Exception: Field required a bean of type that could not be found 

This error “Field required a bean of type that could not be found” indicates that Spring is unable to find a bean of a particular type that is needed for dependency injection. This can happen due to several reasons:

• Missing Bean Definition: You might have forgotten to define the bean in your Spring configuration.
• Component Scanning Issue: Spring might not be scanning the package where the bean is located.
• Incorrect Bean Name or Type: The bean might be defined with a different name or type than what is being expected by the consuming component.

Let’s walk through an example to clarify. Let’s assume you have an interface NotificationService and its implementation NotificationServiceImpl. Your AdminServiceImpl class requires an instance of NotificationService to be injected. First, define your NotificationService interface, then implement this interface with NotificationServiceImpl

public interface NotificationService {
    void sendNotification(String message);
}


@Service // This annotation marks the class as a Spring service
public class NotificationServiceImpl implements NotificationService {
    @Override
    public void sendNotification(String message) {
        
        System.out.println("Sending notification: " + message);
    }
}

Now, let’s say you have your AdminServiceImpl class where you want to inject NotificationService:

@Service 
public class AdminServiceImpl implements AdminService {

    private final NotificationService notificationService;

    @Autowired // This annotation tells Spring to inject the dependency
    public AdminServiceImpl(NotificationService notificationService) {
        this.notificationService = notificationService;
    }

    }

Here, AdminServiceImpl class has a constructor that takes NotificationService as a parameter. The @Autowired annotation tells Spring to inject an instance of NotificationService when creating an instance of AdminServiceImpl. To make Spring aware of NotificationServiceImpl as a bean, you typically need to enable component scanning in your Spring configuration.