CODEWITHSUNDEEP
javaspringlistgenericsdependency-injection
manuna
manuna

Reputation: 861

Injecting list of generics in Spring 4.1

The following issue was encountered while upgrading Spring 3.2 -> 4.1

There is a Metadata hierarchy, like: AMetadata extends Metadata, BMetadata extends Metadata etc.

There is a Processor hierarchy, like:

abstract Processor<M extends Metadata>,

AProcessor extends Processor<AMetadata>,

BProcessor extends Processor<BMetadata> etc

There is a service containing an injected List of processors, like this:

@Inject private List<Processor<Metadata>> processors;

While this worked perfectly in Spring 3.2, with Spring 4.1.0 (and 4.0 as well) it fails to inject list members. Going into debug, it was discovered that: Processor<Metadata>.isAssignableFrom(BProcessor) == false and this causes Processor beans not to be matched as eligible candidates for injection.

A possible hack-looking solution is to declare Processors as follows:

BProcessor<Metadata> extends Processor<BMetadata> - that works, but looks a bit weird. Another option is to use List<Processor<? extends Metadata>>, but this requires some code changes elsewhere to be compilable and causes a lot of type-safety-check warnings in classes which relied on generics.

So the question is, how to handle this case properly? Did anyone encountered something similar?

Upvotes: 0

Views: 285

Answers (2)

alexsmail
alexsmail

Reputation: 5813

Also see video here Spring Framework on Java 8 https://www.youtube.com/watch?v=-_aWK8T_YMI that explains this. Generic type information was ingored till 4.0 Spring release. Now, they are taking into account.

Upvotes: 0

luboskrnac
luboskrnac

Reputation: 24561

Autowiring based on generics was one of the new features in Spring4. AFAIK they were ignored in previous versions. More info here: https://spring.io/blog/2013/12/03/spring-framework-4-0-and-java-generics

So I can't think of any other solution than you already pointed out: List<Processor<? extends Metadata>>.

Upvotes: 1

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