Commonly Used Spring Annotations – Core, MVC/REST, Boot, Stereotype, Transaction, Scheduling and Testing

Since Java 5 introduced annotations, the Spring framework has gradually moved from XML configuration to annotation‑based configuration. This guide summarises the most useful Spring annotations, grouped by their purpose.

1. Core Annotations

@Required Applied to a bean's setter method to indicate that the property must be injected; otherwise a BeanInitializationException is thrown.

@Autowired Used on fields, setter methods, or constructors to declare a dependency that Spring should inject by type.

@Component
public class User {
    @Autowired
    private Address address;
}

The annotation can also be placed on a setter to add custom logic:

@Component
public class User {
    private Address address;

    @Autowired
    public void setAddress(Address address) {
        // custom code
        this.address = address;
    }
}

When used on a constructor, only one constructor in a class may be annotated. From Spring 4.3 onward, a single‑constructor class is autowired automatically even without the annotation.

@Qualifier Combined with @Autowired to disambiguate which bean should be injected when multiple candidates of the same type exist.

@Component
public class User {
    @Autowired
    @Qualifier("address1")
    private Address address;
}

@Configuration Marks a class as a source of bean definitions, equivalent to an XML configuration file. Methods annotated with @Bean inside the class define individual beans.

@Configuration
public class SpringCoreConfig {
    @Bean
    public AdminUser adminUser() {
        return new AdminUser();
    }
}

@ComponentScan Used together with @Configuration to specify the packages that Spring should scan for annotated components.

@Lazy Delays bean initialization until the bean is first requested.

@Value Injects values from property files, environment variables, or Spring Expression Language (SpEL) into fields, constructor parameters, or method parameters.

2. Spring MVC and REST Annotations

@Controller and @RestController declare a class as a web controller; the latter combines @Controller and @ResponseBody.

@RequestMapping maps HTTP requests to handler classes or methods. Method‑level attributes such as method = RequestMethod.GET restrict the HTTP verb.

@Controller
@RequestMapping("/users")
public class UserController {
    @RequestMapping(method = RequestMethod.GET)
    public String getUserList() {
        return "users";
    }
}

Since Spring 4.3, shortcut annotations like @GetMapping, @PostMapping, @PutMapping, @PatchMapping, and @DeleteMapping provide concise method mappings.

Other parameter‑binding annotations include @PathVariable, @RequestParam, @RequestBody, @RequestHeader, @CookieValue, @MatrixVariable (requires enabling matrix variables), @RequestAttribute, @SessionAttribute, and @ModelAttribute.

3. Spring Boot Annotations

@SpringBootApplication combines @Configuration, @EnableAutoConfiguration, and @ComponentScan, enabling auto‑configuration and component scanning for the application’s base package.

4. Stereotype Annotations

@Component – generic Spring component. @Service – business‑logic component. @Repository – data‑access component with automatic exception translation.

5. Data Access Annotation

@Transactional marks a class or method as participating in a transaction; the actual transaction handling is performed by Spring’s infrastructure.

6. Task Execution and Scheduling Annotations

@Scheduled schedules a method for periodic execution. fixedDelay waits for the previous execution to finish, while fixedRate does not.

@Scheduled(fixedDelay = 1000)
public void schedule() {
    // task logic
}

@Scheduled(fixedRate = 1000)
public void scheduleFixedRate() {
    // task logic
}

@Async runs a method asynchronously in a separate thread; the return type can be void or a Future.

7. Testing Annotations

@ContextConfiguration specifies the configuration classes or XML files for a Spring test context, usually used together with @RunWith(SpringJUnit4ClassRunner.class).

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(classes = SpringCoreConfig.class)
public class UserServiceTest {
    // test methods
}

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