@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
