在 Hibernate 保存数据的时候，提示没有选择数据库。
完整的错误信息为：
09:06:40.277 [http-nio-8080-exec-1] ERROR o.h.id.enhanced.TableStructure - could not read a hi value java.sql.SQLException: (conn=5493) No database selected at org.mariadb.jdbc.internal.util.exceptions.ExceptionMapper.get(ExceptionMapper.java:265) at org.mariadb.jdbc.internal.util.exceptions.ExceptionMapper.getException(ExceptionMapper.java:164) at org.mariadb.jdbc.MariaDbStatement.executeExceptionEpilogue(MariaDbStatement.java:244) at org.mariadb.jdbc.ClientSidePreparedStatement.executeInternal(ClientSidePreparedStatement.java:225) at org.mariadb.jdbc.ClientSidePreparedStatement.execute(ClientSidePreparedStatement.java:145) at org.mariadb.jdbc.ClientSidePreparedStatement.executeQuery(ClientSidePreparedStatement.java:159) at com.mchange.v2.c3p0.impl.NewProxyPreparedStatement.executeQuery(NewProxyPreparedStatement.java:353) at org.hibernate.id.enhanced.TableStructure.executeQuery(TableStructure.java:216) at org.hibernate.id.enhanced.TableStructure.access$300(TableStructure.java:46) at org.hibernate.id.enhanced.TableStructure$1$1.execute(TableStructure.java:138) at org.hibernate.id.enhanced.TableStructure$1$1.execute(TableStructure.java:126) at org.hibernate.jdbc.WorkExecutor.executeReturningWork(WorkExecutor.java:55) at org.hibernate.jdbc.AbstractReturningWork.accept(AbstractReturningWork.java:34) at org.hibernate.resource.transaction.backend.jdbc.internal.JdbcIsolationDelegate.delegateWork(JdbcIsolationDelegate.java:57) at org.hibernate.id.enhanced.TableStructure$1.getNextValue(TableStructure.java:125) at org.hibernate.id.enhanced.NoopOptimizer.generate(NoopOptimizer.java:40) at org.hibernate.id.enhanced.SequenceStyleGenerator.generate(SequenceStyleGenerator.java:523) at org.hibernate.event.internal.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:115) at org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.saveWithGeneratedOrRequestedId(DefaultSaveOrUpdateEventListener.java:194) at org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.entityIsTransient(DefaultSaveOrUpdateEventListener.java:179) at org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.performSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:100) at org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.onSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:75) at org.hibernate.event.service.internal.EventListenerGroupImpl.fireEventOnEachListener(EventListenerGroupImpl.java:102) at org.hibernate.internal.SessionImpl.fireSaveOrUpdate(SessionImpl.java:606) at org.hibernate.internal.SessionImpl.saveOrUpdate(SessionImpl.java:599) at org.hibernate.internal.SessionImpl.saveOrUpdate(SessionImpl.java:594) at sun.reflect.GeneratedMethodAccessor87.invoke(Unknown Source) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:498) at org.hibernate.context.internal.ThreadLocalSessionContext$TransactionProtectionWrapper.invoke(ThreadLocalSessionContext.java:351) at com.sun.proxy.$Proxy141.saveOrUpdate(Unknown Source) at com.ossez.covid19.common.Factory.saveOrUpdate(Factory.java:249) at com.ossez.covid19.common.dao.factories.Covid19Factory.save(Covid19Factory.java:74) at com.ossez.covid19.service.batch.tasklet.Covid19Tasklet.execute(Covid19Tasklet.java:49) at org.springframework.batch.core.step.tasklet.TaskletStep$ChunkTransactionCallback.doInTransaction(TaskletStep.java:407) at org.springframework.batch.core.step.tasklet.TaskletStep$ChunkTransactionCallback.doInTransaction(TaskletStep.java:331) at org.springframework.transaction.support.TransactionTemplate.execute(TransactionTemplate.java:140) at org.springframework.batch.core.step.tasklet.TaskletStep$2.doInChunkContext(TaskletStep.java:273) at org.springframework.batch.core.scope.context.StepContextRepeatCallback.doInIteration(StepContextRepeatCallback.java:82) at org.springframework.batch.repeat.support.RepeatTemplate.getNextResult(RepeatTemplate.java:375) at org.springframework.batch.repeat.support.RepeatTemplate.executeInternal(RepeatTemplate.java:215) at org.springframework.batch.repeat.support.RepeatTemplate.iterate(RepeatTemplate.java:145) at org.springframework.batch.core.step.tasklet.TaskletStep.doExecute(TaskletStep.java:258) at org.springframework.batch.core.step.AbstractStep.execute(AbstractStep.java:208) at org.springframework.batch.core.job.SimpleStepHandler.handleStep(SimpleStepHandler.java:148) at org.springframework.batch.core.job.flow.JobFlowExecutor.executeStep(JobFlowExecutor.java:68) at org.springframework.batch.core.job.flow.support.state.StepState.handle(StepState.java:68) at org.springframework.batch.core.job.flow.support.SimpleFlow.resume(SimpleFlow.java:169) at org.springframework.batch.core.job.flow.support.SimpleFlow.start(SimpleFlow.java:144) at org.springframework.batch.core.job.flow.FlowJob.doExecute(FlowJob.java:137) at org.springframework.batch.core.job.AbstractJob.execute(AbstractJob.java:319) at org.springframework.batch.core.launch.support.SimpleJobLauncher$1.run(SimpleJobLauncher.java:147) at org.springframework.core.task.SyncTaskExecutor.execute(SyncTaskExecutor.java:50) at org.springframework.batch.core.launch.support.SimpleJobLauncher.run(SimpleJobLauncher.java:140) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:498) at org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:344) at org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:198) at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:163) at org.springframework.batch.core.configuration.annotation.SimpleBatchConfiguration$PassthruAdvice.invoke(SimpleBatchConfiguration.java:127) at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:186) at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:212) at com.sun.proxy.$Proxy71.run(Unknown Source) at com.ossez.covid19.service.controller.TaskController.refreshData(TaskController.java:36) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:498) at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:190) at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:138) at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:105) at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:879) at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:793) at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87) at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1040) at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:943) at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1006) at org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:909) at javax.servlet.http.HttpServlet.service(HttpServlet.java:665) at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:883) at javax.servlet.http.HttpServlet.service(HttpServlet.java:750) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:100) at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) at org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:93) at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:201) at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:202) at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96) at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:541) at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:139) at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92) at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:74) at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343) at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:373) at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65) at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:868) at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1594) at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) at java.lang.Thread.run(Thread.java:748) Caused by: java.sql.SQLException: No database selected at org.mariadb.jdbc.internal.protocol.AbstractQueryProtocol.readErrorPacket(AbstractQueryProtocol.java:1599) at org.mariadb.jdbc.internal.protocol.AbstractQueryProtocol.readPacket(AbstractQueryProtocol.java:1461) at org.mariadb.jdbc.internal.protocol.AbstractQueryProtocol.getResult(AbstractQueryProtocol.java:1424) at org.mariadb.jdbc.internal.protocol.AbstractQueryProtocol.executeQuery(AbstractQueryProtocol.java:240) at org.mariadb.jdbc.ClientSidePreparedStatement.executeInternal(ClientSidePreparedStatement.java:216) ... 112 common frames omitted 09:06:40.277 [http-nio-8080-exec-1] WARN o.h.e.jdbc.spi.SqlExceptionHelper - SQL Error: 1046, SQLState: 3D000 09:06:40.277 [http-nio-8080-exec-1] ERROR o.h.e.jdbc.spi.SqlExceptionHelper - (conn=5493) No database selected
这里需要注意一个比较重要的错误信息：
09:06:40.277 [http-nio-8080-exec-1] ERROR o.h.id.enhanced.TableStructure - could not read a hi value
如果光看错误日志的话，你可能会觉得是你的 Hibernate 没有设置正确的数据库，比如说在连接参数中没有设置，比如说没有在 HBM 文件中没有设置分类等。
其实这个没有选择数据库的错误是因为你主键生成策略没有设置正确。
你可能设置了 @GeneratedValue 这个注解说明的主键生成策略是 Auto，会自动尝试使用序列等方式，这个时候会 因为表找不到，而提示错误。
如果你使用的是 MySQL 或者 MariaDB， 你可以在表中设置自增序列，让数据库来帮你处理主键。
但是在 Hibernate 的 @GeneratedValue 注解中，你需要设置为： @GeneratedValue(strategy = GenerationType.IDENTITY)，这样你就不会遇到上面提示的数据库没有找到的错误了。
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