java - How can I mock private static method with PowerMockito?

Question

I'm trying to mock private static method anotherMethod(). See code below

public class Util {
    public static String method(){
        return anotherMethod();
    }

    private static String anotherMethod() {
        throw new RuntimeException(); // logic was replaced with exception.
    }
}

Here is me test code

@PrepareForTest(Util.class)
public class UtilTest extends PowerMockTestCase {

        @Test
        public void should_prevent_invoking_of_private_method_but_return_result_of_it() throws Exception {

            PowerMockito.mockStatic(Util.class);
            PowerMockito.when(Util.class, "anotherMethod").thenReturn("abc");

            String retrieved = Util.method();

            assertNotNull(retrieved);
            assertEquals(retrieved, "abc");
        }    
}

But every tile I run it I get this exception

java.lang.AssertionError: expected object to not be null

I suppose that I'm doing something wrong with mocking stuff. Any ideas how can I fix it?

Answer 1

To to this, you can use PowerMockito.spy(...) and PowerMockito.doReturn(...).

Moreover, you have to specify the PowerMock runner at your test class, and prepare the class for testing, as follows:

@PrepareForTest(Util.class)
@RunWith(PowerMockRunner.class)
public class UtilTest {

   @Test
   public void testMethod() throws Exception {
      PowerMockito.spy(Util.class);
      PowerMockito.doReturn("abc").when(Util.class, "anotherMethod");

      String retrieved = Util.method();

      Assert.assertNotNull(retrieved);
      Assert.assertEquals(retrieved, "abc");
   }
}

Hope it helps you.