CODEWITHSUNDEEP
javagenericsspringdependency-injection
Thomas Lötzer
Thomas Lötzer

Reputation: 25381

Dependency Injection by type using generics - how does it work?

Using Spring, I can get all beans of a certain type that are currently defined using this:

@Resource
private List<Foo> allFoos;

How does Spring do this? I thought type information of generics was erased at runtime. So how does Spring know about the type Foo of the list and only inject dependencies of the correct type?

To illustrate: I don't have a bean of type "List" that contains the other beans. Instead, Spring creates that list and adds all beans of the correct type (Foo) to this list and then injects that list.

Upvotes: 2

Views: 1682

Answers (1)

sfussenegger
sfussenegger

Reputation: 36095

Not all generic information is lost at runtime:

import java.lang.reflect.Field;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.util.List;

public class Main {

    public static List<String> list;

    public static void main(String[] args) throws Exception {
        Field field = Main.class.getField("list");
        Type type = field.getGenericType();

        if (type instanceof ParameterizedType) {
            ParameterizedType pType = (ParameterizedType) type;
            Type[] types = pType.getActualTypeArguments();
            for (Type t : types) {
                System.out.println(t);
            }
        } else {
            System.err.println("not parameterized");
        }
    }

}

Output:

class java.lang.String

Upvotes: 5

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