@Qualifier for ambiguous beans in Spring Boot - Cheat Sheet & Quick Revision

Recall & Review

beginner

What problem does the @Qualifier annotation solve in Spring Boot?

It helps Spring know which bean to inject when there are multiple beans of the same type, avoiding confusion.

beginner

How do you use @Qualifier to specify a bean named 'serviceA' for injection?

You add @Qualifier("serviceA") above the injection point, like: <br>@Autowired<br>@Qualifier("serviceA")<br>private MyService service;

beginner

True or False: @Qualifier can be used with constructor injection, setter injection, and field injection.

True. @Qualifier works with all types of dependency injection to specify which bean to use.

beginner

What happens if you have two beans of the same type but do NOT use @Qualifier when injecting?

Spring throws an error because it doesn't know which bean to pick. This is called an ambiguous bean error.

intermediate

Can @Qualifier be combined with @Primary? How do they work together?

Yes. @Primary marks a default bean to use if no @Qualifier is specified. But if @Qualifier is used, it overrides @Primary.

What does @Qualifier do in Spring Boot?

A Specifies which bean to inject when multiple beans of the same type exist

B Marks a bean as the primary bean

C Creates a new bean instance

D Disables bean injection

@Qualifier tells Spring exactly which bean to use when there are multiple candidates.

If you have two beans of type Service, how do you avoid ambiguity?

A Use @ComponentScan

B Use @Autowired only

C Use @Qualifier to name the bean to inject

D Remove one bean

Using @Qualifier specifies which bean Spring should inject, avoiding ambiguity.

Which of these is a correct way to use @Qualifier with constructor injection?

A public MyClass(MyService service) {}

B @Qualifier("bean1") public MyClass(MyService service) {}

C public MyClass(MyService service) {@Qualifier("bean1")}

D public MyClass(@Qualifier("bean1") MyService service) {}

You place @Qualifier on the constructor parameter to specify the bean.

What annotation marks a bean as the default when no @Qualifier is used?

A @Primary

B @Default

C @Qualifier

D @Autowired

@Primary marks a bean as the default choice if no @Qualifier is specified.

If a bean is marked with @Primary and another bean is injected with @Qualifier, which bean is used?

A The @Primary bean

B The bean specified by @Qualifier

C Both beans

D Injection fails

@Qualifier overrides @Primary and selects the specified bean.

Explain why @Qualifier is important when you have multiple beans of the same type in Spring Boot.

Think about how Spring chooses beans when there are many options.

Describe how you would use @Qualifier in a Spring Boot project with two implementations of the same interface.

Consider how to tell Spring which version to use.