@Async, @ControllerAdvice Global Exception Handling, MockMvc, Cors configuration, AOP Logging

✅ 1. @Async – Asynchronous Execution

🔹 Code:

@Configuration
@EnableAsync
public class AppConfig {}

🔸 Explanation:

• @Configuration: Tells Spring this class defines beans.
• @EnableAsync: Enables Spring’s ability to run methods asynchronously using @Async.

🔹 Service:

@Service
public class EmailService {

    @Async
    public void sendEmail(String to) {
        System.out.println("Sending email to: " + to);
        try {
            Thread.sleep(3000); // simulate delay
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("Email sent to: " + to);
    }
}

🔸 Explanation:

• @Service: Makes this a Spring-managed bean.
• @Async: Runs sendEmail() in a new thread (non-blocking).
• Thread.sleep(3000): Simulates a 3-second delay as if sending an email.

🔹 Controller:

@RestController
public class NotificationController {

    @Autowired
    private EmailService emailService;

    @GetMapping("/notify")
    public String notifyUser() {
        emailService.sendEmail("[email protected]");
        return "Notification triggered!";
    }
}

🔸 Explanation:

• When you access /notify, Spring immediately responds, while sendEmail continues to run in the background thread.

✅ 2. @ControllerAdvice – Global Exception Handling

🔹 Code:

@ControllerAdvice
public class GlobalExceptionHandler {

    @ExceptionHandler(IllegalArgumentException.class)
    public ResponseEntity<String> handleIllegalArg(IllegalArgumentException ex) {
        return new ResponseEntity<>("Invalid input: " + ex.getMessage(), HttpStatus.BAD_REQUEST);
    }

    @ExceptionHandler(Exception.class)
    public ResponseEntity<String> handleAll(Exception ex) {
        return new ResponseEntity<>("Server Error: " + ex.getMessage(), HttpStatus.INTERNAL_SERVER_ERROR);
    }
}

🔸 Explanation:

• @ControllerAdvice: Applies to all controllers.
• @ExceptionHandler: Specifies which exception this method handles.
• ResponseEntity: Allows you to return custom status and message.

🔹 Controller:

@RestController
public class TestController {

    @GetMapping("/error")
    public String throwError() {
        throw new IllegalArgumentException("Bad ID");
    }
}

🔸 Result:

Visiting /error returns 400 Bad Request with "Invalid input: Bad ID"

✅ 3. MockMvc – Controller Testing

🔹 Code:

@RunWith(SpringRunner.class)
@WebMvcTest(HelloController.class)
public class HelloControllerTest {

    @Autowired
    private MockMvc mockMvc;

    @Test
    public void testHelloEndpoint() throws Exception {
        mockMvc.perform(get("/hello"))
               .andExpect(status().isOk())
               .andExpect(content().string("Hello Spring"));
    }
}

🔸 Explanation:

• @WebMvcTest: Loads only web layer (not full context).
• MockMvc: Used to simulate HTTP requests to test controllers.
• get("/hello"): Sends a fake GET request to /hello.

🔹 Controller:

@RestController
public class HelloController {
    @GetMapping("/hello")
    public String hello() {
        return "Hello Spring";
    }
}

✅ 4. CORS Configuration

🔹 Code:

@Configuration
public class CorsConfig implements WebMvcConfigurer {
    @Override
    public void addCorsMappings(CorsRegistry registry) {
        registry.addMapping("/api/**")
                .allowedOrigins("http://localhost:3000")
                .allowedMethods("GET", "POST");
    }
}

🔸 Explanation:

• Allows JavaScript frontend (e.g., React on port 3000) to call Spring backend.
• /api/** matches any endpoint starting with /api.

✅ 5. Custom Converter – String to User

🔹 Converter:

@Component
public class StringToUserConverter implements Converter<String, User> {

    @Autowired
    private UserRepository userRepository;

    @Override
    public User convert(String id) {
        return userRepository.findById(id).orElse(null);
    }
}

🔸 Explanation:

• Converts String ID into User object using repository.
• Spring auto-uses this in @RequestParam, @PathVariable, or form binding.

🔹 Usage:

@GetMapping("/user")
public String getUser(User user) {
    return "User: " + user.getName();
}

✅ 6. Static Resources

🔹 Example Directory:

src/main/resources/static/js/script.js

🔹 Access in Browser:

http://localhost:8080/js/script.js

🔸 Explanation:

• Spring Boot automatically maps /static/** to serve static files.

✅ 7. Custom Annotation + AOP Logging

🔹 Annotation:

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
public @interface LogExecutionTime {}

🔹 Aspect:

@Aspect
@Component
public class LoggingAspect {

    @Around("@annotation(LogExecutionTime)")
    public Object logTime(ProceedingJoinPoint joinPoint) throws Throwable {
        long start = System.currentTimeMillis();
        Object result = joinPoint.proceed();  // Execute method
        long end = System.currentTimeMillis();
        System.out.println("Execution Time: " + (end - start) + " ms");
        return result;
    }
}

🔹 Service:

@Service
public class MyService {

    @LogExecutionTime
    public void process() throws InterruptedException {
        Thread.sleep(2000);
    }
}