@SpringBootApplication

@SpringBootApplication is an annotation in the Spring Framework for Java that indicates that a class is a Spring Boot application. This annotation combines several other annotations, including @Configuration, @EnableAutoConfiguration, and @ComponentScan, into a single convenient annotation.

The @Configuration annotation indicates that the class is a source of bean definitions for the application context.

The @EnableAutoConfiguration annotation enables automatic configuration of the Spring context based on the classpath.

And the @ComponentScan annotation tells Spring where to look for components (i.e., beans).

By using @SpringBootApplication, you can bootstrap a Spring application with minimal configuration, allowing you to quickly get up and running with a Spring Boot application.

@Configuration

@Configuration is an annotation in the Spring Framework for Java that indicates that a class is a source of bean definitions for the application context. This annotation is used in conjunction with other annotations, such as @Bean, @ComponentScan, and @Import, to provide the configuration for a Spring application.

When Spring Boot starts up, it automatically scans for classes annotated with @Configuration and loads them into the application context. These classes can contain methods annotated with @Bean that provide bean definitions. These beans can then be injected into other parts of the
application.

In a Spring Boot application, @Configuration classes contain methods annotated with @Bean that define the different components (beans) that the application needs. These beans can then be used throughout the application.

@EnableAutoConfiguration

@EnableAutoConfiguration is an annotation in the Spring Framework for Java that enables automatic configuration of the Spring context based on the classpath and the dependencies that are present. This annotation is often used in conjunction with @SpringBootApplication, but can also be used on its own.

When you annotate a configuration class with @EnableAutoConfiguration, Spring Boot will attempt to automatically configure the Spring context based on the dependencies that are present on the classpath. For example, if the application has a dependency on the Spring Data JPA module, Spring Boot will automatically configure the data source, transaction manager, and other related components that are needed to use JPA.

This annotation is very useful because it saves you a lot of time and effort in configuring your Spring application. Instead of having to manually configure each component, you can simply include the necessary dependencies on the classpath and let Spring Boot do the rest.

It’s worth noting that @EnableAutoConfiguration works by using Spring Boot’s auto-configuration mechanism, which is based on conditional beans. Conditional beans are beans that are only created if certain conditions are met, such as the presence of certain dependencies or properties in the environment. This mechanism allows Spring Boot to automatically configure the application in a way that makes sense based on the application’s dependencies and the environment it’s running in.

@ComponentScan

@ComponentScan is an annotation in the Spring Framework for Java that tells Spring where to look for Spring-managed components (i.e., beans) in your application. This annotation is often used in conjunction with @Configuration, but can also be used on its own.

When you annotate a configuration class with @ComponentScan, Spring will scan the specified packages and their sub-packages for classes that are annotated with @Component, @Service, @Repository, @Controller, or other annotations that indicate a Spring-managed component.

Spring will then create instances of these components and add them to the application context, where they can be injected into other components using dependency injection. This allows you to easily manage the lifecycle of your components and their dependencies, without having to manually create and wire them together.

For example, if you have a package named com.example.app that contains classes annotated with @Component, and you annotate your configuration class with @ComponentScan(“com.example.app”), Spring will automatically create instances of these components and add them to the application context.

@ComponentScan is a very powerful annotation that enables you to easily manage your components in a Spring application, and is often used in combination with other annotations such as @Configuration and @Autowired.

@Bean

@Bean is an annotation in the Spring Framework that is used to create and configure a Spring-managed bean. It is often used in a @Configuration class to define a bean that can be injected into other components using dependency injection.

Spring also supports other scopes, including singleton, prototype, request, session, and websocket.

The default scope is singleton, which means that a single instance of the bean is created and shared by all the components that need it.

A prototype bean is created each time it is requested, which means that a new instance of the bean is created for each injection point or method call.

A request bean is created once per HTTP request, and is destroyed when the request is completed.

A session bean is created once per HTTP session, and is destroyed when the session is terminated.

A websocket bean is created once per WebSocket session, and is destroyed when the session is closed.

To specify the scope of a bean, you can use the @Scope annotation in conjunction with @Bean. For example, to create a prototype-scoped bean, you can do the following:

@Configuration
public class AppConfig {
    @Bean
    @Scope(ConfigurableBeanFactory.SCOPE_PROTOTYPE)
    public MyPrototypeBean myPrototypeBean() {
        // create and configure a prototype-scoped bean
        return new MyPrototypeBean();
    }
}

@Qualifier

@Qualifier to disambiguate between beans with different names.

@Configuration
public class AppConfig {
    @Bean("dataSource1")
    public DataSource dataSource1() {
        // create and configure a DataSource object
        return dataSource1;
    }

    @Bean("dataSource2")
    public DataSource dataSource2() {
        // create and configure a different DataSource object
        return dataSource2;
    }
}

In this example, the AppConfig class defines two DataSource beans with different names, “dataSource1” and “dataSource2”.

To inject one of these beans into a component, you can use the @Qualifier annotation. For example:

@Component
public class MyComponent {
    private final DataSource dataSource;

    @Autowired
    public MyComponent(@Qualifier("dataSource1") DataSource dataSource) {
        this.dataSource = dataSource;
    }

    // ...
}

@Component

@Component is an annotation in the Spring Framework that is used to mark a Java class as a Spring-managed component.

@Component is a class-level annotation that is used to mark a Java class as a Spring-managed component.

@Controller

@Controller is a Spring Framework annotation used to mark a Java class as a Spring MVC controller. Controllers are responsible for processing HTTP requests and returning an appropriate HTTP response. With @Controller, you can take advantage of Spring MVC’s request handling capabilities to build web applications.

@Service

@Component and @Service are both annotations in the Spring Framework that are used to mark a Java class as a Spring-managed component. However, there is a subtle difference between the two:

• @Component is a generic annotation that can be used to mark any Java class as a Spring-managed component.
• @Service is a specialization of @Component that is specifically used to mark a Java class as a Spring-managed service. By convention, services are used to define the business logic of an application and are typically located in the service layer of the application architecture.
• @Controller is used to mark a Java class as a Spring MVC controller. Controllers are responsible for processing HTTP requests, handling user input, and returning the appropriate HTTP response. They typically contain methods that are mapped to specific URL patterns using annotations like @RequestMapping.