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1. Overview

In this tutorial, we’ll see how to inject Spring beans by generic parameters.

2. Autowiring Generics in Spring 3.2.

Spring supports injection of generic types since version 3.2.

Suppose we have an abstract class called Vehicle and a concrete sub-class of it called Car:

public abstract class Vehicle {
    private String name;
    private String manufacturer;
 
    // ... getters, setters etc
}
public class Car extends Vehicle {
    private String engineType;
 
    // ... getters, setters etc
}

Suppose we want to inject a list of objects of type Vehicle into some handler class:

@Autowired
private List<Vehicle> vehicles;

Spring will autowire all the Vehicle instance beans into this list. It doesn’t matter how we instantiate these beans through Java or XML configuration.

We may also use qualifiers to get only specific beans of the Vehicle type. Then we create @CarQualifier and annotate it with @Qualifier:

@Target({
  ElementType.FIELD, 
  ElementType.METHOD,
  ElementType.TYPE, 
  ElementType.PARAMETER})
@Retention(RetentionPolicy.RUNTIME)
@Qualifier
public @interface CarQualifier {
}

Now we may use this annotation on our list to get only some specific Vehicles:

@Autowired
@CarQualifier
private List<Vehicle> vehicles;

In this case, we may create several Vehicle beans but Spring will only inject those with @CarQualifier into the list above:

public class CustomConfiguration {
    @Bean
    @CarQualifier
    public Car getMercedes() {
        return new Car("E280", "Mercedes", "Diesel");
    }
}

3. Autowiring Generics in Spring 4.0.

Suppose we have another Vehicle sub-class called Motorcycle:

public class Motorcycle extends Vehicle {
    private boolean twoWheeler;
    //... getters, setters etc
}

Now, if we want to inject only the Car beans into our list but no Motorcycle ones, we can do this by using the specific sub-class as a type parameter:

@Autowired
private List<Car> vehicles;

Spring lets us use a generic type as a qualifier without the need for an explicit annotation since version 4.0.

Prior to Spring 4.0, the code above wouldn’t work with beans of multiple subclasses of Vehicle. Without explicit qualifiers, we would receive a NonUniqueBeanDefinitionException.

4. ResolvableType

The generics autowiring feature works with the help of ResolvableType class behind the scenes.

It was introduced in Spring 4.0 to encapsulate Java Type and handle access to supertypes, interfaces, generic parameters and finally resolve to a Class:

ResolvableType vehiclesType = ResolvableType.forField(getClass().getDeclaredField("vehicles"));
System.out.println(vehiclesType);

ResolvableType type = vehiclesType.getGeneric();
System.out.println(type);

Class<?> aClass = type.resolve();
System.out.println(aClass);

The output of the above code would show the corresponding simple and generic types:

java.util.List<com.example.model.Vehicle>
com.example.model.Vehicle
class com.example.model.Vehicle

5. Conclusion

Injection of generic types is a powerful feature, which saves the developer the effort of assigning explicit qualifiers, making code cleaner and much more understandable.

As always the code can be found over on GitHub.

I just announced the new Spring Boot 2 material, coming in REST With Spring:

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