Mockito - NullpointerException when stubbing Method

Question

So I started writing tests for our Java-Spring-project    What I use is JUnit and Mockito  It s said  that when I use the when     thenReturn   option I can mock services  without simulating them or so  So what I want to do is  to set   when classIwantToTest object get   methodWhichReturnsAList input  thenReturn ListcreatedInsideTheTestClass      But no matter which when-clause I do  I always get a NullpointerException  which of course makes sense  because input is null    Also when I try to mock another method from an object    when object method    thenReturn true    There I also get a Nullpointer  because the method needs a variable  which isn t set    But I want to use when    thenReturn   to get around creating this variable and so on  I just want to make sure  that if any class calls this method  then no matter what  just return true or the list above   Is it a basically misunderstanding from my side  or is there something else wrong   Code   public class classIWantToTest implements classIWantToTestFacade           Autowired         private SomeService myService            Override         public Optional lt OutputData gt  getInformations final InputData inputData                final Optional lt OutputData gt  data   myService getListWithData inputData               if  data isPresent                      final List lt ItemData gt  allData   data get   getItemDatas                          do something with the data and allData                 return data                             return Optional absent                     And here is my test class   public class Test        private InputData inputdata       private ClassUnderTest classUnderTest       final List lt ItemData gt  allData   new ArrayList lt ItemData gt           Mock     private DeliveryItemData item1        Mock     private DeliveryItemData item2          Mock     private SomeService myService         Before     public void setUp   throws Exception           classUnderTest   new ClassUnderTest            myService   mock myService class            classUnderTest setService myService           item1   mock DeliveryItemData class           item2   mock DeliveryItemData class                 Test     public void test sort             createData            when myService getListWithData inputdata  get   getItemDatas              when item1 hasSomething    thenReturn true           when item2 hasSomething    thenReturn false               public void createData             item1 setSomeValue  val            item2 setSomeOtherValue  test             item2 setSomeValue  val            item2 setSomeOtherValue  value             allData add item1           allData add item2

User · Answer

Another common gotcha is that the method signature is accidentally declared as 'final'.

This one catches out a lot of people who work on codebases which are subjected to Checkstyle and have internalised the need to mark members as final.

i.e. in the OP's example:

object.method()

Make sure that method() is not declared as final:

public final Object method() {
}

Mockito cannot mock a final method and this will come up as a wrapped NPE:

Suppressed: org.mockito.exceptions.misusing.InvalidUseOfMatchersException:

Buried deep in the error message is the following:

Also, this error might show up because you use argument matchers with methods that cannot be mocked.
Following methods *cannot* be stubbed/verified: final/private/equals()/hashCode().
Mocking methods declared on non-public parent classes is not supported.

User · Answer

In my case  I missed add first   PowerMockito spy ClassWhichNeedToBeStaticMocked class     so this can be helpful to somebody who see such error  java lang NullPointerException     at org powermock api mockito internal expectation PowerMockitoStubberImpl addAnswersForStubbing PowerMockitoStubberImpl java 67      at org powermock api mockito internal expectation PowerMockitoStubberImpl when PowerMockitoStubberImpl java 42      at org powermock api mockito internal expectation PowerMockitoStubberImpl when PowerMockitoStubberImpl java 112

User · Answer

you need to initialize MockitoAnnotations initMocks this  method has to called to initialize annotated fields       Before public void initMocks            MockitoAnnotations initMocks this          for more details see Doc

User · Answer

Ed Webb s answer helped in my case  And instead  you can also try add      Rule public Mocks mocks   new Mocks this     if you  RunWith JUnit4 class

User · Answer

For future readers  another cause for NPE when using mocks is forgetting to initialize the mocks like so    Mock SomeMock someMock    InjectMocks SomeService someService    Before public void setup        MockitoAnnotations initMocks this     without this you will get NPE     Test public void someTest        Mockito when someMock someMethod    thenReturn  some result                  Also make sure you are using JUnit for all annotations  I once accidently created a test with  Test from testNG so the  Before didn t work with it  in testNG the annotation is  BeforeTest

User · Answer

I had the same problem and my issue was simply that I had not annotated the class properly using  RunWith  In your example  make sure that you have    RunWith MockitoJUnitRunner class  public class Test         Once I did that  the NullPointerExceptions disappeared

User · Answer

I had this issue and my problem was that I was calling my method with any   instead of anyInt     So I had   doAnswer      with myMockObject  thisFuncTakesAnInt any      and I had to change it to   doAnswer      with myMockObject  thisFuncTakesAnInt anyInt      I have no idea why that produced a NullPointerException   Maybe this will help the next poor soul

User · Answer

RunWith MockitoJUnitRunner class     OR  PowerMockRunner class   PrepareForTest  UpdateUtil class Log class SharedPreferences class SharedPreferences Editor class   public class InstallationTest extends TestCase    Mock Context mockContext   Mock SharedPreferences mSharedPreferences   Mock SharedPreferences Editor mSharedPreferenceEdtor    Before public void setUp   throws Exception             mockContext   Mockito mock Context class             mSharedPreferences   Mockito mock SharedPreferences class             mSharedPreferenceEdtor   Mockito mock SharedPreferences Editor class       when mockContext getSharedPreferences Mockito anyString   Mockito anyInt     thenReturn mSharedPreferences       when mSharedPreferences edit    thenReturn mSharedPreferenceEdtor       when mSharedPreferenceEdtor remove Mockito anyString     thenReturn mSharedPreferenceEdtor       when mSharedPreferenceEdtor putString Mockito anyString   Mockito anyString     thenReturn mSharedPreferenceEdtor       Test public void deletePreferencesTest   throws Exception           All the above commented codes are not required    mockContext   Mockito mock Context class      if you use  Mock Annotation to Context mockContext     Mock  Context mockContext     But it will work if you use  RunWith MockitoJUnitRunner class  only  As per Mockito you can create mock object by either using  Mock or Mockito mock Context class      I got NullpointerException because of using  RunWith PowerMockRunner class   instead of that I changed to  RunWith MockitoJUnitRunner class  it works fine

User · Answer

When using JUnit 5 or above  You have to inject the class annotated with  Mock in an  BeforeEach setup

User · Answer

As this is the closest I found to the issue I had  it s the first result that comes up and I didn t find an appropriate answer  I ll post the solution here for any future poor souls  any   doesn t work where mocked class method uses a primitive parameter   public Boolean getResult String identifier  boolean switch   The above will produce the same exact issue as OP  Solution  just wrap it   public Boolean getResult String identifier  Boolean switch   The latter solves the NPE

User · Answer

Make sure you initialize your mocks    JUnit4 use  Before   Before public void setup         MockitoAnnotations initMocks this       JUnit5 use  BeforeEach   BeforeEach public void setup         MockitoAnnotations initMocks this       For JUnit5 check  you are using proper imports also    import org junit runner RunWith import org mockito junit MockitoJUnitRunner    RunWith MockitoJUnitRunner class

User · Answer

In my case it was due to wrong import of the  Test annotation Make sure you are using the following import import org junit jupiter api Test

User · Answer

Corner case  If you re using Scala and you try to create an any matcher on a value class  you ll get an unhelpful NPE   So given case class ValueClass value  Int  extends AnyVal  what you want to do is ValueClass anyInt  instead of any ValueClass    when mock someMethod ValueClass anyInt    thenAnswer             val v    ValueClass invocation getArguments   0  asInstanceOf Int              This other SO question is more specifically about that  but you d miss it when you don t know the issue is with value classes

User · Answer

The default return value of methods you haven t stubbed yet is false for boolean methods  an empty collection or map for methods returning collections or maps and null otherwise   This also applies to method calls within when       In you re example when myService getListWithData inputData  get    will cause a NullPointerException because myService getListWithData inputData  is null - it has not been stubbed before   One option is create mocks for all intermediate return values and stub them before use  For example   ListWithData listWithData   mock ListWithData class   when listWithData get    thenReturn item1   when myService getListWithData    thenReturn listWithData     Or alternatively  you can specify a different default answer when creating a mock  to make methods return a new mock instead of null  RETURNS DEEP STUBS  SomeService myService   mock SomeService class  Mockito RETURNS DEEP STUBS   when myService getListWithData   get    thenReturn item1     You should read the Javadoc of Mockito RETURNS DEEP STUBS which explains this in more detail and also has some warnings about its usage   I hope this helps  Just note that your example code seems to have more issues  such as missing assert or verify statements and calling setters on mocks  which does not have any effect

User · Answer

None of the above answers helped me  I was struggling to understand why code works in Java but not in Kotlin   Then I figured it out from this thread   You have to make class and member functions open  otherwise NPE was being thrown   After making function open tests started to pass   You might as well consider using compiler s  all-open  plugin      Kotlin has classes and their members final by default  which makes it inconvenient to use frameworks and libraries such as Spring AOP that require classes to be open  The all-open compiler plugin adapts Kotlin to the requirements of those frameworks and makes classes annotated with a specific annotation and their members open without the explicit open keyword

User · Answer

None of these answers worked for me  This answer doesn t solve OP s issue but since this post is the only one that shows up on googling this issue  I m sharing my answer here   I came across this issue while writing unit tests for Android  The issue was that the activity that I was testing extended AppCompatActivity instead of Activity  To fix this  I was able to just replace AppCompatActivity with Activity since I didn t really need it  This might not be a viable solution for everyone  but hopefully knowing the root cause will help someone

User · Answer

For me the reason I was getting NPE is that I was using Mockito any   when mocking primitives  I found that by switching to using the correct variant from mockito gets rid of the errors   For example  to mock a function that takes a primitive long as parameter  instead of using any    you should be more specific and replace that with any Long class  or Mockito anyLong     Hope that helps someone

User · Answer

faced the same issue  the solution that worked for me   Instead of mocking the service interface  I used  InjectMocks to mock the service implementation    InjectMocks private exampleServiceImpl exampleServiceMock    instead of     Mock private exampleService exampleServiceMock

[java] Mockito - NullpointerException when stubbing Method

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