@RunWith(MockitoJUnitRunner.class)
public class WordCountMapperTest {

    @Mock
    private Mapper.Context mockContext; // declare your mocks

    org.apache.hadoop.examples.WordCount.TokenizerMapper mapper;

    @Before
    public void setUp() throws Exception {
       /*
        * mock and define your components here
        */
        mapper = new WordCount.TokenizerMapper();

        doNothing().when(mockContext.getCounter(TaskCounter.MAP_OUTPUT_RECORDS)).increment(anyLong());
    }

    @Test
    public void testMap() throws IOException, InterruptedException {
       /*
        * whatever you want to test you can write here in the verify statement
        */
        String line1 = "key_value";

        mapper.map(null, new Text(line1), mockContext);
        verify(mockContext.getCounter(TaskCounter.MAP_OUTPUT_RECORDS), times(1)).increment(1);
    }
}