How to use ConstructorInstantiator class of org.mockito.internal.creation.instance package

Lenient mocks are what you want, if you can't just use a real UserKeychain.

Mockito.lenient().when(userKeychain.getUserStatus()).thenReturn(UserState.OK);
Mockito.lenient().when(otherUserKeychain.getUserStatus()).thenReturn(UserState.SUSPENDED);

Mockito is designed to replace systems where you can't use the real system in your test, particularly in systems that predictably invoke services instead of getting properties from data objects (or other idempotent actions). Because your system doesn't call those methods in a deterministic order, and because the calls aren't expensive and don't have side effects, I would recommend just going with the "lenient" option.

Imagine this case instead, where you are testing deleting user 1001:

when(userRpc.deleteUser(1001)).thenReturn(RPC_SUCCESS);
when(userRpc.deleteUser(1002)).thenReturn(RPC_SUCCESS);  // unnecessary

The test might pass if you ever delete the wrong user: Over-stubbing has masked a problem. Compare with this:

when(userRpc.fetchExpensiveUserDetails(1001)).thenReturn(user1001);
when(userRpc.fetchExpensiveUserDetails(1002)).thenReturn(user1002);  // unnecessary

Depending on what you're testing, this might be dangerous, but might not be so bad. Simulating a slow mobile network or with expensive data, maybe it is entirely against spec for you to fetch too much. However, in other cases, it may be acceptable. Finally, compare this case:

when(calculationResult.getRealComponent()).thenReturn(-1d);
when(calculationResult.getComplexComponent()).thenReturn(5);
when(calculationResult.getShortString()).thenReturn("-1 + 5i");

calculationResult looks an awful lot like a data object, and it is probably not a critical part of your test which of your methods to call or whether you're calling all of them. This is a case where Mockito's strict stubbing hinders you rather than helping you, and might be a case where you want to make some of those stubbings lenient. You might also choose to make the entire mock lenient, which particularly makes sense if you were to create a test helper method like stubCalculationResult(-1, 5) that prepared an entire object for you like that.

The only option better than that is to use a real object. In my example, if CalculationResult is an existing well-defined object, it may be lower risk overall to use a real one than to mock the behavior you believe at test-writing time to be correct. Similarly for your case, if you have access to a UserKeychain constructor that populates UserStatus etc, then it may be safer to use that in a test.

Though this might appear at first glance to be a slippery slope into turning a unit test into an integration test, I'd like to clarify that I'm recommending this only for data objects, which have no dependencies, and which ideally are immutable objects that have no methods with side-effects. If you use dependency injection, these are the type of single-implementation data holders that you would call new on rather than getting from your graph. This is also a good reason to separate your data objects so they are immutable and easy to construct, and to shift your services to consume those objects rather than giving methods to the data objects (favoring loginService.login(user) rather than user.login(loginService)).

You should use @SpyBean from spring-boot-test.

This used to be broken (per Spring Boot issue #7033) but it's now been fixed and supported since Spring Boot 2.5.3.

If you still getting this error and can't update your Spring Boot version, you might want to try the work-around proposed by @Mateusz Zając in his answer to this question.

Mockito has some powerful tools for this.

List<MyClass> list = new ArrayList<>();
when(mapper.query(eq(MyClass.class), any(DynamoDBQueryExpression.class)))
    .thenReturn(mock(PaginatedQueryList.class, withSettings().defaultAnswer(new ForwardsInvocations(list))));

This will return a mocked PaginatedQueryList which forwards all method calls to list.

I think you do need to cast it, but it shouldn't be too bad:

Foo<Bar> mockFoo = (Foo<Bar>) mock(Foo.class);
when(mockFoo.getValue()).thenReturn(new Bar());

You can't do this with Mockito, but Apache Commons actually has a way to do this using one of its built in utilities. You can put this in a function in JUnit that is run after Mockito injects the rest of the mocks but before your test cases run, like this:

@InjectMocks
MyClass myClass;

@Before
public void before() throws Exception {
    FieldUtils.writeField(myClass, "fieldName", fieldValue, true);
}

You don't generally mock constructors. You can do with tools like PowerMock, but I'd generally suggest you don't.

Currently, your code isn't actually testable, if you want to control what happens when a new Bicycle is constructed. Is constructing a Bicycle actually a complex operation? Perhaps you want a BicycleFactory, which can be passed into your class as a dependency, for example - then you could mock BicycleFactory.createBicycle or whatever you call it.

Constructors are like static methods - when you use them, you're tightly bound to the specific code you're calling; there's no clean way to inject other behaviour without approaches like PowerMock's.

The best solution I have found is to reverse the order of the stubs:

Map<String, String> map = mock(Map.class);
when(map.get(anyString())).thenReturn("I don't know that string");
when(map.get("abcd")).thenReturn("defg");
when(map.get("defg")).thenReturn("ghij");

When the default is to throw an exception you can just use doThrow and doReturn

doThrow(new RuntimeException()).when(map).get(anyString());
doReturn("defg").when(map).get("abcd");
doReturn("ghij").when(map).get("defg");

https://static.javadoc.io/org.mockito/mockito-core/2.18.3/org/mockito/Mockito.html#12

Make sure you add @RunWith(PowerMockRunner.class) to the top of your class as well.

::edit:: two years later...

Don't ever use PowerMockito, you shouldn't need to.

If you do need to, you have most likely broken the SOLID principles and your design is wrong.

Fix your design instead.

Hamcrest matcher methods return Matcher<T> and Mockito matchers return T. So, for example: org.hamcrest.Matchers.any(Integer.class) returns an instance of org.hamcrest.Matcher<Integer>, and org.mockito.Matchers.any(Integer.class) returns an instance of Integer.

That means that you can only use Hamcrest matchers when a Matcher<?> object is expected in the signature - typically, in assertThat calls. When setting up expectations or verifications where you are calling methods of the mock object, you use the Mockito matchers.

For example (with fully qualified names for clarity):

@Test
public void testGetDelegatedBarByIndex() {
    Foo mockFoo = mock(Foo.class);
    // inject our mock
    objectUnderTest.setFoo(mockFoo);
    Bar mockBar = mock(Bar.class);
    when(mockFoo.getBarByIndex(org.mockito.Matchers.any(Integer.class))).
        thenReturn(mockBar);

    Bar actualBar = objectUnderTest.getDelegatedBarByIndex(1);
    
    assertThat(actualBar, org.hamcrest.Matchers.any(Bar.class));
    verify(mockFoo).getBarByIndex(org.mockito.Matchers.any(Integer.class));
}

If you want to use a Hamcrest matcher in a context that requires a Mockito matcher, you can use the org.mockito.Matchers.argThat (or org.mockito.hamcrest.MockitoHamcrest.argThat in Mockito 2). It converts a Hamcrest matcher into a Mockito matcher. So, say you wanted to match a double value with some precision (but not much). In that case, you could do:

when(mockFoo.getBarByDouble(argThat(is(closeTo(1.0, 0.001))))).
    thenReturn(mockBar);