@SpringBootApplication – It is a combination of three annotations @ComponentScan, @EnableAutoConfiguration, and @Configuration.
@Required –
It applies to the bean setter method.
It indicates that the beans with annotation must populate at configuration time with the required property, else it throws an exception BeanInitilizationException.
@Configuration –
It is a class-level annotation.
It generates bean definitions and service requests.
@EnableAutoConfiguration – It auto-configures the bean that is present in the classpath and configures it to run the methods.
@Component –
It is a class-level annotation.
It marks Java class as a bean.
A Java class with @Component annotation is searched in the classpath.
If a class is found, the Spring Framework will configure it in the application context as a Spring Bean.
@ComponentScan –
It scans a package for beans.
One can use this annotation along with the @Configuration annotation.
@Autowired –
It auto wires spring bean on setter methods, instance variable, and constructor.
In other words, it allows automatic dependency injection. The spring container injects the dependent beans by matching data-type.
@Controller –
The @Controller is a class-level annotation.
It marks the Java class as a web request handler.
By default, it returns a string that tells the route to redirect.
It is mostly used with @RequestMapping annotation.
Some other annotations –
@Bean –
The @Bean is a method-level annotation.
It act as an alternative of XML <bean> tag.
This annotation tells the method to produce a bean that a Spring Container can manage.
@Service –
It is also used at the class level.
It tells the Spring that class contains business logic.
@Repository –
It is a class-level annotation.
The repository is a DAO(Data Access Object) that access the database directly and perform all the operations related to the database.
@RequestMapping –
It is used for mapping the web requests.
It consists of optional elements. For Instance consumes, header, method, name, params, path, produce, and value.
@GetMapping – It maps the HTTP GET requests on the specific handler method.
@PostMapping – It maps the HTTP POSTrequests on the specific handler method.
@PutMapping – It maps the HTTP PUT requests on the specific handler method.
@DeleteMapping – It maps the HTTP DELETE requests on the specific handler method.
@PatchMapping – It maps the HTTP PATCH requests on the specific handler method.
@RequestBody – It is used to bind HTTP request with an object in a method parameter.
@ResponseBody – It binds the method return value to the response body.
@PathVariable – It is used to extract the values from the URI.
@RequestParam – It is used to extract the query parameters from the URL.
@RequestHeader – It is used to get the details about the HTTP request headers.
@RestController – It can be considered as a combination of @Controller and @ResponseBody annotations.
How does it work ?
@EnableAutoConfiguration annotation allows Spring Boot to automatically configure your application based on the dependencies you have added to the project.
The class that contains @SpringBootApplication annotation is the entry point of the spring boot application.
To run the Spring Boot application, this class should have the main method.
@SpringBootApplication annotation includes Auto- Configuration, Component Scan, and Spring Boot Configuration.
SpringApplication.run() inside the main method will bootstrap String application as a stand-alone application.
It creates an instance of ApplicationContext and loads all the beans.
