javaspringgenericsautowiredspring-annotations

How to Autowire Bean of generic type <T> in Spring?

I have a bean Item<T> which is required to be autowired in a @Configuration class.

@Configuration
public class AppConfig {

    @Bean
    public Item<String> stringItem() {
        return new StringItem();
    }

    @Bean
    public Item<Integer> integerItem() {
        return new IntegerItem();
    }

}

But when I try to @Autowire Item<String>, I get following exception.

"No qualifying bean of type [Item] is defined: expected single matching bean but found 2: stringItem, integerItem"

How should I Autowire generic type Item<T> in Spring?

Solution

  • Simple solution is to upgrade to Spring 4.0 as it will automatically consider generics as a form of @Qualifier, as below:

    @Autowired
private Item<String> strItem; // Injects the stringItem bean

@Autowired
private Item<Integer> intItem; // Injects the integerItem bean

    Infact, you can even autowire nested generics when injecting into a list, as below:

    // Inject all Item beans as long as they have an <Integer> generic
// Item<String> beans will not appear in this list
@Autowired
private List<Item<Integer>> intItems;

    How this Works?

    The new ResolvableType class provides the logic of actually working with generic types. You can use it yourself to easily navigate and resolve type information. Most methods on ResolvableType will themselves return a ResolvableType, for example:

    // Assuming 'field' refers to 'intItems' above
ResolvableType t1 = ResolvableType.forField(field); // List<Item<Integer>> 
ResolvableType t2 = t1.getGeneric(); // Item<Integer>
ResolvableType t3 = t2.getGeneric(); // Integer
Class<?> c = t3.resolve(); // Integer.class

// or more succinctly
Class<?> c = ResolvableType.forField(field).resolveGeneric(0, 0);

    Check out the Examples & Tutorials at below links.