Consider marking one of the beans as @Primary

Question

In my Java spring application, I have

public class BinarySearchImpl {

    @Autowired
    @Qualifier("Quick")
    SortAlgorithem sorter;

    Log log=LogFactory.getLog(BinarySearchImpl.class);

    public BinarySearchImpl(SortAlgorithem sorter) {
        log.info("Binary Search Bean is created");
        this.sorter=sorter;
    }

SortAlgorithem is an interface which makes my application loosely coupled:

public interface SortAlgorithem {

    public int[] sort(int[] arrayNumbers);

}

And then there are 2 implementations for this interface. One is BubbleSort:

@Component
@Qualifier("Bubble")
public class BubbleSort implements SortAlgorithem {

    Log log=LogFactory.getLog(BubbleSort.class);

    public int[] sort(int[] numbers) {
        log.info("Bubble sort is called");
        return numbers;
    }
}

and the other is QuickSort:

@Component
@Qualifier("Quick")
//@Primary
public class QuickSort implements SortAlgorithem{

    Log log= LogFactory.getLog(QuickSort.class);

    public int[] sort(int[] numbers) {
        log.info("Quick Sort is called");
        return numbers;
    }

}

At the end, when I call my app it complains with this message:

Consider marking one of the beans as @Primary, updating the consumer to accept multiple beans, or using @Qualifier to identify the bean that should be consumed

I am wondering... Why @Qualifier annotation does not work?

Answer 1

You have annotated a field with @Autowired and @Qualifier, but you have also created a constructor which sets the field.

I think that Spring is using the constructor, but doesn't automatically know that the constructor parameter corresponds to the annotated field.

So move the annotations into the constructor declaration:

private SortAlgorithm sorter;

@Autowired 
public BinarySearchImpl(@Qualifier("quick") SortAlgorithm sorter) {
     this.sorter = sorter;
}

Alternatively, you could use a zero-arg constructor, keep your field annotation and let Spring inject using reflection. However in my opinion constructor-injection is better -- it allows you to unit test cleanly, without involving Spring or reflection.

As other answers point out, there are other ways to disambiguate autowired beans -- and the Spring docs explain them all -- but using qualifiers like this does work.

Answer 2

The correct approach:

public interface SortAlgorithem {
    public int[] sort(int[] arrayNumbers);
}

@Component("Bubble")
public class BubbleSort implements SortAlgorithem {
    Log log = LogFactory.getLog(BubbleSort.class);

    public int[] sort(int[] numbers) {
        log.info("Bubble sort is called");
        return numbers;
   }
}

@Primary
@Component("Quick")
public class QuickSort implements SortAlgorithem {
    Log log = LogFactory.getLog(QuickSort.class);

    public int[] sort(int[] numbers) {
        log.info("Quick Sort is called");
        return numbers;
    }
}

and then you need to use your implementations like this:

@Autowired
@Qualifier(value = "Bubble")
private SortAlgorithem bubbleSort;

@Autowired
@Qualifier(value = "Quick")
private SortAlgorithem quickSort;