]at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:151) [mybatis-3.5.9.jar:3.5.9]at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:145) [mybatis-3.5.9.jar:3.5.9]at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:140) [mybatis-3.5.9.jar:3.5.9]at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_152]at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_152]at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_152]at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_152]at org.mybatis.spring.SqlSessionTemplate$SqlSessionInterceptor.invoke(SqlSessionTemplate.java:427) [mybatis-spring-2.0.7.jar:2.0.7]at com.sun.proxy.$Proxy56.selectList(Unknown Source) [na:na]at org.mybatis.spring.SqlSessionTemplate.selectList(SqlSessionTemplate.java:224) [mybatis-spring-2.0.7.jar:2.0.7]at org.apache.ibatis.binding.MapperMethod.executeForMany(MapperMethod.java:147) [mybatis-3.5.9.jar:3.5.9]at org.apache.ibatis.binding.MapperMethod.execute(MapperMethod.java:80) [mybatis-3.5.9.jar:3.5.9]at org.apache.ibatis.binding.MapperProxy$PlainMethodInvoker.invoke(MapperProxy.java:145) [mybatis-3.5.9.jar:3.5.9]at org.apache.ibatis.binding.MapperProxy.invoke(MapperProxy.java:86) [mybatis-3.5.9.jar:3.5.9]at com.sun.proxy.$Proxy57.selectAll(Unknown Source) [na:na]at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_152]at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_152]at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_152]at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_152]at org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:344) [spring-aop-5.2.9.RELEASE.jar:5.2.9.RELEASE]at org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:198) [spring-aop-5.2.9.RELEASE.jar:5.2.9.RELEASE]at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:163) [spring-aop-5.2.9.RELEASE.jar:5.2.9.RELEASE]at org.springframework.dao.support.PersistenceExceptionTranslationInterceptor.invoke(PersistenceExceptionTranslationInterceptor.java:139) [spring-tx-5.2.9.RELEASE.jar:5.2.9.RELEASE]at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:186) [spring-aop-5.2.9.RELEASE.jar:5.2.9.RELEASE]at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:212) [spring-aop-5.2.9.RELEASE.jar:5.2.9.RELEASE]at com.sun.proxy.$Proxy58.selectAll(Unknown Source) [na:na]at com.example.elasticsearch.controller.SongController.list(SongController.java:28) [classes/:na]at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_152]at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_152]at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_152]at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_152]at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:190) [spring-web-5.2.9.RELEASE.jar:5.2.9.RELEASE]at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:138) [spring-web-5.2.9.RELEASE.jar:5.2.9.RELEASE]at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:105) [spring-webmvc-5.2.9.RELEASE.jar:5.2.9.RELEASE]at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:878) [spring-webmvc-5.2.9.RELEASE.jar:5.2.9.RELEASE]at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:792) [spring-webmvc-5.2.9.RELEASE.jar:5.2.9.RELEASE]at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87) [spring-webmvc-5.2.9.RELEASE.jar:5.2.9.RELEASE]at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1040) [spring-webmvc-5.2.9.RELEASE.jar:5.2.9.RELEASE]at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:943) [spring-webmvc-5.2.9.RELEASE.jar:5.2.9.RELEASE]at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1006) [spring-webmvc-5.2.9.RELEASE.jar:5.2.9.RELEASE]at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:898) [spring-webmvc-5.2.9.RELEASE.jar:5.2.9.RELEASE]at javax.servlet.http.HttpServlet.service(HttpServlet.java:626) [tomcat-embed-core-9.0.38.jar:4.0.FR]at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:883) [spring-webmvc-5.2.9.RELEASE.jar:5.2.9.RELEASE]at javax.servlet.http.HttpServlet.service(HttpServlet.java:733) [tomcat-embed-core-9.0.38.jar:4.0.FR]at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) [tomcat-embed-core-9.0.38.jar:9.0.38]at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-9.0.38.jar:9.0.38]at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53) [tomcat-embed-websocket-9.0.38.jar:9.0.38]at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-9.0.38.jar:9.0.38]at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-9.0.38.jar:9.0.38]at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:100) [spring-web-5.2.9.RELEASE.jar:5.2.9.RELEASE]at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) [spring-web-5.2.9.RELEASE.jar:5.2.9.RELEASE]at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-9.0.38.jar:9.0.38]at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-9.0.38.jar:9.0.38]at org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:93) [spring-web-5.2.9.RELEASE.jar:5.2.9.RELEASE]at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) [spring-web-5.2.9.RELEASE.jar:5.2.9.RELEASE]at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-9.0.38.jar:9.0.38]at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-9.0.38.jar:9.0.38]at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:201) [spring-web-5.2.9.RELEASE.jar:5.2.9.RELEASE]at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) [spring-web-5.2.9.RELEASE.jar:5.2.9.RELEASE]at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-9.0.38.jar:9.0.38]at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-9.0.38.jar:9.0.38]at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:202) [tomcat-embed-core-9.0.38.jar:9.0.38]at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:97) [tomcat-embed-core-9.0.38.jar:9.0.38]at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:541) [tomcat-embed-core-9.0.38.jar:9.0.38]at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:143) [tomcat-embed-core-9.0.38.jar:9.0.38]at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92) [tomcat-embed-core-9.0.38.jar:9.0.38]at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:78) [tomcat-embed-core-9.0.38.jar:9.0.38]at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343) [tomcat-embed-core-9.0.38.jar:9.0.38]at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:374) [tomcat-embed-core-9.0.38.jar:9.0.38]at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65) [tomcat-embed-core-9.0.38.jar:9.0.38]at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:868) [tomcat-embed-core-9.0.38.jar:9.0.38]at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1590) [tomcat-embed-core-9.0.38.jar:9.0.38]at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) [tomcat-embed-core-9.0.38.jar:9.0.38]at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [na:1.8.0_152]at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [na:1.8.0_152]at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-embed-core-9.0.38.jar:9.0.38]at java.lang.Thread.run(Thread.java:748) [na:1.8.0_152]
Caused by: java.lang.ClassNotFoundException: com.mysql.cj.jdbc.Driverat java.net.URLClassLoader.findClass(URLClassLoader.java:381) ~[na:1.8.0_152]at java.lang.ClassLoader.loadClass(ClassLoader.java:424) ~[na:1.8.0_152]at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:338) ~[na:1.8.0_152]at java.lang.ClassLoader.loadClass(ClassLoader.java:357) ~[na:1.8.0_152]at java.lang.Class.forName0(Native Method) ~[na:1.8.0_152]at java.lang.Class.forName(Class.java:264) ~[na:1.8.0_152]at com.alibaba.druid.util.JdbcUtils.createDriver(JdbcUtils.java:623) ~[druid-1.1.21.jar:1.1.21]... 93 common frames omitted

通过断点发现加载数据库驱动的时候传进来的类加载器居然是null，惊呆了我的小伙伴，不知道看这篇文章的童鞋有没有遇到过这种问题，反正我是第一次遇到。

正在我百思不得其解的时候，我突然想到了，是不是少了什么依赖没导入，每错，就是因为没有导入jdbc连接mysql的依赖。

最后在pom.xml文件中添加依赖，然后刷新Maven，问题就解决了。

    <properties><java.version>1.8</java.version><mysql.version>8.0.28</mysql.version></properties><dependencies>        <dependency><groupId>mysql</groupId><artifactId>mysql-connector-java</artifactId><version>${mysql.version}</version></dependency><dependencies>

总结：以前会直接把其他项目上的依赖批量复制过去，这次因为整合elasticsearch，只是想把mysql里的数据导入elasticsearch，所以用不到那么多依赖，因此是一个个复制的，mybatis的都复制过去了，但是把mysql给忘了。