How to use shouldFailOnWrongMethodCalledOnMockTwo method of org.mockitousage.verification.BasicVerificationInOrderTest class

To test the lambdas behavior, the underlayingApi has to be mocked where the lambda is invoked via the InvoactionOnMock object like this.

    `when`(underlayingApi.post(eq("some/rest/url"),
                               any())).thenAnswer {
        val argument = it.arguments[1]
        val completion = argument as ((rawResult: String?, exception: Exception?) -> Unit)
        completion.invoke("result", null)
    }

This leads to the invocation of the callback within the object under test. Now to check if the callback out of the object under test is working verify it like that.

    objUnderTest.loadData(id,
                          { json, exception ->
                              assert....
                          })

@Spy doc says:

A field annotated with @Spy can be initialized explicitly at declaration point. Alternatively, if you don't provide the instance Mockito will try to find zero argument constructor (even private) and create an instance for you.

@SpyBean doc says:

Annotation that can be used to apply Mockito spies to a Spring ApplicationContext.

All beans in the context of the same type will be wrapped with the spy. If no existing bean is defined a new one will be added.

So the main difference is @SpyBean is a Spring Boot specific annotation but @Spy is part of Mockito itself. @SpyBean and @Spy basically do the same, but @SpyBean can resolve the Spring specific dependencies, e.g. @Autowired, @Spy can only create object with empty constructor.

Take a look at the Mockito API docs. As the linked document mentions (Point # 12) you can use any of the doThrow(),doAnswer(),doNothing(),doReturn() family of methods from Mockito framework to mock void methods.

For example,

Mockito.doThrow(new Exception()).when(instance).methodName();

or if you want to combine it with follow-up behavior,

Mockito.doThrow(new Exception()).doNothing().when(instance).methodName();

Presuming that you are looking at mocking the setter setState(String s) in the class World below is the code uses doAnswer method to mock the setState.

World mockWorld = mock(World.class); 
doAnswer(new Answer<Void>() {
    public Void answer(InvocationOnMock invocation) {
      Object[] args = invocation.getArguments();
      System.out.println("called with arguments: " + Arrays.toString(args));
      return null;
    }
}).when(mockWorld).setState(anyString());

Solution 1 : add a caller function in the calling class

public class DummyWrapper {
val foo get() = DummyCompanion.Companion

public void callAStaticMethod(){
    foo.staticMechod();
}

public void callCompanionMethod(){
    foo.someCompanionMethod();
}
}

In the test class, we can use Mockito to provide a stub for the get() function and verified it is called.

@Test
fun testCase{
....
val mockCompanionObj: DummyCompanion.Companion = mock()
val wrapper = DummyWrapper()

whenever(wrapper.foo).thenReturn(mockCompanionObj)
wrapper.callCompanionMethod()
verify(mockCompanionObj).someCompanionMethod()
....
}

Solution 2: using Mockk Mocking companion object in Mockk is easy. There is no need to insert a test interfacing object in the source code.

 @Test
 fun testCompanionObject() {
    //Mock the companion object
    mockkObject(DummyCompanion.Companion)

    //define the stubbing bechavior of a companion object method
    every { DummyCompanion.Companion.companionMethod() } answers { stubMethod() }

    val testObject = DummyWrapper()

    //Call a method that calls the companion object method
    //You can verify stubMethod() is called
    testObject.callCompanionMethod()

    verify(exactly = 1) { DummyCompanion.someCompanionMethod() }
}

For details see Mockk

How about

when( method-call ).thenReturn( value1, value2, value3 );

You can put as many arguments as you like in the brackets of thenReturn, provided they're all the correct type. The first value will be returned the first time the method is called, then the second answer, and so on. The last value will be returned repeatedly once all the other values are used up.

For the future I would recommend Eran Harel's answer (refactoring moving new to factory that can be mocked). But if you don't want to change the original source code, use very handy and unique feature: spies. From the documentation:

You can create spies of real objects. When you use the spy then the real methods are called (unless a method was stubbed).

Real spies should be used carefully and occasionally, for example when dealing with legacy code.

In your case you should write:

TestedClass tc = spy(new TestedClass());
LoginContext lcMock = mock(LoginContext.class);
when(tc.login(anyString(), anyString())).thenReturn(lcMock);

Make sure you have the following dependency in your pom file

        <dependency>
            <groupId>org.mockito</groupId>
            <artifactId>mockito-core</artifactId>
            <version>3.5.15</version>
            <scope>test</scope>
        </dependency>

Not possible through mockito. From their wiki

Why Mockito doesn't mock private methods?

Firstly, we are not dogmatic about mocking private methods. We just don't care about private methods because from the standpoint of testing private methods don't exist. Here are a couple of reasons Mockito doesn't mock private methods:

It requires hacking of classloaders that is never bullet proof and it changes the api (you must use custom test runner, annotate the class, etc.).

It is very easy to work around - just change the visibility of method from private to package-protected (or protected).

It requires me to spend time implementing & maintaining it. And it does not make sense given point #2 and a fact that it is already implemented in different tool (powermock).

Finally... Mocking private methods is a hint that there is something wrong with OO understanding. In OO you want objects (or roles) to collaborate, not methods. Forget about pascal & procedural code. Think in objects.

I suggest to use Mockito Framework it is very easy to use and understand.

@Mock
private EntityManager entityManager; 

If you want to use any method that belongs to entityManager, you should call.

Mockito.when(METHOD_EXPECTED_TO_BE_CALLED).thenReturn(AnyObjectoftheReturnType);

When you run your test, any call previosly declared in the Mockito.when for the EntityManager will return the value put in the declaration..

Read full documentation here.

https://static.javadoc.io/org.mockito/mockito-core/2.12.0/org/mockito/Mockito.html#stubbing