How to use isVarArgs method of org.mockito.internal.creation.DelegatingMethod class

Not sure why need all that vintage and platform stuff.. Your tests are org.junit.jupiter.api.Test;

I would do this:

1) Remove all the annotations from the class level

2) Init Mockito:

@BeforeEach
public void setup() {
    MockitoAnnotations.initMocks(this);
}

3) These deps should be sufficient:

junit-jupiter 5.5.0
mockito-core 2.28.2

Yes, you are on the right track, putting a mock @Bean in a @Configuration class is one approach, and I'll describe my experience:

The trick is that you need to use a different set of .xml files purely for testing which exclude the live versions of those beans.

@ContextConfiguration(locations = {"context1-test.xml", "context2-test.xml", ...})

And the "-test-xml" files go in src/test/resources.

At least that was my experience in doing the same thing. Maybe there is some way to "override" the beans with the mock versions, but as yet I am not aware of it.

I also chose to put the mocks (I had 5 of them) all together in an own configuration:

@Configuration
public class MockServicesProvider {
     @Bean
     public AnotherBean myMock() { return mock(AnotherBean.class); }
}

Another interesting part of this problem is the common usage of initMocks(this); in the @Before method of your test class.

If the mocks are being used in other places (and they are, that's why you are wiring them up...) then initMocks(this) will blow them away between tests (not literally - just that new mocks will be created and any other mocks wired up in other objects will be "lost").

The solution to this was to call mockito's reset(mockObject) in the @Before method before each test. The same mocks are reset (all the when's and interactions), without creating new mocks.

Note that the Mockito docs for reset say very sternly that this method should not commonly be used, except in the context of mocks being applied via dependency injection, as we are indeed doing in this case :)

Have fun!

Try Cntrl + Shift + T on your eclipse screen. Check what are your results. Ideally eclipse should resolve this class only from one dependency. If you are getting multiple results, you may need to add exclusion on certain dependencies.

Also check if you are able to execute your test cases from command prompt. If you are able to do it, then it's not a maven problem. But if something is failing here, check your dependencies.

I think your results are compatible with the result that would happen if dbCollection is not a Mockito-mock (or your method is static or final). That would mean that a matcher is being used where none can be used; hence the "0 matchers expected, 1 recorded".

TL-DR

• write plain unit tests for components that you can straightly test without loading a Spring container (run them in local and in CI build).

• write partial integration tests/slicing unit test for components that you cannot straightly test without loading a Spring container such as components related to JPA, controllers, REST clients, JDBC ... (run them in local and in CI build)

• write some full integration tests (end-to-end tests) for some high-level components where it brings values (run them in CI build).

---

3 main ways to test a component

• plain unit test (doesn't load a Spring container)
• full integration test (load a Spring container with all configuration and beans)
• partial integration test/ test slicing (load a Spring container with very restricted configurations and beans)

Can all components be tested in these 3 ways ?

In a general way with Spring any component can be tested in integration tests and only some kinds of components are suitable to be tested unitary(without container).
But note that with or without spring, unitary and integration tests are not opposed but complementary.

How to determine if a component can be plain tested (without spring) or only tested with Spring?

You recognize a code to test that doesn't have any dependencies from a Spring container as the component/method doesn't use Spring feature to perform its logical.
Take that FooService class :

@Service
public class FooService{

   private FooRepository fooRepository;
   
   public FooService(FooRepository fooRepository){
       this.fooRepository = fooRepository;
   }

   public long compute(...){
      List<Foo> foos = fooRepository.findAll(...);
       // core logic
      long result = 
           foos.stream()
               .map(Foo::getValue)
               .filter(v->...)
               .count();
       return result;
   }
}

FooService performs some computations and logic that don't need Spring to be executed.
Indeed with or without container the compute() method contains the core logic we want to assert.
Reversely you will have difficulties to test FooRepository without Spring as Spring Boot configures for you the datasource, the JPA context, and instrument your FooRepository interface to provide to it a default implementation and multiple other things.
Same thing for testing a controller (rest or MVC).
How could a controller be bound to an endpoint without Spring? How could the controller parse the HTTP request and generate an HTTP response without Spring? It simply cannot be done.

1)Writing a plain unit test

Using Spring Boot in your application doesn't mean that you need to load the Spring container for any test class you run.
As you write a test that doesn't need any dependencies from the Spring container, you don't have to use/load Spring in the test class.
Instead of using Spring you will instantiate yourself the class to test and if needed use a mock library to isolate the instance under test from its dependencies.
That is the way to follow because it is fast and favors the isolation of the tested component.
Here how to unit-test the FooService class presented above.
You just need to mock FooRepository to be able to test the logic of FooService.
With JUnit 5 and Mockito the test class could look like :

import org.mockito.junit.jupiter.MockitoExtension;
import org.mockito.Mock;
import org.mockito.Mockito;
import org.junit.jupiter.api.extension.ExtendWith;
import org.junit.jupiter.api.Assertions;
import org.junit.jupiter.api.BeforeEach;


@ExtendWith(MockitoExtension.class)
class FooServiceTest{

    FooService fooService;  

    @Mock
    FooRepository fooRepository;

    @BeforeEach 
    void init{
        fooService = new FooService(fooRepository);
    }

    @Test
    void compute(){
        List<Foo> fooData = ...;
        Mockito.when(fooRepository.findAll(...))
               .thenReturn(fooData);
        long actualResult = fooService.compute(...);
        long expectedResult = ...;
        Assertions.assertEquals(expectedResult, actualResult);
    }

}

2)Writing a full integration test

Writing an end-to-end test requires to load a container with the whole configuration and beans of the application.
To achieve that @SpringBootTest is the way :

The annotation works by creating the ApplicationContext used in your tests through SpringApplication

You can use it in this way to test it without any mock :

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import org.junit.jupiter.api.Test;

@SpringBootTest
public class FooTest {

   @Autowired
   Foo foo;

   @Test
   public void doThat(){
      FooBar fooBar = foo.doThat(...);
      // assertion...
   }    
   
}

But you can also mock some beans of the container if it makes sense :

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.mock.mockito.MockBean;
import org.springframework.boot.test.context.SpringBootTest;
import org.junit.jupiter.api.Test;
import org.mockito.Mockito;

@SpringBootTest
public class FooTest {

   @Autowired
   Foo foo;

   @MockBean
   private Bar barDep;

   @Test
   public void doThat(){
      Mockito.when(barDep.doThis()).thenReturn(...);
      FooBar fooBar = foo.doThat(...);
      // assertion...
   }    
   
}

Note the difference for mocking as you want to mock a plain instance of a Bar class (org.mockito.Mock annotation)and that you want to mock a Bar bean of the Spring context (org.springframework.boot.test.mock.mockito.MockBean annotation).

Full integration tests have to be executed by the CI builds

Loading a full spring context takes time. So you should be cautious with @SpringBootTest as this may make unit tests execution to be very long and generally you don't want to strongly slow down the local build on the developer's machine and the test feedback that matters to make the test writing pleasant and efficient for developers.
That's why "slow" tests are generally not executed on the developer's machines.
So you should make them integration tests (IT suffix instead of Test suffix in the naming of the test class) and make sure that these are executed only in the continuous integration builds.
But as Spring Boot acts on many things in your application (rest controllers, MVC controllers, JSON serialization/deserialization, persistence, and so for...) you could write many unit tests that are only executed on the CI builds and that is not fine either.
Having end-to-end tests executed only on the CI builds is ok but having also persistence, controllers or JSON tests executed only on the CI builds is not ok at all.
Indeed, the developer build will be fast but as drawback the tests execution in local will detect only a small part of the possible regressions...
To prevent this caveat, Spring Boot provides an intermediary way : partial integration test or the slice testing (as they call it) : the next point.

3)Writing a partial integration test focusing on a specific layer or concern thanks to slice testing

As explained in the point "Recognizing a test that can be plain tested (without spring))", some components can be tested only with a running container.
But why using @SpringBootTest that loads all beans and configurations of your application while you would need to load only a few specific configuration classes and beans to test these components?
For example why loading a full Spring JPA context (beans, configurations, in memory database, and so forth) to test the controller part?
And reversely why loading all configurations and beans associated to Spring controllers to test the JPA repository part?
Spring Boot addresses this point with the slice testing feature.
These are not as much as fast than plain unit tests (that is without container) but these are really much faster than loading a whole spring context. So executing them on the local machine is generally very acceptable.
Each slice testing flavor loads a very restricted set of auto-configuration classes that you can modify if needed according to your requirements.

Some common slice testing features :

• Auto-configured JSON Tests : @JsonTest

To test that object JSON serialization and deserialization is working as expected, you can use the @JsonTest annotation.

• Auto-configured Spring MVC Tests : @WebMvcTest

To test whether Spring MVC controllers are working as expected, use the @WebMvcTest annotation.

• Auto-configured Spring WebFlux Tests : @WebFluxTest

To test that Spring WebFlux controllers are working as expected, you can use the @WebFluxTest annotation.

• Auto-configured Data JPA Tests : @DataJpaTest

You can use the @DataJpaTest annotation to test JPA applications.

And you have still many other slice flavors that Spring Boot provides to you.
See the testing part of the documentation to get more details.
Note that if you need to define a specific set of beans to load that the built-in test slice annotations don't address, you can also create your own test slice annotation(https://spring.io/blog/2016/08/30/custom-test-slice-with-spring-boot-1-4).

4)Writing a partial integration test focusing on specific beans thanks to lazy bean initialization

Some days ago, I have encountered a case where I would test in partial integration a service bean that depends on several beans that themselves also depend on other beans. My problem was that two deep dependency beans have to be mocked for usual reasons (http requests and a query with large data in database).
Loading all the Spring Boot context looked an overhead, so I tried to load only specific beans. To achieve that, I annotation the test class with @SpringBootTest and I specified the classes attribute to define the configuration/beans classes to load.
After many tries I have gotten something that seemed working but I had to define an important list of beans/configurations to include.
That was really not neat nor maintainable.
So as clearer alternative, I chose to use the lazy bean initialization feature provided by Spring Boot 2.2 :

@SpringBootTest(properties="spring.main.lazy-initialization=true")
public class MyServiceTest { ...}

That has the advantage to load only beans used at runtime.
I don't think at all that using that property has to be the norm in test classes but in some specific test cases, that appears the right way.

You can try the following idea (taken from similar discussion on github):

.doAnswer { throw ServiceException("Bad Request") }

I will answer the question in two parts. First, yes it is code smell to mock or spy the class under test. That does not mean that it cannot be done correctly but that it is risk prone and should be avoided whenever possible.

WRT your specific example, I would see how the spy could be correctly used but that would be predicated on the assertion that you have elsewhere fully unit tested getStringMetadata. This then begs the question, if you have fully unit tested getStringMetadata elsewhere then you must know how to test it and therefore why not test getNumLines without the spy.

All this being said, millhouse makes a good point but either way you have to unit test the expensive code somewhere. His suggestion goes a long way to help isolate the expensive code and ensure that you only have to test / exercise it once.

What you are asking is not possible because your legacy code relies on a static method getInstance() and Mockito does not allow to mock static methods, so the following line won't work

when(FormatterService.getInstance()).thenReturn(formatter);

There are 2 ways around this problem:

1. Use a different mocking tool, such as PowerMock, that allows to mock static methods.

2. Refactor your code, so that you don't rely on the static method. The least invasive way I can think of to achieve this is by adding a constructor to DriverSnapshotHandler that injects a FormatterService dependency. This constructor will be only used in tests and you production code will continue to use the real singleton instance.

   public static class DriverSnapshotHandler {
   
       private final FormatterService formatter;
   
       //used in production code
       public DriverSnapshotHandler() {
           this(FormatterService.getInstance());
       }
   
       //used for tests
       DriverSnapshotHandler(FormatterService formatter) {
           this.formatter = formatter;
       }
   
       public String getImageURL() {
           return formatter.formatTachoIcon();
       }
   }
   

Then, your test should look like this :

FormatterService formatter = mock(FormatterService.class);
when(formatter.formatTachoIcon()).thenReturn("MockedURL");
DriverSnapshotHandler handler = new DriverSnapshotHandler(formatter);
handler.getImageURL();
verify(formatter, atLeastOnce()).formatTachoIcon();

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).