Spring嵌套的自动装配泛型抛出NoUniqueBeanDefinitionException

Question

Is it possible to autowire a generic type into a different generic type in the constructor? I currently have a structure like this:

@Service
public class ExampleService {
@Autowired
ServiceA<Integer> servicea;

}

and the services:

@Component
@Scope(SCOPE_PROTOTYPE)
public class ServiceA<S> {
  private final ServiceB<S,String> dependentServiceB;

  public ServiceA (ServiceB<S,String> dependentServiceB){
     this.dependentServiceB = dependentServiceB;
  }
}

@Configuration
public class ServiceBConfig {
  @Bean
  ServiceB<Integer,String> serviceBwithInt (){
      return new ServiceBImplInt();
  }

  @Bean
  ServiceB<Long,String> serviceBwithLong (){
      return new ServiceBImplLong();
  }

}

Now if I try to autowire the ExampleService class somewhere else, it throws a NoUniqueBeanDefinitionException containing "No qualifying bean of type '....ServiceB< ? >' available: expected single matching bean but found 2: serviceBwithInt ,serviceBwithLong".

Is this possible to solve it like this or do I have to implement subclasses of ServiceA for each of the different generic types?

Answer 1

Given how type erasure works, both of those beans are eligible to be injected simply because Java would accept both of those as valid arguments to the constructor.

If you want to be explicit about which is injected, use a @Qualifier .

@Component
@Scope(SCOPE_PROTOTYPE)
public class ServiceA<S> {
  private final ServiceB<S,String> dependentServiceB;

  public ServiceA (@Qualifier("serviceBWithInt")ServiceB<S,String> dependentServiceB){
     this.dependentServiceB = dependentServiceB;
  }
}

If you can't be explicit about which bean you want, you may want to reconsider adding generics here. There are niche reasons to want to do this in the first place, and the flow of dependency injection is still wholly predictable, even if you don't control the lifecycle of the objects.