今天项目 springboot 1.* 升级到 2.0.4，以至于将 Mybatis-Plus 升级，springBoot 启动之后报错Unsatisfied dependency expressed through field ‘baseMapper’，详细错误信息如下

org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'userController': Unsatisfied dependency expressed through field 'userService'; nested exception is org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'userServicesImpl': Unsatisfied dependency expressed through field 'baseMapper'; nested exception is org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'userMapper' defined in file [E:\IntelliJIDEAProject\xsblog\target\classes\com\xsit\blog\mapper\UserMapper.class]: Unsatisfied dependency expressed through bean property 'sqlSessionFactory'; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'sqlSessionFactory' defined in class path resource [com/baomidou/mybatisplus/autoconfigure/MybatisPlusAutoConfiguration.class]: Bean instantiation via factory method failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [org.apache.ibatis.session.SqlSessionFactory]: Factory method 'sqlSessionFactory' threw exception; nested exception is org.springframework.core.NestedIOException: Failed to parse mapping resource: 'file [E:\IntelliJIDEAProject\xsblog\target\classes\mapper\UserMapper.xml]'; nested exception is java.lang.NullPointerException
	at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:586)
	at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:91)
	at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:372)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1341)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:572)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:495)
	at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:317)
	at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222)
	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:315)
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:199)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:759)
	at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:869)
	at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:550)
	at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:140)
	at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:762)
	at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:398)
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:330)
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1258)
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1246)
	at com.xsit.blog.BlogApplication.main(BlogApplication.java:10)
	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.boot.devtools.restart.RestartLauncher.run(RestartLauncher.java:49)
Caused by: org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'userServicesImpl': Unsatisfied dependency expressed through field 'baseMapper'; nested exception is org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'userMapper' defined in file [E:\IntelliJIDEAProject\xsblog\target\classes\com\xsit\blog\mapper\UserMapper.class]: Unsatisfied dependency expressed through bean property 'sqlSessionFactory'; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'sqlSessionFactory' defined in class path resource [com/baomidou/mybatisplus/autoconfigure/MybatisPlusAutoConfiguration.class]: Bean instantiation via factory method failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [org.apache.ibatis.session.SqlSessionFactory]: Factory method 'sqlSessionFactory' threw exception; nested exception is org.springframework.core.NestedIOException: Failed to parse mapping resource: 'file [E:\IntelliJIDEAProject\xsblog\target\classes\mapper\UserMapper.xml]'; nested exception is java.lang.NullPointerException
	at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:586)
	at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:91)
	at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:372)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1341)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:572)
	at 

仔细看错误什么也没发现，最后空指针更是奇怪，把官网源码下载下来运行居然成功，作对比后发现一个问题，项目中之前引入的包中主键用的 @Id 注解，升级之后用的是 @TableId 注解，用官网demo 试了一下，还果真如此。

然后去百度了很多，

解决方法

（1）是版本问题（我遇到的问题），其实可以看到下面这两个，mybatis-plus，及 generator 版本是同步的。所以说这俩版本要一致，关于 mybatis-plus 的依赖版本要一致，不然就会出现问题。

https://mvnrepository.com/artifact/com.baomidou/mybatis-plus-boot-starter
https://mvnrepository.com/artifact/com.baomidou/mybatis-plus-generator

（2）如果你是用的 boot，个人建议 mybatis，mybatisplus 那一堆堆包全都不要了，只要用
mybatis-plus-boot-starter 这一个 starter 包即可。免去所有版本冲突

（3）常见于 service，serviceImpl。如果是其他文件，创建 bean 错误，出现问题极有可能是扫描不到包，没有用 controller，service，repository，mapper，component 这些注解去标识到需要的包。当然也可能是你包的位置的问题，可能根本没有去扫描那个包。

（4）其他就是没有找到 mapper.xml 文件比较常见，这个一般是因为，mapper.xml 文件放在那里都有，你可以放在 java 目录，也可以放在 resources 但是要注意，去指定所在目录，比如说：

mybatis-plus.mapper-locations=classpath:/mapper/*Mapper.xml
#实体扫描，多个package用逗号或者分号分隔
mybatis-plus.type-aliases-package=com.xiaopengwei.springboot.mybatisplus.entity