Trying to autowire an interface. Error: Consider defining a bean of type "interface"

Question

So I'm trying to autowire an interface.

MyApp:

@SpringBootApplication(scanBasePackages={"com.B","com.C"})
public class MyApp {
...
}

MyController (which is in package com.B):

@RestController
@RequestMapping("/users")
public class UserController
{
    @Autowired
    MyInterface myInterface;
}

MyInterface (which is in package com.C):

@Service
public interface MyInterface
{
...
}

But I'm getting this error:

Consider defining a bean of type 'C.MyInterface' in your configuration.

even though I have:

The package included here: @SpringBootApplication(scanBasePackages={"com.B","com.C"})
@Service in the interface

Is it even possible to autowire an interface?

You can perfectly well autowire an interface, but you will obviously need an implementation. How should Spring create an instance of `MyInterface`? — M. Deinum, Feb 10 '22 at 11:02
@M.Deinum Are you suggesting me to do this: https://stackoverflow.com/questions/12899372/spring-why-do-we-autowire-the-interface-and-not-the-implemented-class — Jenga, Feb 10 '22 at 11:47
I'm suggesting that you need an implementation, how you do that is up to you. You cannot create an instance of an interface. — M. Deinum, Feb 10 '22 at 13:02

Michael Gantman · Accepted Answer · 2022-02-12T20:17:18.107

Yes you can inject an interface. But there should be at least one implementation that is declared as bean, so Spring manages it and will know what to inject. If you have more then one implementation you will need to specify which one you want. Read about annotation @Resource and about and it's property "name". A good article to read about it would be Wiring in Spring: @Autowired, @Resource and @Inject. Also, If you have many implementations of the same interface and want to inject them all you can do this:

@Autowired
private List<DataSource> dataSources;

If DataSource is an interface the list will contain all of its implementations. You can read about it here and here

Trying to autowire an interface. Error: Consider defining a bean of type "interface"

1 Answers1