Unit testing without dependency injection

Question

Code from Spring in Action :

public class DamselRescuingKnight implements Knight {
    private RescueDamselQuest quest;
    public DamselRescuingKnight() {
        this.quest = new RescueDamselQuest();
    }
    public void embarkOnQuest() {
        quest.embark();
    }
}

public class BraveKnight implements Knight {
    private Quest quest;
    public BraveKnight(Quest quest) {
        this.quest = quest;
    }
    public void embarkOnQuest() {
        quest.embark();
    }
}


public class BraveKnightTest {
    @Test
    public void knightShouldEmbarkOnQuest() {
        Quest mockQuest = mock(Quest.class);
        BraveKnight knight = new BraveKnight(mockQuest);
        knight.embarkOnQuest();
        verify(mockQuest, times(1)).embark();
    }
}

I understand the use of dependency injection, which allows us to switch implementation without modifying the depending code.

The book says "terribly difficult to write a unit test ...".

However, I am not able to understand how it will be very difficult for unit-testing without dependency injection! My intuition refuses to co-operate !

Can you start writing junit/unit testing for the class "DamselRescuingKnight" and for any other better example class (without DI), to make me realize the point/stage at which DI makes unit testing easier ?

user_2781431 · Answer

This perplexed me as well when I was reading this in Spring in Action. After reading above answers I wanted to add that when DI is not used then Junit method need to call method of object which is private (which is in accessible) and this object Quest is created in constructor of DamselRescuingKnight so test case for embarkQuest() can't be written. On contrary when using DI then you are externalizing object creation and Junit method can create that object so it will be accessible to it then can test emabarkQuest() which eventually need to test quest method

Unit testing without dependency injection

3 Answers