Difference between Mock and InjectMocks

Question

What is the difference between  Mock and  InjectMocks in Mockito framework

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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

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Notice that that  InjectMocks are about to be deprecated      deprecate  InjectMocks and schedule for removal in Mockito 3 4   and you can follow  avp answer and link on       Why You Should Not Use InjectMocks Annotation to Autowire Fields

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InjectMocks annotation can be used to inject mock fields into a test object automatically   In below example  InjectMocks has used to inject the mock dataMap into the dataLibrary     Mock Map lt String  String gt  dataMap     InjectMocks DataLibrary dataLibrary   new DataLibrary           Test     public void whenUseInjectMocksAnnotation              Mockito when dataMap  get  aData    thenReturn  aMeaning             assertEquals  aMeaning   dataLibrary  getMeaning  aData

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Though the above answers have covered  I have just tried to add minute detail s which i see missing  The reason behind them The Why        Illustration   Sample java ---------------     public class Sample          DependencyOne dependencyOne          DependencyTwo dependencyTwo            public SampleResponse methodOfSample                dependencyOne methodOne                dependencyTwo methodTwo                                  return sampleResponse                      SampleTest java -----------------------  RunWith PowerMockRunner class   PrepareForTest  ClassA class   public class SampleTest        InjectMocks     Sample sample        Mock     DependencyOne dependencyOne        Mock     DependencyTwo dependencyTwo        Before     public void init             MockitoAnnotations initMocks this              public void sampleMethod1 Test              Arrange the dependencies         DependencyResponse dependencyOneResponse   Mock sampleResponse class           Mockito doReturn dependencyOneResponse  when dependencyOne  methodOne             DependencyResponse dependencyTwoResponse   Mock sampleResponse class           Mockito doReturn dependencyOneResponse  when dependencyTwo  methodTwo               call the method to be tested         SampleResponse sampleResponse   sample methodOfSample               Assert          lt assert the SampleResponse here gt            Reference

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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 lt String gt  enemies   new ArrayList lt  gt         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 lt String gt  opponents     public Game Player player  List lt String gt  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 lt String gt

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Many people have given a great explanation here about  Mock vs  InjectMocks  I like it  but I think our tests and application should be written in such a way that we shouldn t need to use  InjectMocks   Reference for further reading with examples  https   tedvinke wordpress com 2014 02 13 mockito-why-you-should-not-use-injectmocks-annotation-to-autowire-fields

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Mock annotation mocks the concerned object    InjectMocks annotation allows to inject into the underlying object the different  and relevant  mocks created by  Mock   Both are complementary

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Mock is used to declare mock the references of the dependent beans  while  InjectMocks is used to mock the bean for which test is being created   For example   public class A      public class B b      public void doSomething               test for class A   public class TestClassA       Mocks    public class B b       InjectMocks    public class A a       Test    public testDoSomething

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A  mocking framework   which Mockito is based on  is a framework that gives you the ability to create Mock objects   in old terms these objects could be called shunts  as they work as shunts for dependend functionality    In other words  a mock object is used to imitate the real object your code is dependend on  you create a proxy object with the mocking framework  By using mock objects in your tests you are essentially going from normal unit testing to integrational testing  Mockito is an open source testing framework for Java released under the MIT License  it is a  mocking framework   that lets you write beautiful tests with clean and simple API  There are many different mocking frameworks in the Java space  however there are essentially two main types of mock object frameworks  ones that are implemented via proxy and ones that are implemented via class remapping   Dependency injection frameworks like Spring allow you to inject your proxy objects without modifying any code  the mock object expects a certain method to be called and it will return an expected result   The  InjectMocks annotation tries to instantiate the testing object instance and injects fields annotated with  Mock or  Spy into private fields of the testing object   MockitoAnnotations initMocks this  call  resets testing object and re-initializes mocks  so remember to have this at your  Before    BeforeMethod annotation

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In your test class  the tested class should be annotated with  InjectMocks  This tells Mockito which class to inject mocks into    InjectMocks private SomeManager someManager    From then on  we can specify which specific methods or objects inside the class  in this case  SomeManager  will be substituted with mocks    Mock private SomeDependency someDependency    In this example  SomeDependency inside the SomeManager class will be mocked

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Mock creates a mock implementation for the classes you need   InjectMock  creates an instance of the class and injects the mocks that are marked with the annotations  Mock into it    For example   Mock StudentDao studentDao    InjectMocks StudentService service    Before public void setUp   throws Exception       MockitoAnnotations initMocks this       Here we need the DAO class for the service class  So  we mock it and inject it in the service class instance  Similarly  in Spring framework all the  Autowired beans can be mocked by  Mock in jUnits and injected into your bean through  InjectMocks   MockitoAnnotations initMocks this  method initialises these mocks and injects them for every test method so it needs to be called in the setUp   method   This link has a good tutorial for Mockito framework

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One advantage you get with the approach mentioned by  Tom is that you don t have to create any constructors in the SomeManager  and hence limiting the clients to instantiate it     RunWith MockitoJUnitRunner class  public class SomeManagerTest         InjectMocks     private SomeManager someManager        Mock     private SomeDependency someDependency     this will be injected into someManager        You don t need to instantiate the SomeManager with default contructor at all      SomeManager someManager   new SomeManager             Or SomeManager someManager   new SomeManager someDependency           tests         Whether its a good practice or not depends on your application design

[java] Difference between @Mock and @InjectMocks

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