Difference between @mock and @Injectmocks!

Mock & InjectMocks.txt

Sample code on how @Mock and @InjectMocks works.

Say we have Game and Player class.

class Game {

    private Player player;

    public Game(Player player) {
        this.player = player;
    }

    public String attack() {
        return "Player attack with: " + player.getWeapon();
    }

}

class Player {

    private String weapon;

    public Player(String weapon) {
        this.weapon = weapon;
    }

    String getWeapon() {
        return weapon;
    }
}
As you see, Game class need Player to perform an attack.

@RunWith(MockitoJUnitRunner.class)
class GameTest {

    @Mock
    Player player;

    @InjectMocks
    Game game;

    @Test
    public void attackWithSwordTest() throws Exception {
        Mockito.when(player.getWeapon()).thenReturn("Sword");

        assertEquals("Player attack with: Sword", game.attack());
    }

}
Mockito will mock a Player class and it's behaviour using when and thenReturn method. Lastly, using @InjectMocks Mockito will put that Player into Game.

Notice that you don't even have to create a new Game object. Mockito will inject it for you.

// you don't have to do this
Game game = new Game(player);
We will also get same behaviour using @Spy annotation. Even if the attribute name is different.

@RunWith(MockitoJUnitRunner.class)
public class GameTest {

  @Mock Player player;

  @Spy List<String> enemies = new ArrayList<>();

  @InjectMocks Game game;

  @Test public void attackWithSwordTest() throws Exception {
    Mockito.when(player.getWeapon()).thenReturn("Sword");

    enemies.add("Dragon");
    enemies.add("Orc");

    assertEquals(2, game.numberOfEnemies());

    assertEquals("Player attack with: Sword", game.attack());
  }
}

class Game {

  private Player player;

  private List<String> opponents;

  public Game(Player player, List<String> opponents) {
    this.player = player;
    this.opponents = opponents;
  }

  public int numberOfEnemies() {
    return opponents.size();
  }

  // ...
That's because Mockito will check the Type Signature of Game class, which is Player and List<String>.