Bean not found from another module

Question

I am struggling with a problem and I don't know how to solve it.

This is the structure of my project using Maven :

myProject:

• moduleA <-- has dependencies on moduleB and moduleC
• moduleB <-- has no dependency
• moduleC <-- has dependency on moduleB

I wrote two interfaces in moduleB, one of them is implemented in moduleB, the other one is implemented in moduleC.

Greeting interface :

public interface Greeting {
    WelcomeMessage greet();
}

Its implementation written on moduleB :

@Component
public class HelloWorld implements Greeting {

    private final Messages messages;

    public HelloWorld(Messages messages) {
        this.messages = messages;
    }

    @Override
    public WelcomeMessage greet() {
        return messages.getWelcomeMessage();
    }
}

Messages interface:

public interface Messages {
    WelcomeMessage getWelcomeMessage();
}

Its implementation written on moduleC :

@Component
public class MessagesImpl implements Messages {

    private final Mapper mapper;
    private final MessageRepository messageRepository;

    public MessagesImpl(Mapper mapper, MessageRepository messageRepository) {
        this.mapper = mapper;
        this.messageRepository = messageRepository;
    }

    @Override
    public WelcomeMessage getWelcomeMessage() {
        // do something with repository and mapper
    }
}

My problem is Spring does not seem to use the implementation of Messages from moduleC, this error shows when I start the application :

**required a bean of type 'fr.mypackage.core.spi.Messages' that could not be found**

Also IntelliJ keeps telling me MessagesImpl is never used.

Answer 1

Here what I do for my libraries:

• Every library has a configuration class at top package level with @ComponentScan
• It can be empty or have additional beans if necessary
• Then, either the library is made an autoconfiguration with a file META-INF/spring.factories in src/main/resouces with the content:

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
  com.example.my.library.MyLibraryConfiguration

• Or I manually import the configuration class in my main application configuration class with @Import(com.example.my.library.MyLibraryConfiguration)

For a module, I recommend the autoconfiguration approach. You can @ComponentScan from the main application the packages of your library, but I don't recommend that, it give you less control and can be messy.