How to use ExceptionFactoryTest.class.getClassLoader method of org.mockito.internal.junit.ExceptionFactoryTest class

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.

There are different ways to use Mockito - I'll go through them one by one.

Manually

Creating mocks manually with Mockito::mock works regardless of the JUnit version (or test framework for that matter).

Annotation Based

Using the @Mock-annotation and the corresponding call to MockitoAnnotations::initMocks to create mocks works regardless of the JUnit version (or test framework for that matter but Java 9 could interfere here, depending on whether the test code ends up in a module or not).

Mockito Extension

JUnit 5 has a powerful extension model and Mockito recently published one under the group / artifact ID org.mockito : mockito-junit-jupiter.

You can apply the extension by adding @ExtendWith(MockitoExtension.class) to the test class and annotating mocked fields with @Mock. From MockitoExtension's JavaDoc:

@ExtendWith(MockitoExtension.class)
public class ExampleTest {

    @Mock
    private List list;

    @Test
    public void shouldDoSomething() {
        list.add(100);
    }

}

The MockitoExtension documentation describes other ways to instantiate mocks, for example with constructor injection (if you rpefer final fields in test classes).

No Rules, No Runners

JUnit 4 rules and runners don't work in JUnit 5, so the MockitoRule and the Mockito runner can not be used.

O.K, I got my mistake!!! I've used the @InjectMocks but initialized the same variable in the init() method... So what happened was that mockito injected the mock objects to my variable - but seconds later I ran it over - initializing that very same variable!!!

You haven't mocked the behavior of getId in externalDependencyObject therefore it is returning null and giving you the NPE when toString() is called on that null.

You need a when(externalDependencyObject.getId()).then...

Weird that your solution seems to work.
According to their documentation on Github it says.

Mocking of final classes and methods is an incubating, opt-in feature. It uses a combination of Java agent instrumentation and subclassing in order to enable mockability of these types. As this works differently to our current mechanism and this one has different limitations and as we want to gather experience and user feedback, this feature had to be explicitly activated to be available ; it can be done via the mockito extension mechanism by creating the file src/test/resources/mockito-extensions/org.mockito.plugins.MockMaker containing a single line:

mock-maker-inline

After you created this file, Mockito will automatically use this new engine and one can do :

 final class FinalClass {
   final String finalMethod() { return "something"; }
 }

 FinalClass concrete = new FinalClass(); 

 FinalClass mock = mock(FinalClass.class);
 given(mock.finalMethod()).willReturn("not anymore");

 assertThat(mock.finalMethod()).isNotEqualTo(concrete.finalMethod());

In subsequent milestones, the team will bring a programmatic way of using this feature. We will identify and provide support for all unmockable scenarios. Stay tuned and please let us know what you think of this feature!

My working structure now looks like this.

I have a method with definition like this:

class Template{
   public void process(Object rootMap, StringWriter out){
 .......   
  }
 }

I will show you how you can change/modify the "out"(StringWriter) reference.

private final Template mocktTemplate = mock(Template.class);
doAnswer(new Answer<StringWriter>() {

            public StringWriter answer(InvocationOnMock invocation)
                    throws Throwable {
                Object[] args = invocation.getArguments();
                if (args[1] instanceof StringWriter) {
                    StringWriter stringWriter = (StringWriter) args[1];
                    stringWriter.write("Email message");
                }
                return null;
            }
        }).when(this.mocktTemplate).process(anyObject(),any(StringWriter.class));

Now when you do make the actual call like:

msgBodyTemplate.process(model, msgBodyWriter);

the value of Stringbuffer ref in msgBodyWriter will be "Email message"; irrespective of it earlier value.

The exception is being thrown because you are using a Junit Jupiter @Test annotation but the Junit 4 @Runwith annotation. You should either use org.junit.Test from Junit 4 or annotate the class with @ExtendWith(SpringExtension.class) to use Junit Jupiter.

@WebMvcTest is probably what you want (it implies @AutoConfigureMockMvc, amongst other things).

Relevant docs

JUnit 5 has @TestInstance(PER_CLASS) annotation that can be used for this purpose. One of the features that it enables is non-static BeforeAll and AfterAll methods:

@TestInstance(PER_CLASS)
class BeforeAllTests {

    lateinit var isInit = false

    @BeforeAll
    fun setup() {
        isInit = true
    }

   @TestFactory
   fun beforeAll() = listOf(
       should("initialize isInit in BeforeAll") {
           assertTrue(isInit)
       }
   )
}

fun should(name: String, test: () -> Unit) = DynamicTest.dynamicTest("should $name", test)

Try anyInt():

when(candidateService.findById(anyInt())).thenReturn(new Candidate());

For example I have anyLong() in my project:

when(dao.getAddress(anyLong())).thenReturn(Arrays.asList(dto));

EDIT: You must import:

import static org.mockito.Matchers.anyInt;