Spring Boot：Consider defining a bean of type '*.*.*' in your configuration

Description:
Field mapper in com.demo.service.impl.UserServiceImpl required a bean of type 'com.demo.mapper.UserMapper' that could not be found.
Action:
Consider defining a bean of type 'com.demo.mapper.UserMapper' in your configuration.

mapper(Dao层)

 8 //@Component
 9 @Repository
10 public interface UserMapper {
11 
12     public User gYeMian(User u);
13 
14     public int sYeMian(User u);
15 
16 }

解决方案一：

mapper(Dao层)

 1
 7 @Mapper
 8 public interface UserMapper {
 9 
10     public User gYeMian(User u);
11 
12     public int sYeMian(User u);
13 
14 }

 解决方案二：

Application(启动类)

 1 package com.demo;
 2 
 3 import org.mybatis.spring.annotation.MapperScan;
 4 import org.springframework.boot.SpringApplication;
 5 import org.springframework.boot.autoconfigure.SpringBootApplication;
 6 
 7 @SpringBootApplication
 8 @MapperScan(value = "com.demo.mapper")
 9 public class App 
10 {
11     public static void main(String[] args) throws Exception {
12         SpringApplication.run(App.class, args);
13     }
14 }

原因：在mybatis-spring-boot-autoconfigure的jar包中有一个类 MybatisAutoConfiguration，在这个类中的registerBeanDefinitions方法告诉了我们

 1 @Override
 2     public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
 3 
 4       logger.debug("Searching for mappers annotated with @Mapper");
 5 
 6       ClassPathMapperScanner scanner = new ClassPathMapperScanner(registry);
 7 
 8       try {
 9         if (this.resourceLoader != null) {
10           scanner.setResourceLoader(this.resourceLoader);
11         }
12 
13         List<String> packages = AutoConfigurationPackages.get(this.beanFactory);
14         if (logger.isDebugEnabled()) {
15           for (String pkg : packages) {
16             logger.debug("Using auto-configuration base package '{}'", pkg);
17           }
18         }
19 
20         scanner.setAnnotationClass(Mapper.class);
21         scanner.registerFilters();
22         scanner.doScan(StringUtils.toStringArray(packages));
23       } catch (IllegalStateException ex) {
24         logger.debug("Could not determine auto-configuration package, automatic mapper scanning disabled.", ex);
25       }
26     }