How to understand the annotation @ComponentScan work when wiring Bean without application.xml?

Question

1, When my dictionary tree likes these, the error "@EnableAsync annotation metadata was not injected" occurs。

2, But when my dictionary tree likes this, the error doesn't happen.

3, When I wire Bean with application.xml, and the dictionary tree likes paragraph one, but the error also doesn't happen.

So what I am so confirmed is that @ComponentScan's work when wiring Bean without application.xml?

Answer 1

When a class does not include a package declaration, it is considered to be in the “default package” (src.main.java). The use of the “default package” is generally discouraged and should be avoided. It can cause particular problems for Spring Boot applications that use the @ComponentScan, @ConfigurationPropertiesScan, @EntityScan, or @SpringBootApplication annotations, since every class from every jar is read.

Spring Boot favors Java-based configuration. Although it is possible to use SpringApplication with XML sources, we generally recommend that your primary source be a single @Configuration class. Usually the class that defines the main method is a good candidate as the primary @Configuration.

This is a classic bug when first making a spring application, it is better to have all your classes in a package, never put your classes in the "default" (src.main.java) package.

Read more here from chapter "2. Structuring Your Code" to "3. Configuration Classes":

https://docs.spring.io/spring-boot/docs/current/reference/html/using-spring-boot.html#using-boot-configuration-classes