Spring Alllow @Primary to specify a list of interface/class on which it applies to limit his scope

Hello,

First of all thanks for this magnificent framework.

Now let's get to the point with a sample. Let's say we have 3 interfaces First, Second and Third like below:

public interface First {}
public interface Second extends First {}
public interface Third extends Second {}

And, at least for now, two implementations like below:

@Component
@Primary
 public class FirstImpl implements First {}

@Component
public class SecondImpl implements Second {}

Everything is perfect, i can bind both interfaces with their implementations :

public SomeConstructor(First first) {} // OK, it works 
public SomeOtherConstructor(Second second) {} // OK, it works 

But here comes the Third implementation :

@Component
public class ThirdImpl implements Third {}

And now the pain that may force us to fall back to qualifiers:

public SomeConstructor(First first) {} // OK, it works 
public SomeOtherConstructor(Second second) {} // KO, NoUniqueBeanDefinitionException,Third is a Second
public SomeOtherOtherConstructor(Third third) {} // OK, it works

To solve this issue i would like to allow @Primary annotation to specify on which class or interface it applies then we could use it as follow:

@Component
@Primary
 public class FirstImpl implements First {}

@Component
@Primary(for = {Second.class})
public class SecondImpl implements Second {}

@Component
@Primary(for = {Third.class})
public class ThirdImpl implements Third {}

Another use case would be to select primary interfaces on the same bean like:

@Component
@Primary(for = {Interface1.class, Interface3.class})
public class SomeImpl implements Interface1,Interface2,Interface3 {}

I hope it makes sense for who will read it

Comment From: snicoll

Thanks for the suggestion but we're not keen in making the annotation model more complex. There are some discussions about adding a fallback, see #26241 that would be quite difficult to implement if the scope of @Primary was extended.

Besides the core container doesn't have a notion of restricted type. The visible type in the bean factory is a candidate for injection in general so the kind of restriction that you're alluding would break with regular @Bean semantic. Qualifiers are probably a good candidate for the use case above.