Using the @Lookup Annotation in Spring

The @Lookup annotation is an injection (like @Inject, @Resource, @Autowired) annotation used at the method level. This annotation tells Spring to overwrite the method, redirecting to the bean factory to return a bean matching the return type of the method. This can be useful for some bean scopes, such as the prototype scope, which will return a new instance of the bean each time it is requested. By combining it with the @Lookup annotation, you can make things more dynamic.

Use Cases for @Lookup

Imagine that you have a bean that handles user-specific session data. Each request should have its own instance to avoid data conflicts.

Code example:

Let's say you have a prototype bean that gets the exact moment a user of your system has made a request.

@Component
@Scope("prototype")
public class UserSessionBean {

    private String userData;

    public UserSessionBean() {
        this.userData = "Session data for " + System.currentTimeMillis();
    }

    public String getUserData() {
        return userData;
    }

}

Now you want Spring to get you a new bean to process a request and use this new bean's information to do some processing. In this example, it will look like a session object that can hold user information during the request context.

@Component
public class UserService {

    @Lookup
    public UserSessionBean getUserSessionBean() {
        // Spring will override this method to return a new instance of UserSessionBean
        return null;
    }

    public String processUserSession() {
        UserSessionBean userSessionBean = getUserSessionBean();
        return "Processing user session with data: " + userSessionBean.getUserData();
    }
}

@RestController
@RequestMapping("/users")
public class UserController {

    private final UserService userService;

    public UserController(UserService userService) {
        this.userService = userService;
    }

    @GetMapping("/session")
    public String getUserSession() {
        return userService.processUserSession();
    }
}

Conclusion

By using the @Lookup annotation, you can make things more dynamic. In this example, it allows you to handle user-specific sessions, but you can be creative and create many more scenarios using this annotation.

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