MeterRegistry counter test case failing

Question

I have implemented micro-meter Prometheus counter in my service by injecting MeterRegistry and incrementing the count as shown below , and I have written a test case as well , but when I am running the test case , I am getting

"java.lang.NullPointerException: Cannot invoke "io.micrometer.core.instrument.MeterRegistry.counter(String, String[])" because "this.meterRegistry" is null".

Please can someone help

Service file:

    @Autowired
    private MeterRegistry meterRegistry;
    
    public void counterIncrement()
    { 
    meterRegistry.counter("test_count").increment();
    }

Test case file:

    @MockBean
    private MeterRegistry registry;
     
    @Test
    void testCounter()
   {
    //  invoking counterIncrement();
    }

Answer 1

Depending on which junit version you are using you need to add the annotation to your test class. Junit 5: @ExtendWith(MockitoExtension.class) or for Junit 4: @RunWith(MockitoJUnitRunner.class)

Answer 2

How do you create your class under test? Since the registry is never instantiated, something seems up with how you setup your test.

Check that you are using the @MockBean in the correct way. This will replace the bean in the application context and if you do not spin up a spring context i your test, it will not work. See this post for more info

A different approach would be to use @Mock and inject the registry in the constructor, example:

@ExtendWith(MockitoExtension.class)
public class MyServiceTest {

  @Mock
  private MeterRegistry registry;

  private MyService myService;

  @BeforeEach
  void setup() {
    myService = new MyService(registry);
  }

  @Test
  void testCounter() {
    var counter = mock(Counter.class);
    given(registry.counter(any(String.class))).willReturn(counter);
    myService.counterIncrement();
  }