Mocking member variables of a class using Mockito

Question

I am a newbie to development and to unit tests in particular    I guess my requirement is pretty simple  but  I am keen to know others thoughts on this    Suppose I have two classes like so -   public class First        Second second        public First            second   new Second               public String doSecond            return second doSecond             class Second        public String doSecond            return  Do Something             Let s say I am writing unit test to test First doSecond   method  However  suppose  i want to Mock Second doSecond   class like so   I am using Mockito to do this    public void testFirst        Second sec   mock Second class       when sec doSecond    thenReturn  Stubbed Second         First first   new First        assertEquals  Stubbed Second   first doSecond         I am seeing that the mocking does not take effect and the assertion fails   Is there no way to mock the member variables of a class that I want to test

User · Answer

Lots of others have already advised you to rethink your code to make it more testable - good advice and usually simpler than what I'm about to suggest.

If you can't change the code to make it more testable, PowerMock: https://code.google.com/p/powermock/

PowerMock extends Mockito (so you don't have to learn a new mock framework), providing additional functionality. This includes the ability to have a constructor return a mock. Powerful, but a little complicated - so use it judiciously.

You use a different Mock runner. And you need to prepare the class that is going to invoke the constructor. (Note that this is a common gotcha - prepare the class that calls the constructor, not the constructed class)

@RunWith(PowerMockRunner.class)
@PrepareForTest({First.class})

Then in your test set-up, you can use the whenNew method to have the constructor return a mock

whenNew(Second.class).withAnyArguments().thenReturn(mock(Second.class));

User · Answer

I had the same issue where a private value was not set because Mockito does not call super constructors  Here is how I augment mocking with reflection   First  I created a TestUtils class that contains many helpful utils including these reflection methods  Reflection access is a bit wonky to implement each time  I created these methods to test code on projects that  for one reason or another  had no mocking package and I was not invited to include it   public class TestUtils          get a static class value     public static Object reflectValue Class lt   gt  classToReflect  String fieldNameValueToFetch            try               Field reflectField    reflectField classToReflect  fieldNameValueToFetch               reflectField setAccessible true               Object reflectValue   reflectField get classToReflect               return reflectValue            catch  Exception e                fail  Failed to reflect   fieldNameValueToFetch                     return null               get an instance value     public static Object reflectValue Object objToReflect  String fieldNameValueToFetch            try               Field reflectField    reflectField objToReflect getClass    fieldNameValueToFetch               Object reflectValue   reflectField get objToReflect               return reflectValue            catch  Exception e                fail  Failed to reflect   fieldNameValueToFetch                     return null               find a field in the class tree     public static Field reflectField Class lt   gt  classToReflect  String fieldNameValueToFetch            try               Field reflectField   null              Class lt   gt  classForReflect   classToReflect              do                   try                       reflectField   classForReflect getDeclaredField fieldNameValueToFetch                     catch  NoSuchFieldException e                        classForReflect   classForReflect getSuperclass                                    while  reflectField  null    classForReflect  null               reflectField setAccessible true               return reflectField            catch  Exception e                fail  Failed to reflect   fieldNameValueToFetch    from    classToReflect                     return null               set a value with no setter     public static void refectSetValue Object objToReflect  String fieldNameToSet  Object valueToSet            try               Field reflectField    reflectField objToReflect getClass    fieldNameToSet               reflectField set objToReflect  valueToSet             catch  Exception e                fail  Failed to reflectively set    fieldNameToSet       valueToSet                        Then I can test the class with a private variable like this  This is useful for mocking deep in class trees that you have no control as well    Test public void testWithRectiveMock   throws Exception          mock the base class using Mockito     ClassToMock mock   Mockito mock ClassToMock class       TestUtils refectSetValue mock   privateVariable    newValue           and this does not prevent normal mocking     Mockito when mock somthingElse    thenReturn  anotherThing               then do your asserts     I modified my code from my actual project here  in page  There could be a compile issue or two  I think you get the general idea  Feel free to grab the code and use it if you find it useful

User · Answer

If you can t change the member variable  then the other way around this is to use powerMockit and call   Second second   mock Second class  when second doSecond    thenReturn  Stubbed Second    whenNew Second class  withAnyArguments thenReturn second     Now the problem is that ANY call to new Second will return the same mocked instance  But in your simple case this will work

User · Answer

Yes  this can be done  as the following test shows  written with the JMockit mocking API  which I develop     Test public void testFirst  Mocked final Second sec        new NonStrictExpectations      sec doSecond    result    Stubbed Second            First first   new First        assertEquals  Stubbed Second   first doSecond         With Mockito  however  such a test cannot be written  This is due to the way mocking is implemented in Mockito  where a subclass of the class to be mocked is created  only instances of this  mock  subclass can have mocked behavior  so you need to have the tested code use them instead of any other instance

User · Answer

This is not possible if you can t change your code  But I like dependency injection and Mockito supports it   public class First            Resource     Second second       public First             second   new Second               public String doSecond             return second doSecond              Your test    RunWith MockitoJUnitRunner class  public class YourTest       Mock    Second second       InjectMocks    First first   new First        public void testFirst          when second doSecond    thenReturn  Stubbed Second          assertEquals  Stubbed Second   first doSecond              This is very nice and easy

User · Answer

You can mock member variables of a Mockito Mock with ReflectionTestUtils ReflectionTestUtils setField yourMock   quot memberFieldName quot   value

User · Answer

If you look closely at your code you ll see that the second property in your test is still an instance of Second  not a mock  you don t pass the mock to first in your code   The simplest way would be to create a setter for second in First class and pass it the  mock explicitly  Like this  public class First        Second second        public First            second   new Second               public String doSecond            return second doSecond               public void setSecond Second second            this second   second             class Second        public String doSecond            return  quot Do Something quot                  public void testFirst    Second sec   mock Second class   when sec doSecond    thenReturn  quot Stubbed Second quot      First first   new First    first setSecond sec  assertEquals  quot Stubbed Second quot   first doSecond        Another would be to pass a Second instance as First s constructor parameter  If you can t modify the code  I think the only option would be to use reflection  public void testFirst        Second sec   mock Second class       when sec doSecond    thenReturn  quot Stubbed Second quot          First first   new First        Field privateField   PrivateObject class          getDeclaredField  quot second quot         privateField setAccessible true        privateField set first  sec        assertEquals  quot Stubbed Second quot   first doSecond        But you probably can  as it s rare to do tests on code you don t control  although one can imagine a scenario where you have to test an external library cause it s author didn t

User · Answer

If you want an alternative to ReflectionTestUtils from Spring in mockito  use  Whitebox setInternalState first   second   sec

User · Answer

You need to provide a way of accessing the member variables so you can pass in a mock  the most common ways would be a setter method or a constructor which takes a parameter     If your code doesn t provide a way of doing this  it s incorrectly factored for TDD  Test Driven Development

[java] Mocking member variables of a class using Mockito

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