学习SpringBoot遇到的问题或错误

记录学习SpringBoot遇到的错误，方便再次遇到时回头检查。

错误场景：MainApplication类不应放在默认的src.main.java下。

Error：

** WARNING ** : Your ApplicationContext is unlikely to start due to a @ComponentScan of the default package.


2022-04-25 12:13:48.622  WARN 14856 --- [           main] ConfigServletWebServerApplicationContext : Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.BeanDefinitionStoreException: Failed to read candidate component class: URL [jar:file:/D:/Maven/repository/org/springframework/boot/spring-boot-autoconfigure/2.5.3/spring-boot-autoconfigure-2.5.3.jar!/org/springframework/boot/autoconfigure/jdbc/DataSourceAutoConfiguration$EmbeddedDatabaseConfiguration.class]; nested exception is java.lang.IllegalStateException: Could not evaluate condition on org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration$EmbeddedDatabaseConfiguration due to org/springframework/dao/DataAccessException not found. Make sure your own configuration does not rely on that class. This can also happen if you are @ComponentScanning a springframework package (e.g. if you put a @ComponentScan in the default package by mistake)
2022-04-25 12:13:48.627  INFO 14856 --- [           main] ConditionEvaluationReportLoggingListener : 

Error starting ApplicationContext. To display the conditions report re-run your application with 'debug' enabled.
2022-04-25 12:13:48.640 ERROR 14856 --- [           main] o.s.boot.SpringApplication               : Application run failed

org.springframework.beans.factory.BeanDefinitionStoreException: Failed to read candidate component class: URL [jar:file:/D:/Maven/repository/org/springframework/boot/spring-boot-autoconfigure/2.5.3/spring-boot-autoconfigure-2.5.3.jar!/org/springframework/boot/autoconfigure/jdbc/DataSourceAutoConfiguration$EmbeddedDatabaseConfiguration.class]; nested exception is java.lang.IllegalStateException: Could not evaluate condition on org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration$EmbeddedDatabaseConfiguration due to org/springframework/dao/DataAccessException not found. Make sure your own configuration does not rely on that class. This can also happen if you are @ComponentScanning a springframework package (**e.g. if you put a @ComponentScan in the default package by mistake**)
	at org.springframework.context.annotation.ClassPathScanningCandidateComponentProvider.scanCandidateComponents(ClassPathScanningCandidateComponentProvider.java:452) ~[spring-context-5.3.9.jar:5.3.9]
	at org.springframework.context.annotation.ClassPathScanningCandidateComponentProvider.findCandidateComponents(ClassPathScanningCandidateComponentProvider.java:315) ~[spring-context-5.3

解决：

错误场景：
下面图1中的@Configuration注解会默认生成一个以无参构造函数创建的Bean加入到容器中，且这个bean的名字是与类同名，开头字母小写，即person。图2，@Configuration搭配@Bean注解使用，也是生成一个Bean放到容器中，这个bean名字是如方法同名，即person。
这样就会有冲突。
图1：

图2：

Error：

Description:

The bean 'person', defined in class path resource [spring/boot/study/config/MyConfig.class], could not be registered. A bean with that name has already been defined in file [D:SourceTreerepositoryjavapracticeSpringBootSpringBootStudytargetclassesspringbootstudymodelPerson.class] and overriding is disabled.

Action:

Consider renaming one of the beans or enabling overriding by setting spring.main.allow-bean-definition-overriding=true

解决：
其实上面的Action也有讲了解决方法，重新命名或者设置bean名可以覆盖。

错误场景：
下图有3个Bean都是Person类型，但是Bean名字不同。第一个是无参构造函数创建的，@Autowired默认是以类型注入Bean，当我们简单用以下person注入Bean时，得到的Bean就是这个无参构造函数创建的Bean；当用person3获取Bean时，同样是存在3个相同类型的Bean，但前者(person)不报错，后者(person3)报错，尚不知为何。突然get到一个idea：难道是当同类型有多个Bean时，会判断容器中的Bean是否存在与我们定义的对象名（如person，person3）同名的Bean，存在则获取同名的，不存在就报错。
用下面三处代码测试了，果真如此（只是不知源码是哪个）。
这样的缺点是对象名与Bean名耦合了。我们还可以使用@Qualifier(“person”)指定注入的Bean的名字。也可以在我们声明Bean时使用@Primary指定一个主要的Bean，注入时使用的就是@Primary修饰的Bean。

    @Autowired
    Person person;

// 有错误
    @Autowired
    Person person3;
    
// 容器中存在于对象person_zhangSan同名的Bean，则注入同名的那个
    @Autowired
    Person person_zhangSan;
    
// 指定注入的Bean的名字
    @Autowired
    @Qualifier("person")
    Person person4;

// 使用@Primary，存在多个Bean时默认使用Primary的
    @Bean
    @Primary
    public Person person_zhangSan(){
        System.out.println("开始创建Person实例！");
        return new Person("张三", "男", 19);
    }

Error：

Description:

Field person3 in spring.boot.study.controller.HelloController required a single bean, but 3 were found:
	- person: defined by method 'person' in class path resource [spring/boot/study/config/MyConfig.class]
	- person_zhangSan: defined by method 'person_zhangSan' in class path resource [spring/boot/study/config/MyConfig.class]
	- person_liSi: defined by method 'person_liSi' in class path resource [spring/boot/study/config/MyConfig.class]


Action:

Consider marking one of the beans as @Primary, updating the consumer to accept multiple beans, or using @Qualifier to identify the bean that should be consumed

解决：
使用@Primary定义个以Bean为primary；使用@Qualifier指定注入的Bean的名字；定义consumer的名字与容器中Bean名一致。