Partner – Microsoft – NPI (cat= Spring)
announcement - icon

Azure Spring Apps is a fully managed service from Microsoft (built in collaboration with VMware), focused on building and deploying Spring Boot applications on Azure Cloud without worrying about Kubernetes.

And, the Enterprise plan comes with some interesting features, such as commercial Spring runtime support, a 99.95% SLA and some deep discounts (up to 47%) when you are ready for production.

>> Learn more and deploy your first Spring Boot app to Azure.

You can also ask questions and leave feedback on the Azure Spring Apps GitHub page.

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:

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:

public @interface CarQualifier {

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

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 {
    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:

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"));

ResolvableType type = vehiclesType.getGeneric();

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

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

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.

Course – LS (cat=Spring)

Get started with Spring and Spring Boot, through the Learn Spring course:

res – REST with Spring (eBook) (everywhere)
Comments are open for 30 days after publishing a post. For any issues past this date, use the Contact form on the site.