Spring

Spring is a very large topic, so we will only be covering the essentials and hand-picked topics.

SpringBootApplication

@SpringBootApplication - A class annotated as an application should be defined as an entry point. The annotation includes:

• @EnableAutoConfiguration - Enable the automatic configuration feature, a core part of Spring Boot.
• @ComponentScan - Automatically scan this package and sub packages for components.
• @Configuration - This class is a configuration and will automatically load any beans defined.

As with any entry point, a main method needs to be defined - in this case calling the SpringApplication run method.

@SpringBootApplication
public class WebServerApp {

   public static void main(String[] args) {
      SpringApplication.run(WebServerApp.class, args);
   }
}

Components

@Component - A class annotated as a component is automatically instantiated and registered as a bean if it is imported using @ComponentScan. Note that any parameters to the constructor selected will be injected by spring as if they were autowired, so need to be defined in a configuration.

There are a few special types of component:

• @Controller - Marks the class as part of the presentation layer.
• @Repository - Marks the class as part of the persistence layer.
• @Service - Marks the class as part of the service layer.

Controller

@Controller - A class annotated as a controller is enabled for Spring MVC.

Configurations

@Configuration - A class annotated as a configuration is used to define configuration for an aspect of the application. Configurations can be imported by the spring application and by each other using the @Import annotation.

The entire configuration can also be made optional, even if imported, using one of the conditional annotations, for example:

• @ConditionalOnProperty - if a property is set to a specific value
• @ConditionalOnExpression - if a spring expression evaluates to true
• @ConditionalOnResource - if a resource exists
• @ConditionalOnBean / @ConditionalOnMissingBean - if a bean has or has not been defined
• @ConditionalOnClass / @ConditionalOnMissingClass - if a class is or is not on the classpath

Beans

@Bean - A method annotated as a bean will have its return value automatically registered as a spring bean. Note that any parameters to the method will be injected by spring as if they were autowired, so need to be defined in a configuration.