Question

Is the @Qualifier annotation needed at all? Can’t we just inject an instance of particular type? It seems like a little extra work, since we have to create annotation type for each implementing class. To show what I mean, here is the example below:

@Documented
@Retention(RUNTIME)
@Qualifier
public @interface AppleQ { }

@Documented
@Retention(RUNTIME)
@Qualifier
public @interface CheeseQ { }

public interface Eatable { }

@AppleQ
public class Apple implements Eatable { }

@CheeseQ
public class Cheese implements Eatable { }

public class Breakfast {
  @Inject @AppleQ Eatable somethingToEat;
}

vs

public interface Eatable { }

public class Apple implements Eatable { }

public class Cheese implements Eatable { }

public class Breakfast {
  @Inject Apple somethingToEat;
}

Answer 1

You are (kind of) right, your example works without qualifiers. But its rather that your example is a bit misleading than that you don’t need qualifiers.

In general, you will need qualifiers whenever you have have more than one managed bean of a certain type eligible for injection. This is not the case in your example, but would easily be if you wrote your code like this:

public class Breakfast {
  @Inject Eatable somethingToEat;
}

(This gives you the flexibility to change your implementation later, for the same reason as you normally write List list = new ArrayList())

You will not need qualifiers if you have just one managed bean of a certain type eligible for injection.

More serious examples where you want to use qualifiers would look like this:

Imagine you want have a class Locale in your system. Using different qualifiers (together with different producer methods) would allow you to write code like this:

...
@Inject
@DefaultLocale
Locale theDefaultLocale;
...
@Inject
@StandardLocale
Locale theStandardLocale;
...
@Inject 
Instance<Locale> allLocales;
...

To summarize: You need qualifiers if and only if you have more than one bean of a type. This makes qualifiers redundant for the overwhelming majority of you beans – but you will certainly need them.

All this and much more is best read here.