错误如下：
java.lang.RuntimeException: java.lang.reflect.InvocationTargetException at tk.mybatis.spring.mapper.SpringBootBindUtil$SpringBoot2Bind.bind(SpringBootBindUtil.java:129) ~[mapper-spring-boot-autoconfigure-1.2.3.jar:na] at tk.mybatis.spring.mapper.SpringBootBindUtil.bind(SpringBootBindUtil.java:58) ~[mapper-spring-boot-autoconfigure-1.2.3.jar:na] at tk.mybatis.spring.mapper.ClassPathMapperScanner.setMapperProperties(ClassPathMapperScanner.java:48) ~[mapper-spring-boot-autoconfigure-1.2.3.jar:na] at tk.mybatis.spring.annotation.MapperScannerRegistrar.registerBeanDefinitions(MapperScannerRegistrar.java:33) ~[mapper-spring-boot-autoconfigure-1.2.3.jar:na] at org.springframework.context.annotation.ConfigurationClassBeanDefinitionReader.lambda$loadBeanDefinitionsFromRegistrars$1(ConfigurationClassBeanDefinitionReader.java:364) ~[spring-context-5.1.9.RELEASE.jar:5.1.9.RELEASE] at java.util.LinkedHashMap.forEach(LinkedHashMap.java:684) ~[na:1.8.0_131] at org.springframework.context.annotation.ConfigurationClassBeanDefinitionReader.loadBeanDefinitionsFromRegistrars(ConfigurationClassBeanDefinitionReader.java:363) ~[spring-context-5.1.9.RELEASE.jar:5.1.9.RELEASE] at org.springframework.context.annotation.ConfigurationClassBeanDefinitionReader.loadBeanDefinitionsForConfigurationClass(ConfigurationClassBeanDefinitionReader.java:145) ~[spring-context-5.1.9.RELEASE.jar:5.1.9.RELEASE] at org.springframework.context.annotation.ConfigurationClassBeanDefinitionReader.loadBeanDefinitions(ConfigurationClassBeanDefinitionReader.java:117) ~[spring-context-5.1.9.RELEASE.jar:5.1.9.RELEASE] at org.springframework.context.annotation.ConfigurationClassPostProcessor.processConfigBeanDefinitions(ConfigurationClassPostProcessor.java:327) ~[spring-context-5.1.9.RELEASE.jar:5.1.9.RELEASE] at org.springframework.context.annotation.ConfigurationClassPostProcessor.postProcessBeanDefinitionRegistry(ConfigurationClassPostProcessor.java:232) ~[spring-context-5.1.9.RELEASE.jar:5.1.9.RELEASE] at org.springframework.context.support.PostProcessorRegistrationDelegate.invokeBeanDefinitionRegistryPostProcessors(PostProcessorRegistrationDelegate.java:275) ~[spring-context-5.1.9.RELEASE.jar:5.1.9.RELEASE] at org.springframework.context.support.PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(PostProcessorRegistrationDelegate.java:95) ~[spring-context-5.1.9.RELEASE.jar:5.1.9.RELEASE] at org.springframework.context.support.AbstractApplicationContext.invokeBeanFactoryPostProcessors(AbstractApplicationContext.java:705) ~[spring-context-5.1.9.RELEASE.jar:5.1.9.RELEASE] at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:531) ~[spring-context-5.1.9.RELEASE.jar:5.1.9.RELEASE] at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:141) ~[spring-boot-2.1.7.RELEASE.jar:2.1.7.RELEASE] at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:743) [spring-boot-2.1.7.RELEASE.jar:2.1.7.RELEASE] at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:390) [spring-boot-2.1.7.RELEASE.jar:2.1.7.RELEASE] at org.springframework.boot.SpringApplication.run(SpringApplication.java:312) [spring-boot-2.1.7.RELEASE.jar:2.1.7.RELEASE] at org.springframework.boot.SpringApplication.run(SpringApplication.java:1214) [spring-boot-2.1.7.RELEASE.jar:2.1.7.RELEASE] at org.springframework.boot.SpringApplication.run(SpringApplication.java:1203) [spring-boot-2.1.7.RELEASE.jar:2.1.7.RELEASE] at com.qst.gmall.user.GmallUserApplication.main(GmallUserApplication.java:12) [classes/:na] Caused by: java.lang.reflect.InvocationTargetException: null 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 tk.mybatis.spring.mapper.SpringBootBindUtil$SpringBoot2Bind.bind(SpringBootBindUtil.java:127) ~[mapper-spring-boot-autoconfigure-1.2.3.jar:na] ... 21 common frames omitted Caused by: java.util.NoSuchElementException: No value bound at org.springframework.boot.context.properties.bind.BindResult.get(BindResult.java:56) ~[spring-boot-2.1.7.RELEASE.jar:2.1.7.RELEASE] ... 26 common frames omitted
解决方法：
在 application.properties 配置文件中，加一行
mapper.identity=MYSQL
问题解决