Consider the following ways of doing Dependency Injection in a pseudo-singleton, by using Lombok‘s @ApplicationScoped :

It will not work.

❌

@ApplicationScoped
public class MyService {

    private final Database db;

    @Inject
    public MyService (final Database db) {
        this.db= db;
    } 
...
}

^ That code will crash at startup because the DI engine demands a parameter-less constructor.

This one :

❌

@ApplicationScoped
@RequiredArgsConstructor(onConstructor = @__(@Inject))
public class MyService {
    private final Database db;
}

^ …will crash at startup for the same reasons (missing parameter-less constructor)

I cannot do this :

❌

@ApplicationScoped
public class MyService {

    @Inject
    private Database db;

}

^ …because my unit tests need to be able to pass the db’s mock explicitly.

So, I try to add the parameter-less constructor:

⚠️

@ApplicationScoped
@RequiredArgsConstructor(onConstructor = @__(@Inject))
@NoArgsConstructor(force = true)
public class MyService {
    private final Database db;
}

That works, but it makes the linter paranoid : It warns me that db could be null wherever I use it.

I’m aiming at zero warnings. What’s the correct way of achieving that?

I’ve seen some examples where people added a “getInstance” method that checks for null but my intuition is that there is a more straightforward/more compact solution that fits well with the annotations.