Spring annotations

@Component is added at a class, meaning that this class will be scanned by the Spring container and loaded in the Spring context.  @Component is the most generic decoration. A @Service is a component; A @Controller is a component too. 

@Service means actually a specific @Component, which stands for a business logic layer.  So when a class is added with @Service, it will be scanned by Spring container and loaded in the context. 

@RestController  is also a specific @Component when a Spring context is initiated. These controllers will be instantiated and start waiting for the requests and making responses. 

@Autowired is specific to the Spring framework; it is equivalent to the Java @Injected 

@Configuration and its Relevant Annotations

@Configuration working with @Bean, @Value, @PropertySource, @ComponentScan and @Profile

@Configuration together with @Bean defines a bean blueprint, which tells a bean factory how to create a bean and its dependencies(DI).  In a simple word, it depicts a bean graph. 

@Configuration indicates a class, in which one or more @Beans methods are declared. 

What is a @Bean? @Bean is used to decorate a method. It tells Spring that this method is used to return an instantiated  @component or @Service. @Bean is used together with @Configuration class to tell Spring framework how to do DI. 

@Cofiguration working with externalized values

@Configuration class may need external env properties or other values defined in the other property files. 

Using  Environment API

Via Spring Environment injection, we may access properties. 

@Configuration
 public class AppConfig {

     @Autowired Environment env;

     @Bean
     public MyBean myBean() {
         MyBean myBean = new MyBean();
         myBean.setName(env.getProperty("bean.name"));
         return myBean;
     }
 }

Using Property Sources

Properties resolved via Environment reside in one or more "property source" objects; @Configuration may work with specific property files. 

@Configuration
@PropertySource("classpath:/com/acme/app.properties")
 public class AppConfig {

     @Inject Environment env;

     @Bean
     public MyBean myBean() {
         return new MyBean(env.getProperty("bean.name"));
     }
 }

Using Value Injection

Without bypass the Environment, externalized values may be injected into @Configuration via @Value.

@Configuration
 @PropertySource("classpath:/com/acme/app.properties")
 public class AppConfig {

     @Value("${bean.name}") String beanName;

     @Bean
     public MyBean myBean() {
         return new MyBean(beanName);
     }
 }

Specifying @Component Location

@ComponentScan configures component scanning directives for use with @Configuration classes. 

@Configuration
 @ComponentScan("com.acme.app.services")
 public class AppConfig {
     // various @Bean definitions ...
 }

?About proxyBeanMethods

By default  @Configuration(proxyBeanMethods=true); it specifies whether @Bean methods should get proxied in order to enforce bean lifecycle behaviour, e.g to return shared singleton bean instances even in direct @Bean method calls in user code. This feature requires method interception, implemented through a runtime-generated CGLIB subclass which comes with limitations such as configuration class and its methods not being allowed to declare final.  

public abstract boolean proxyBeanMethods