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.
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@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.
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@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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