Writing JUnit test cases for a Spring Boot Service Application with autowired components

it helps some times Ok. So there are several things that could / should be done.
Unit test the Controller directly. Mock all other classes. Attempt to use any framework provided testing utilities to test how the controller interacts with the framework. An example of this is Spring's MockMVC. For an example, check out the "correct" answer to this question: JUnit test for ExceptionHandler

will help you My JUnit test is failing with the following error: "java.lang.IllegalArgumentException: Could not find field [userRepository] of type [null] on target [com.clearsoft.demo.UserResource@756b2d90]" , Exception comes from this line:

ReflectionTestUtils.setField(userResource, "userRepository", userRepository);

help you fix your problem Mockito 1 runner (MockitoJUnitRunner class) is not designed to run JUnit 5 tests.
So annotating your class with :

import org.mockito.junit.MockitoJUnitRunner;

@RunWith(MockitoJUnitRunner.class) 
public class MyJUnit5Test {

 MockitoAnnotations.initMocks(this);

@Component
public class SomeDependency {

    public String returnThat() {
        return "that";
    }
}

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

import davidhxxx.example.angularsboot.SomeDependency;

@ExtendWith(MockitoExtension.class)    
public class SomeClassTest {

    @Mock
    SomeDependency someDependencyMock;

    private SomeClass someClass;

    @BeforeEach
    public void setup(){
      someClass = new SomeClass(someDependencyMock);
    }

    @Test 
    void myFirstTest() {
      Mockito.when(someDependencyMock.returnThat()).thenReturn("mock result");
      Assertions.assertEquals("mock result", someClass.inc());
    }

}

<dependencies>
    ...
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-test</artifactId>
        <scope>test</scope>
    </dependency>
   ...
</dependencies>

<dependencies>
    ...
    <dependency>
        <groupId>org.junit.jupiter</groupId>
        <artifactId>junit-jupiter-engine</artifactId>
        <scope>test</scope>
    </dependency>
    <dependency>
        <groupId>org.mockito</groupId>
        <artifactId>mockito-junit-jupiter</artifactId>
        <version>2.18.0</version> <!-- version to specify as not provided by Spring Boot dependencies -->
        <scope>test</scope>
    </dependency>
</dependencies>

    <plugins>
        ...
        <plugin>
            <artifactId>maven-surefire-plugin</artifactId>
            <version>2.19</version>
            <dependencies>
                <dependency>
                    <groupId>org.junit.platform</groupId>
                    <artifactId>junit-platform-surefire-provider</artifactId>
                    <version>1.0.0</version>
                </dependency>
            </dependencies>
        </plugin>
     ...
   <plugins>

it helps some times I have managed to make your code to work. I will post only the changed classes:
The interface:

public interface DisplayRepository extends CrudRepository<Display, Long> {

   Optional<Display> findByTitle(String name);
}

@RunWith(SpringRunner.class)
@AutoConfigureTestDatabase(replace= AutoConfigureTestDatabase.Replace.NONE)
@DataJpaTest
public class DisplayRepositoryTest {

@Autowired
private TestEntityManager testEntityManager;

@Autowired
private DisplayRepository productRespository;

@Before()
public void setUp(){

    Display m = new Display();
    // m.setId(2L); // The ID is autogenerated; can retrieve it from the persistAndFlush result
    m.setCategory("Group1");
    m.setTitle("Product2");

    testEntityManager.persistAndFlush(m);
}

@Test
public void whenFindByName_thenReturnProduct() {
    // when
    Display product = productRespository.findByTitle("Product2").orElseThrow(() -> new RuntimeException("Product not found"));

    // then
    assertThat(product.getTitle()).isEqualTo("Product2");
}

@Test
public void whenFindAll_thenReturnProductList() {
    // when
    List<Display> products = (List<Display>) productRespository.findAll();

    // then
    assertThat(products).hasSize(1);
}
}

@Before
public void setUp() {
    MockitoAnnotations.initMocks(this);
}

may help you . You should avoid using field injection (even though is possible) and use constructor injection. This will solve this problem as you will be able to pass the service from the constructor but it will also be useful in the future as you can locate usages and trace objects in your code way better than field injections which are "hidden"
So I recommend instead of trying to solve your problem here to refactor your class in constructor injection and pass the service from there either by directly creating the object in your test or by creating a configuration for your test that will generate the object and give the arguments it requires

@ContextConfiguration(classes = { GeneralTester.TestConfig.class })
@RunWith(SpringRunner.class)
public class GeneralTester {
  @TestConfiguration
  public static class TestConfig {
    @Bean
    public IProductService productService(final IProductRepository productRepository){
     return new ProductService(productRepository);
    }
    @Bean
    public IProductRepository productRepository(){
      return mock(IProductRepository.class);
    }
  }

  @Autowire
  public IProductService productService;

  @Autowire
  public IProductRepository productRepository;

  @Before
  public void setUp() {
    reset(productRepository);
  }

  @After
  public void tearDown() {
    verifyNoMoreInteractions(productRepository);
  }

  @Test
  public void doSmth() {
    //... your setup
    when(productRepository.save(any())).thenReturn("something");
    //... your call and assertions
    verify(productRepository).save(any());
  }
}