MockMvc Controller Testing and return NullPointerException

I am facing a problem with my MockMvc instatnce in the RestController test,
I created a test for PostMapping and GetMapping. In the setup a created a MockMvc of my controller but when I use it inside the test of my methods, I keep receiving a NullPointerException.
I am new in testing, Can anyone please help me with this, Thanks

This is my controller

    @RestController
    @RequestMapping("/api/courses")
    public class CourseController {
        @Autowired
        private CourseService courseService;
    
        @GetMapping
        public List<Course> GetAllCourses() {
            return courseService.AllCourses();
        }
    
        @GetMapping("/{id}")
        public ResponseEntity<Course> GetOneCourseByID(@PathVariable Long id) {
            Course course = courseService.findOneCourse(id);
            if(course == null){
                return new ResponseEntity<Course>(HttpStatus.NOT_FOUND);
            }
            return new
                    ResponseEntity<Course>(course, HttpStatus.OK);
        }
    
        @PostMapping
        public Course AddCourse(@RequestBody Course course){
          courseService.addCourse(course);
          return course;
        }
    
        @DeleteMapping("/{id}")
        public String deleteCourse(@PathVariable Long id) {
            return courseService.deleteCourse(id);
        }
    
        @PutMapping("/{id}")
        public Course updateCourse(@RequestBody Course course) {
            return courseService.updateCourse(course);
        }
    }

This is my service

    
    @Transactional
    @Service
    public class CourseService {
    
        @Autowired
        private CourseRepository courseRepository;
    
        public List<Course> AllCourses() {
            return courseRepository.findAll();
        }
    
        public Course findOneCourse(Long id)  {
            return courseRepository.findOneById(id);
        }
    
        public Course addCourse(Course course) {
            courseRepository.save(course);
            return course;
        }
    
        public Course updateCourse(Course course) {
            courseRepository.save(course);
            return course;
        }
    
        public String deleteCourse(Long id) {
            courseRepository.deleteById(id);
            return "Deleted";
    
        }
    
    }

I have created a Unit test with Mockito for my Controller:

    @RunWith(MockitoJUnitRunner.class)
    @SpringBootTest
    public class CourseControllerTest {

    private static Course course1;
    private static List<Course> courseList = new ArrayList<>();

    // inject the mock on the controller
    @InjectMocks
    private CourseController courseController;
    // define mock MVC

    private MockMvc mockMvc;

    // mock the respository
    @Mock
    private CourseRepository courseRepository;

    @Before
    public void setUp() throws Exception {
        mockMvc = MockMvcBuilders.standaloneSetup(courseController).build();
    }

    @BeforeEach
    public void setupMethods() {
        course1 = new Course();
        course1.setId(13L);
        course1.setName("Java Script");
        course1.setDescription("Web Developing with Java Script");

        Teacher teacher1 = new Teacher("Koen", "Groffieon", 26, "koen@capgemini.com");
        Section section1 = new Section("Programming");

        course1.getSection().add(section1);
        course1.setTeacher(teacher1);
        section1.getCourses().add(course1);
        teacher1.getCourses().add(course1);
        courseList.add(course1);
        courseRepository.save(course1);
    }

    @Test
    public void GetCourseTest() throws Exception {

        when(courseRepository.findAll()).thenReturn(courseList);
        mockMvc.perform(get("/api/courses"))
                .andDo(print())
                .andExpect(jsonPath("$", Matchers.hasSize(1)))
                .andExpect(jsonPath("$.[0].id", is(13)))
                .andExpect(jsonPath("$.[0].name", is("Java Script")))
                .andExpect(MockMvcResultMatchers.status().isOk());

    }

    @Test
    public void postCourseTest() throws Exception {

        // define a mapper for json data
        ObjectMapper mapper = new ObjectMapper();
        String json = mapper.writeValueAsString(course1);

        when(courseRepository.save(Mockito.any(Course.class))).thenReturn(course1);

        this.mockMvc.perform(MockMvcRequestBuilders.post("/api/courses")
                .contentType(MediaType.APPLICATION_JSON)
                .content(json))
                .andDo(print())
                .andExpect(jsonPath("$.id", Matchers.is((course1.getId().intValue()))))
                .andExpect(jsonPath("$.name", Matchers.is(course1.getName())))
                .andExpect(status().isOk()

                );
        // verify(courseRepository,times(1)).save(Mockito.any(Course.class));
    }
}

But I am facing a problem as following:

java.lang.NullPointerException
at com.mockitoexample.controllers.CourseControllerTest.postCourseTest(CourseControllerTest.java:110)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:566)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
at org.mockito.internal.runners.DefaultInternalRunner$1$1.evaluate(DefaultInternalRunner.java:46)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.mockito.internal.runners.DefaultInternalRunner$1.run(DefaultInternalRunner.java:77)
at org.mockito.internal.runners.DefaultInternalRunner.run(DefaultInternalRunner.java:83)
at org.mockito.internal.runners.StrictRunner.run(StrictRunner.java:39)
at org.mockito.junit.MockitoJUnitRunner.run(MockitoJUnitRunner.java:163)
at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
at com.intellij.rt.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:33)
at com.intellij.rt.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:230)
at com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:58)

You need to inject a CourseService instead of CourseRepository into your Controller, and then do some mocking tests on the service like:

@Mock
private CourseService courseService;

//inside your GetCourseTest()
when(courseService.AllCourses()).thenReturn(courseList);