Spring Bean annotation: why only one Qualifier is needed for several beans of the same type

Dog.java:

@Component
public class Dog {

    @Autowired
    @Qualifier("dogName")
    public String dogNameop;

    public Dog()  {
        
    }
    
     public Dog(String dogNameop) {
        this.dogNameop = dogNameop;
    }

    public String getDogNameop() {
        return dogNameop;
    }

    public void setDogNameop(String dogNameop) {
        this.dogNameop = dogNameop;
    }
}

Cat.java:

@Component
public class Cat {
    private String catName;
    
    public Cat()  {
        
    }

    @Autowired
    public Cat(String catName) {
        this.catName = catName;
    }

    public String getCatName() {
        return catName;
    }

    public void setCatName(String catName) {
        this.catName = catName;
    }
}

Bear.java:

@Component
public class Bear {
    private String  bearName;

    public Bear()  {
        
    }
    
    @Autowired
    public Bear(String bearName) {
        this.bearName = bearName;
    }

    public String getBearName() {
        return bearName;
    }

    public void setBearName(String bearName) {
        this.bearName = bearName;
    }
}

Config.java:

@Configuration
public class Config {
    @Bean
    String dogName()  {
        return "oof";
    }
    
    @Bean
    String catName()  {
        return "catto";
    }

    @Bean
    String bearName()  {
        return "bear";
    }
}

DemoApplication.java:

@SpringBootApplication(exclude = {SecurityAutoConfiguration.class})
public class DemoApplication {
	 

	public static void main(String[] args) {
		SpringApplication.run(DemoApplication.class, args);
	}
	
 

 	@Bean
	CommandLineRunner commandLineRunner(Dog dog, Cat cat, Bear bear)  {
		return args ->  {
 			System.out.println(dog.dogNameop);
			System.out.println(cat.getCatName());
			System.out.println(bear.getBearName());
		};
	} 
}

Output:

oof
catto
bear

If I remove @Qualifier("dogName") from Dog.java, I get the error:

Field dogNameop in com.example.demo.Dog required a single bean, but 3 were found:

However with only one qualifier it manages to correctly assign the fields in Cat and Bear classes. How? I would expect that each class: Dog, Cat, and Bear would require a @Qualifier inside.

EDIT:

Changing the field name in Cat from catName to catNameHm makes the error to appear again (independently chaning bearName in Bear class to bearNameOh also triggers the error).

So it looks like once a single @Qualifier is applied it makes Spring to always apply autowiring of type String by the name of the field - in all classes, rather than just the class that has @Qualifier annotation?

Spring resolves that dependency injection because:

1. @Bean method name is the default bean name.
   Baeldung:

> when we use the @Bean annotation on a method, Spring uses the method
> name as a bean name.

2. If no qualifier is specified and multiple beans of a same class are available, Spring @Autowire falls back to parameter name as a qualifier.
   Baeldung (see example after quote):
   > Spring uses the bean's name as a default qualifier value

official docs:

> For a fallback match, the bean name is considered a default qualifier
> value. Thus, you can define the bean with an id of main instead of the
> nested qualifier element, leading to the same matching result.

Thus, to resolve @Autowired public Bear(String bearName) Spring looks up for a bean named bearName, which is present due to defining function naming. Likewise for catName.

Motivation around is about how Spring resolves the dependency injection.

Spring can resolve via type, name or explicit qualifier. Since all three beans are Strings, the only way for SprinfFramework can resolve the dependency injection is via name or @Qualifier

that's way you have to put the @Qualifier otherwise you can evaluate to refactor your class and change dogNameop to dogName like below:

@Component
public class Dog {

    @Autowired
    public String dogName;

    public Dog()  {
        
    }
    
     public Dog(String dogName) {
        this.dogName = dogName;
    }

    public String getDogName() {
        return dogName;
    }

    public void setDogName(String dogName) {
        this.dogName = dogName;
    }
}

1. You are @Autowire'ing a String

   So, in your application context, there are three beans with that type.

2. Spring got to choose, since you're injecting a single bean, but without qualifiers, there is nothing more Spring can do to narrow down the search.

   End of the story: error.

> NOTE: If you wanted all beans of type String, you could @Autowire on a List<String>, then decide for yourself which one suits your need.