Difference between @Mock and @InjectMocks

Question

What is the difference between @Mock and @InjectMocks in Mockito framework?

Answer 1

@Mock creates a mock. @InjectMocks creates an instance of the class and injects the mocks that are created with the @Mock (or @Spy) annotations into this instance.

Note you must use @RunWith(MockitoJUnitRunner.class) or Mockito.initMocks(this) to initialize these mocks and inject them (JUnit 4).

With JUnit 5, you must use @ExtendWith(MockitoExtension.class).

@RunWith(MockitoJUnitRunner.class) // JUnit 4 // @ExtendWith(MockitoExtension.class) for JUnit 5 public class SomeManagerTest {      @InjectMocks     private SomeManager someManager;      @Mock     private SomeDependency someDependency; // this will be injected into someManager        // tests...  } 

Answer 2

This is a 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>.