How to Autowire Bean of generic type in Spring

问题:

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?

回答1:

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.

• Spring Framework 4.0 and Java Generics
• Spring and Autowiring of Generic Types

Hope this helps you.

回答2:

If you dont want to upgrade to Spring 4 you have to autowire by name as below :

@Autowired
@Qualifier("stringItem")
private Item<String> strItem; // Injects the stringItem bean

@Autowired
@Qualifier("integerItem")
private Item<Integer> intItem; // Injects the integerItem bean

回答3:

Spring autowired strategy is defined in your configration file(application.xml).

if you don't defined, default is by Type, spring inject use JDK reflect mechanism.

so List?String? and List?Item?, the type is same List.class, so spring confused how to inject.

and as above persons response, you should be point @Qualifier to tell spring which bean should be inject.

i like spring configration file to define bean rather then Annotation.

<bean>
 <property name="stringItem">
        <list>
              <....>
        </list>
 </property>

回答4:

Spring 4.0 is the answer with the @Qualifier annotation usage. Hope this helps

回答5:

I believe it has nothing to with generics... If you are injecting two different beans of same type then you need to provide a qualifier to help Spring identify them;

... Elsewhere

@Configuration
@Bean
public Item stringItem() {
    return new StringItem();
}

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

If you have a non-generic declarations like these then you need to add qualifier to help Spring identify them...

@Autowired
**@Qualifier("stringItem")**
private Item item1;

@Autowired
**@Qualifier("integerItem")**
private Item item2;

Of course, in versions 4 and above spring considers the Generic Types through the resolvers which is very cool...