How to provide converter service to POJO class without passing it as method argument

I have ShoppingList service which is responsible for generating shopping list and a IngredientConverter service which is a helping tool for converting objects. My current implementation looks like this

@Service
@AllArgsConstructor
public class ShoppingListService {

    private final RecipeService recipeService;
    private final IngredientConverter ingredientConverter;

    public ShoppingList generateShoppingList(List<UUID> uuidsOfRecipes) {
        List<Recipe> recipes = recipeService.getAllByIDIn(uuidsOfRecipes);
        ShoppingList shoppingList = ShoppingList.empty();

        for (Recipe recipe : recipes) {
            shoppingList.addIngredients(recipe.getIngredients());
        }
        shoppingList.finishAddition(ingredientConverter);

        return shoppingList;
    }

}

@RequiredArgsConstructor
public class ShoppingList {

    @Getter
    private final List<IngredientQuantity> optimizedList;
    private final Map<Ingredient, Integer> ingredientAmountMap;

    public static ShoppingList empty() {
        return new ShoppingList(new ArrayList<>(), new HashMap<>());
    }

    public void addIngredients(List<IngredientQuantity> ingredients) { ... }

    public void addIngredient(IngredientQuantity ingredientQuantity) { ... }

    public void finishAddition(IngredientConverter ingredientConverter) {
        for (Ingredient ingredient : ingredientAmountMap.keySet()) {
            IngredientQuantity ingredientQuantity = ingredientConverter.convertWithAmount(
                    ingredient.getName(),
                    ingredientAmountMap.get(ingredient),
                    ingredient.getUnit());

            optimizedList.add(ingredientQuantity);
        }
    }
}

@Service 
public class IngredientConverter {

    public IngredientQuantity convertWithAmount(String name, int amount, Unit unit) { ... }
}

Is there a better strategy for providing IngredientConverter service to this class? Could I Autowire it somehow despite ShoppingList being POJO class? Should ShoppingList be marked as Component maybe? Not sure what is the best approach.

You cannot autowire service class into POJO. Autowire can be done only within spring managed classes. I can see that ShoppingList is not a spring managed class. Adding @Component will also not be ideal solution. AFAIK, The best solution to use here would be mapStruct. mapStruct can be used to map fields between entity and POJO. And in cases where any field has to be calculated separately, you can write your custom logic and autowire services. Below are steps

1. Add mapStruct library to pom.xml
2. Add below mapper class to your project. componentModel="spring" tells the system that this mapper is managed by spring.
3. All the fields that have same name will be automapped.
4. For fields which require conversions, you can write @BeforeMapping

Mapper(componentModel="spring")
public abstract class ShoppingListMapper 
{
	@Autowired
	IngredientConverter ingredientConverter; //autowire method you use.
	
	public abstract shoppingListToShoppingListDTO(ShoppingList shoppingList) throws Exception;
	public abstract List<ShoppingList> mapShoppingListsToDTOs(List<ShoppingList> shoppingLists) throws Exception;
	
	@BeforeMapping
    public void convertLogic(ShoppingList la, @MappingTarget ShoppingListDTO slDto) throws Exception 
	{
		//your logic to set required shoppinglist field using converter
    }
	
}

If this example is not clear, you can refer to web for various mapstruct examples. Let me know if you need further help