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When and where to use JUnit, Mockito and Integration testing in Spring Boot Application?

I'm very novice to Unit Testing, I have created a Spring Boot Application and now I want to do some testing what's confusing me is where to use what e.g. I have classes and interfaces Controllers, Service, Repository I know each will have it's own Test class, so what will I be using let's say in Controllers, JUnit or Mockito ? similar question for Service and Repository.

conclusion: JUnit and Mockito can be used at same time, JUnit is used to do common test, and Mockito is used to mock objects.
In my view, JUnit is used more often, and Mockito is only used when I want to mock object, such as the object which is not finished yet. If you just want to test your Controller, Service and Repository, I recommend you to use JUnit. But if you need to mock some objects, you can use Mockito.
For example, when you test ServiceA, which depends on ServiceB, and ServiceB is not finished yet, you can use Mockito to mock ServiceB just to satisfy your requirements to test ServiceA
Hope this answer helps you! you can go to mockito and JUnit5 to learn more about them.

Integrate Mockito with CDI

I'm currently thinking about expanding unit tests for a server and a client application. Mockito is already in use and - I think - very well suited for the task at hand. However, as the Mockito documentation itself admits:
Mockito is not an dependency injection framework, don't expect [the #InjectMocks annotation] to inject a complex graph of objects be it mocks/spies or real objects.
The server-side of course already has CDI via annotations at some places, the client will probably be extended in some place to use CDI for JavaSE in the near future. There is/will be a wild mix of field- and constructor-injection + #postconstruct methods, which is already too complicated for Mockito. So I'm looking for something that will allow me to easily use CDI annotations to inject Mockito's mocks/spies/real objects where needed.
Can Mockito's functionality be expanded via plugins or something similar to enable a dependency resolution closer to what is specified by CDI (I don't think I need the full spec, but something closer to it)? Is there another library that integrates with Mockito and JUnit5 that does that?

weld supports this out of the box, specifically by weld-junit. It supports both junit4 and junit5. In both cases one can define producer methods for the required injection points in which one can freely use mockito or powermock or any other mocking mechanism to create mocks which weld then injects into the test object.

If you wish to mock EJB/CDI beans with OpenEJB, you can do it very easily:
http://tomee.apache.org/master/examples/rest-applicationcomposer-mockito.html

cdi-unit and ioc-unit have modules which support that.
Just add
#Produces
#Mock
ClassName mockedObject;
and the framework will make it an injectable Bean.

How to test a component / bean in Spring Boot

To test a component/bean in a Spring Boot application, the testing part of the Spring Boot documentation provides much information and multiple ways :
#Test, #SpringBootTest, #WebMvcTest, #DataJpaTest and still many other ways.
Why provide so many ways ?
How decide the way to favor ?
Should I consider as integration tests my test classes annotated with Spring Boot test annotations such as #SpringBootTest, #WebMvcTest, #DataJpaTest ?
PS : I created this question because I noticed that many developers (even experienced) don't get the consequences to use an annotation rather than another.

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.

what is AbstractTestNGSpringContextTests

I have a TestNG class which is like the following:
public class WebAPITestCase extends AbstractTestNGSpringContextTests{.....}
I was trying to understand what this means extends AbstractTestNGSpringContextTests.
How does it work and what is the use of it?

Please read the javadoc:
AbstractTestNGSpringContextTests is an abstract base test class that
integrates the Spring TestContext Framework with explicit
ApplicationContext testing support in a TestNG environment. When you
extend AbstractTestNGSpringContextTests, you can access a protected
applicationContext instance variable that you can use to perform
explicit bean lookups or to test the state of the context as a whole.
Basically a spring application context will be setup for the test class.
If that still doesn't make sense I'd recommend you read this.

First, TestNG (stands for Test Next Generation) is a testing framework inspired from JUnit and NUnit but introducing some new functionalities that make it more powerful and easier to use like test that your code is multithread safe, powerful execution model, etc.
The class AbstractTestNGSpringContextTests includes the spring ApplicationContext. To make it available when executing TestNG test, AbstractTestNGSpringContextTests has methods annotated with TestNG annotations like #BeforeClass and #BeforeMethod.
So to have this functionality of running TestNG with Spring components, all it left to do is to extend AbstractTestNGSpringContextTests.
BTW, AbstractTransactionalTestNGSpringContextTests extends AbstractTestNGSpringContextTests. It not only provides transactional support but also has some convenience functionality for JDBC access.

advice on integration testing my dao layer

I have a spring mvc application, and I am using jdbctemplate for my database Dao objects.
How should I go about integration testing?
Where should I put my integration testing files, is this layout correct:
/src/main/test/integration/...
or
/src/main/integration/...
How will this work, for my test cases, I should have a base class or some code in my setup that will run once before starting my testing where it loads the spring configuration/application context.
How should I do this?

If You are using Maven, tests should go to src/test/java. If You're not, it seems like a reasonable place anyway.
To set up spring context You should use #RunWith(SpringJUnit4ClassRunner.class) together with #ContextConfiguration, no initialization code necessary.