Powermock, mockStatic of UUID.getRandom works in test class but doesn't in the class tested

Question

Strange case: using Powermock to mock out UUID.getRandom(). This works within the JUnit test class but when the class under test (a Filter, if that matters) calls UUID.getRandom, a unique UUID is produced.

Simple test case

@RunWith(PowerMockRunner.class)
@PrepareForTest({UUID.class})
public class MyTest {

    private MyFilter filter;

    @Before
    public void setup() {

        //The most convenient way to get a UUID, have also tried creating one manually
        UUID uuid = UUID.randomUUID();

        mockStatic(UUID.class);

        PowerMockito.when(UUID.randomUUID()).thenReturn(uuid);

        filter = new MyFilter();
    }

    @Test
    public void testMyUUID() {

        //This test works
        assertEquals(UUID.randomUUID(), UUID.randomUUID());

    }

    @Test
    public void testFilterUUID() {

        //This test fails
        assertEquals(UUID.randomUUID(), filter.getUUID());
    }
}

Simple class being tested

public class MyFilter implements Filter {

    public UUID getUUID() {

        return UUID.randomUUID();
    }
}

Pretty simple stuff, have done mocking like this before, just can't figure out why this case doesn't work.

Using Powermock version 1.5.

Answer 1

I believe UUID falls under the "system classes" category, so you need to prepare for test the class calling UUID.randomUUID()(related github issue)., thus changing to @PrepareForTest({MyFilter.class}) should fix things. The following works as expected with JUnit 4.4 & Powermock 1.5, as well as JUnit 4.12 and Powermock 1.7.3:

@RunWith(PowerMockRunner.class)
@PrepareForTest(MyFilter.class)
public class MyTest {

    private MyFilter filter;

    @Before
    public void setup() {
        //The most convenient way to get a UUID, have also tried creating one manually
        UUID uuid = UUID.randomUUID();
        mockStatic(UUID.class);
        PowerMockito.when(UUID.randomUUID()).thenReturn(uuid);
        filter = new MyFilter();
    }

    @Test
    public void testMyUUID() {
        //This test works
        assertEquals(UUID.randomUUID(), UUID.randomUUID());

    }

    @Test
    public void testFilterUUID() {
        //This test fails
        assertEquals(UUID.randomUUID(), filter.getUUID());
    }
}