Where should @Service annotation be kept? Interface or Implementation?

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I never put @Component (or @Service, …) at an interface, because this make the interface useless. Let me explain why.

claim 1: If you have an interface then you want to use that interface for the injection point type.

claim 2: The purpose of an interface is that it define a contract that can been implemented by several implementations. On the other side you have your injection point (@Autowired). Having just one interface and only one class that implement it, is (IMHO) useless, and violates YAGNI.

fact: When you put:

  • @Component (or @Service, …) at an interface,
  • have multiple classes that implements it,
  • at least two classes become Spring Beans, and
  • have an injection point that use the interface for type based injection,

then you will get and NoUniqueBeanDefinitionException
(or you have a very special configurations setup, with Environment, Profiles or Qualifiers …)

Conclusion: If you use @Component (or @Service, …) at an interface then you must violate at least one of the two clains. Therefore I think it is not useful (except some rare scenarios) to put @Component at interface level.

Spring-Data-JPA Repository interfaces are something complete different

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