错误原因如下 ：

org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'helloWord': Unsatisfied dependency expressed through field 'userDOMapper'; nested exception is org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'userDOMapper' defined in file [D:\programme\codeworkspace\eclipse-workspace-springboot\miaosha\target\classes\com\miaoshaproject\dao\UserDOMapper.class]: Unsatisfied dependency expressed through bean property 'sqlSessionFactory'; nested exception is org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'sqlSessionFactory' defined in class path resource [org/mybatis/spring/boot/autoconfigure/MybatisAutoConfiguration.class]: Unsatisfied dependency expressed through method 'sqlSessionFactory' parameter 0; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'dataSource' defined in class path resource [org/springframework/boot/autoconfigure/jdbc/DataSourceConfiguration$Generic.class]: Bean instantiation via factory method failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [javax.sql.DataSource]: Factory method 'dataSource' threw exception; nested exception is java.lang.IllegalStateException: Cannot load driver class: com.mysql.cj.jdbc.Driver

at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:584) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:90) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:370) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1336) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:572) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:495) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:317) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:315) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:199) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:759) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:867) ~[spring-context-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:548) ~[spring-context-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:142) ~[spring-boot-2.0.8.RELEASE.jar:2.0.8.RELEASE]

at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:754) [spring-boot-2.0.8.RELEASE.jar:2.0.8.RELEASE]

at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:386) [spring-boot-2.0.8.RELEASE.jar:2.0.8.RELEASE]

at org.springframework.boot.SpringApplication.run(SpringApplication.java:307) [spring-boot-2.0.8.RELEASE.jar:2.0.8.RELEASE]

at org.springframework.boot.SpringApplication.run(SpringApplication.java:1242) [spring-boot-2.0.8.RELEASE.jar:2.0.8.RELEASE]

at org.springframework.boot.SpringApplication.run(SpringApplication.java:1230) [spring-boot-2.0.8.RELEASE.jar:2.0.8.RELEASE]

at com.miaoshaproject.MiaoshaApplication.main(MiaoshaApplication.java:15) [classes/:na]

Caused by: org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'userDOMapper' defined in file [D:\programme\codeworkspace\eclipse-workspace-springboot\miaosha\target\classes\com\miaoshaproject\dao\UserDOMapper.class]: Unsatisfied dependency expressed through bean property 'sqlSessionFactory'; nested exception is org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'sqlSessionFactory' defined in class path resource [org/mybatis/spring/boot/autoconfigure/MybatisAutoConfiguration.class]: Unsatisfied dependency expressed through method 'sqlSessionFactory' parameter 0; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'dataSource' defined in class path resource [org/springframework/boot/autoconfigure/jdbc/DataSourceConfiguration$Generic.class]: Bean instantiation via factory method failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [javax.sql.DataSource]: Factory method 'dataSource' threw exception; nested exception is java.lang.IllegalStateException: Cannot load driver class: com.mysql.cj.jdbc.Driver

at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireByType(AbstractAutowireCapableBeanFactory.java:1431) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1319) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:572) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:495) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:317) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:315) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:199) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.config.DependencyDescriptor.resolveCandidate(DependencyDescriptor.java:251) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1135) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1062) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:581) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

... 19 common frames omitted

Caused by: org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'sqlSessionFactory' defined in class path resource [org/mybatis/spring/boot/autoconfigure/MybatisAutoConfiguration.class]: Unsatisfied dependency expressed through method 'sqlSessionFactory' parameter 0; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'dataSource' defined in class path resource [org/springframework/boot/autoconfigure/jdbc/DataSourceConfiguration$Generic.class]: Bean instantiation via factory method failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [javax.sql.DataSource]: Factory method 'dataSource' threw exception; nested exception is java.lang.IllegalStateException: Cannot load driver class: com.mysql.cj.jdbc.Driver

at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:733) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.ConstructorResolver.instantiateUsingFactoryMethod(ConstructorResolver.java:475) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateUsingFactoryMethod(AbstractAutowireCapableBeanFactory.java:1246) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1096) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:535) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:495) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:317) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:315) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:199) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.config.DependencyDescriptor.resolveCandidate(DependencyDescriptor.java:251) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1135) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1062) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireByType(AbstractAutowireCapableBeanFactory.java:1416) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

... 30 common frames omitted

Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'dataSource' defined in class path resource [org/springframework/boot/autoconfigure/jdbc/DataSourceConfiguration$Generic.class]: Bean instantiation via factory method failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [javax.sql.DataSource]: Factory method 'dataSource' threw exception; nested exception is java.lang.IllegalStateException: Cannot load driver class: com.mysql.cj.jdbc.Driver

at org.springframework.beans.factory.support.ConstructorResolver.instantiateUsingFactoryMethod(ConstructorResolver.java:591) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateUsingFactoryMethod(AbstractAutowireCapableBeanFactory.java:1246) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1096) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:535) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:495) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:317) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:315) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:199) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.config.DependencyDescriptor.resolveCandidate(DependencyDescriptor.java:251) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1135) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1062) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.ConstructorResolver.resolveAutowiredArgument(ConstructorResolver.java:819) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:725) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

... 43 common frames omitted

Caused by: org.springframework.beans.BeanInstantiationException: Failed to instantiate [javax.sql.DataSource]: Factory method 'dataSource' threw exception; nested exception is java.lang.IllegalStateException: Cannot load driver class: com.mysql.cj.jdbc.Driver

at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:185) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.beans.factory.support.ConstructorResolver.instantiateUsingFactoryMethod(ConstructorResolver.java:583) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

... 56 common frames omitted

Caused by: java.lang.IllegalStateException: Cannot load driver class: com.mysql.cj.jdbc.Driver

at org.springframework.util.Assert.state(Assert.java:94) ~[spring-core-5.0.12.RELEASE.jar:5.0.12.RELEASE]

at org.springframework.boot.autoconfigure.jdbc.DataSourceProperties.determineDriverClassName(DataSourceProperties.java:224) ~[spring-boot-autoconfigure-2.0.8.RELEASE.jar:2.0.8.RELEASE]

at org.springframework.boot.autoconfigure.jdbc.DataSourceProperties.initializeDataSourceBuilder(DataSourceProperties.java:176) ~[spring-boot-autoconfigure-2.0.8.RELEASE.jar:2.0.8.RELEASE]

at org.springframework.boot.autoconfigure.jdbc.DataSourceConfiguration$Generic.dataSource(DataSourceConfiguration.java:120) ~[spring-boot-autoconfigure-2.0.8.RELEASE.jar:2.0.8.RELEASE]

at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_181]

at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_181]

at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_181]

at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_181]

at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:154) ~[spring-beans-5.0.12.RELEASE.jar:5.0.12.RELEASE]

... 57 common frames omitted

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我是按照视频一步步的敲的，但是失败了，不知道为什么，有遇到过此问题的吗？