What happens when i using #Bean and #Component annotations together? - spring

Question

I'm trying to configure my spring security application.
I want to create my own UserDetailsService.
For that i do something like this:
public class ApplicationUserService implements UserDetailsService {
#Override
public UserDetails loadUserByUsername(String username) throws UsernameNotFoundException {
return this.someUser();
}
}
I got 2 ways to add this UserService to Spring Security
Add it to configuration class. Something like this:
#Configuration
#EnableWebSecurity
public class SecurityConfig extends WebSecurityConfigurerAdapter {
#Bean
#Override
protected UserDetailsService userDetailsService() {
return applicationUserService;
}
}
Or add annotaion #Component, or #Service on my class.
Everything is working fine when i chose only 1 way, but i got an question: why when i trying to use both variants (add #Service and add #Bean to config) nothing is working?
I got no exceptions, error or something like this in console:
2021-09-11 17:26:16.755 INFO 15819 --- [ main] com.example.test.TestApplication : Starting TestApplication using Java 16.0.2 on aleksander-MS-7A71 with PID 15819 (/home/aleksander/programming/java/4fun/test/target/classes started by aleksander in /home/aleksander/programming/java/4fun/test)
2021-09-11 17:26:16.756 INFO 15819 --- [ main] com.example.test.TestApplication : No active profile set, falling back to default profiles: default
2021-09-11 17:26:17.402 INFO 15819 --- [ main] o.s.b.w.embedded.tomcat.TomcatWebServer : Tomcat initialized with port(s): 8080 (http)
2021-09-11 17:26:17.409 INFO 15819 --- [ main] o.apache.catalina.core.StandardService : Starting service [Tomcat]
2021-09-11 17:26:17.409 INFO 15819 --- [ main] org.apache.catalina.core.StandardEngine : Starting Servlet engine: [Apache Tomcat/9.0.52]
2021-09-11 17:26:17.442 INFO 15819 --- [ main] o.a.c.c.C.[Tomcat].[localhost].[/] : Initializing Spring embedded WebApplicationContext
2021-09-11 17:26:17.442 INFO 15819 --- [ main] w.s.c.ServletWebServerApplicationContext : Root WebApplicationContext: initialization completed in 630 ms
2021-09-11 17:26:17.555 INFO 15819 --- [ main] o.s.s.web.DefaultSecurityFilterChain : Will secure any request with [org.springframework.security.web.context.request.async.WebAsyncManagerIntegrationFilter#6981f8f3, org.springframework.security.web.context.SecurityContextPersistenceFilter#38bb9d7a, org.springframework.security.web.header.HeaderWriterFilter#62db3891, org.springframework.security.web.authentication.logout.LogoutFilter#48528634, org.springframework.security.web.authentication.www.BasicAuthenticationFilter#80bfdc6, org.springframework.security.web.savedrequest.RequestCacheAwareFilter#78d6447a, org.springframework.security.web.servletapi.SecurityContextHolderAwareRequestFilter#5e65afb6, org.springframework.security.web.authentication.AnonymousAuthenticationFilter#623dcf2a, org.springframework.security.web.session.SessionManagementFilter#2819c460, org.springframework.security.web.access.ExceptionTranslationFilter#6f49d153, org.springframework.security.web.access.intercept.FilterSecurityInterceptor#60bbacfc]
2021-09-11 17:26:17.676 INFO 15819 --- [ main] o.s.b.w.embedded.tomcat.TomcatWebServer : Tomcat started on port(s): 8080 (http) with context path ''
2021-09-11 17:26:17.682 INFO 15819 --- [ main] com.example.test.TestApplication : Started TestApplication in 1.215 seconds (JVM running for 1.794)

Answer 1

The way you've described the question, the application will definitely throw an exception unless you've defined the bean preference.
First case:
Basically, UserDetailsService is an interface and you've provided the implementation of it by declaring the bean as
#Configuration
#EnableWebSecurity
public class SecurityConfig extends WebSecurityConfigurerAdapter {
#Bean
#Override
protected UserDetailsService userDetailsService() {
return new ApplicationUserService();
}
}
Second case: You want to check the behaviour by declaring another bean using #Service or #Component annotation as following
#Service
public class ApplicationUserService implements UserDetailsService {
#Override
public UserDetails loadUserByUsername(String username) throws UsernameNotFoundException {
return new UserDetails();
}
}
If you try to use the above cases together, it won't work. The case is very simple you are providing two beans of type UserDetailsService to the spring container and hence it won't be able to identify which one it should use.
If you want to check the behaviour with both the cases you've to set the priority for beans, so in that case you can mark one of the bean with #Primary annotation.
#Configuration
#EnableWebSecurity
public class SecurityConfig extends WebSecurityConfigurerAdapter {
#Primary
#Bean
#Override
protected UserDetailsService userDetailsService() {
return new ApplicationUserService();
}
}