@Qualifier for ambiguous beans in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when two beans of the same type exist without @Qualifier?

Consider a Spring Boot application with two beans of the same type but no @Qualifier annotation. What will happen when Spring tries to inject this type?

Spring Boot

public interface Service { void execute(); }

@Service
public class ServiceA implements Service { public void execute() { System.out.println("ServiceA"); } }

@Service
public class ServiceB implements Service { public void execute() { System.out.println("ServiceB"); } }

@Component
public class Client {
    private final Service service;
    public Client(Service service) { this.service = service; }
}

A Spring will throw a NoUniqueBeanDefinitionException due to ambiguity.

B Spring will inject ServiceA by default without error.

C Spring will inject both beans as a list automatically.

D Spring will inject ServiceB by default without error.

💡 Hint

Think about what happens when Spring finds multiple beans of the same type but no qualifier to distinguish them.

When multiple beans of the same type exist and no @Qualifier is used, Spring cannot decide which one to inject and throws a NoUniqueBeanDefinitionException.

📝 Syntax

intermediate

Which @Qualifier usage correctly injects the 'serviceB' bean?

Given two beans named 'serviceA' and 'serviceB', which constructor injection correctly uses @Qualifier to inject 'serviceB'?

Spring Boot

@Service("serviceA")
public class ServiceA implements Service {}

@Service("serviceB")
public class ServiceB implements Service {}

@Component
public class Client {
    private final Service service;
    public Client(@Qualifier("serviceB") Service service) {
        this.service = service;
    }
}

A public Client(@Qualifier(serviceB) Service service) { this.service = service; }

B public Client(@Qualifier("serviceB") Service service) { this.service = service; }

C public Client(@Qualifier(ServiceB.class) Service service) { this.service = service; }

D public Client(@Qualifier("ServiceB") Service service) { this.service = service; }

💡 Hint

Remember @Qualifier requires a string matching the bean name.

The @Qualifier annotation expects a string literal matching the bean name. Option B correctly uses @Qualifier("serviceB"). Options A and C are invalid syntax or wrong type. Option B uses incorrect case.

❓ state_output

advanced

What is printed when injecting with @Qualifier in this example?

Given these beans and injection, what will be printed when client.run() is called?

Spring Boot

@Service("serviceA")
public class ServiceA implements Service {
    public void execute() { System.out.println("Running ServiceA"); }
}

@Service("serviceB")
public class ServiceB implements Service {
    public void execute() { System.out.println("Running ServiceB"); }
}

@Component
public class Client {
    private final Service service;
    public Client(@Qualifier("serviceB") Service service) {
        this.service = service;
    }
    public void run() { service.execute(); }
}

A Running ServiceA

B Compilation error due to missing @Autowired

C No output, NullPointerException occurs

D Running ServiceB

💡 Hint

Look at which bean is injected by @Qualifier and what its execute() method prints.

The @Qualifier("serviceB") injects the ServiceB bean, so calling execute() prints 'Running ServiceB'.

🔧 Debug

advanced

Why does this code cause a NoSuchBeanDefinitionException?

This code tries to inject a bean with @Qualifier("serviceC"), but no such bean exists. What error occurs and why?

Spring Boot

@Service("serviceA")
public class ServiceA implements Service {}

@Service("serviceB")
public class ServiceB implements Service {}

@Component
public class Client {
    private final Service service;
    public Client(@Qualifier("serviceC") Service service) {
        this.service = service;
    }
}

A NullPointerException because service is null.

B NoUniqueBeanDefinitionException because multiple beans exist.

C NoSuchBeanDefinitionException because no bean named 'serviceC' exists.

D BeanCreationException due to circular dependency.

💡 Hint

Check if the bean name in @Qualifier matches any defined bean.

The @Qualifier("serviceC") requests a bean named 'serviceC' which is not defined, causing NoSuchBeanDefinitionException.

🧠 Conceptual

expert

How does @Qualifier help resolve ambiguity in Spring bean injection?

Why is @Qualifier important when multiple beans of the same type exist? Choose the best explanation.

A @Qualifier specifies the exact bean name to inject, resolving ambiguity when multiple beans of the same type exist.

B @Qualifier automatically merges all beans of the same type into a collection for injection.

C @Qualifier disables dependency injection for ambiguous beans to avoid errors.

D @Qualifier renames beans at runtime to avoid conflicts.

💡 Hint

Think about how Spring decides which bean to inject when there are many candidates.

@Qualifier tells Spring exactly which bean to use by name, so it does not get confused when multiple beans of the same type exist.