How to Manage Multiple Implementations of a Spring Bean or Interface?

What are the best practices for managing multiple implementations of a single Spring bean or interface?

@Service
public class UserServiceImpl implements UserService {
    // implementation details
}

@Service
public class AdminUserServiceImpl implements UserService {
    // implementation details
}

@Autowired
private UserService userService;

In a Spring application, you may encounter scenarios where multiple implementations of an interface or bean are required. This can pose challenges when autowiring dependencies, leading to ambiguity. Here are effective strategies to manage such situations.

@Service
@Primary
public class DefaultUserService implements UserService {
    // default implementation
}

@Service
@Qualifier("admin")
public class AdminUserService implements UserService {
    // admin implementation
}

@Autowired
@Qualifier("admin")
private UserService userService;

Causes

• Ambiguity in dependency injection when multiple beans of the same type are available.
• Specific cases where different implementations are needed based on application logic.

Solutions

• Use the `@Qualifier` annotation to specify which bean to inject.
• Implement primary beans using the `@Primary` annotation to set a default choice.
• Leverage profiles to denote which implementation to load under certain conditions, especially useful in distinct environments (e.g., dev vs. production).

Mistake: Using multiple implementations without specifying a qualifier or primary bean.

Solution: Always use the `@Primary` or `@Qualifier` annotation when there are multiple candidates for injection.

Mistake: Not considering the scope of beans which may lead to unexpected behavior.

Solution: Ensure the scopes of the beans are appropriately defined to avoid conflicts during runtime.