Mocking final methods or classes

Quick summary

1. Use the @RunWith(PowerMockRunner.class) annotation at the class-level of the test case.
2. Use the @PrepareForTest(ClassWithFinal.class) annotation at the class-level of the test case.
3. Use PowerMock.createMock(ClassWithFinal.class) to create a mock object for all methods of this class (let's call it mockObject).
4. Use PowerMock.replay(mockObject) to change the mock object to replay mode.
5. Use PowerMock.verify(mockObject) to change the mock object to verify mode.

Example

Let's assume you're using a third-party library that contains a class called StateHolder. For whatever reason this class is final and not only that, the methods are final as well. Imagine also that you need to use this class in your code and you'd still like to be able to unit test your own code in separation. StateHolder looks like this:

public final class StateHolder extends JdbcDaoSupport {

	public final String getState() {
		/* Query a database for state, 
                   this is not something we want to do in a unit test */
		getJdbcTemplate().queryForObject("some sql", String.class);
	}
}

In this example we imagine having a class called StateFormatter that interacts with the StateHolder instance. It looks like this:

public class StateFormatter {

	private final StateHolder stateHolder;

	public StateFormatter(StateHolder stateHolder) {
		this.stateHolder = stateHolder;
	}

	public String getFormattedState() {
		String safeState = "State information is missing";
		final String actualState = stateHolder.getState();
		if (actualState != null) {
			safeState = actualState;
		}
		return safeState;
	}
}

If we are to test the getFormattedState method in separation we need to create a mock of the StateHolder class and be able to expect the call to stateHolder.getState(). PowerMock's createMock method can be used to create a mock object of any class even though it's final. This means that you can use expectations as usually. A unit test of getFormattedState method looks something like this:

        @Test
	public void testGetFormattedState_actualStateExists() throws Exception {
		final String expectedState = "state";

                // We use PowerMock.createMock(..) to create the mock object. 
		StateHolder stateHolderMock = createMock(StateHolder.class);
		StateFormatter tested = new StateFormatter(stateHolderMock);

		expect(stateHolderMock.getState()).andReturn(expectedState);

                // PowerMock.replay(..) must be used. 
		replay(stateHolderMock);

		final String actualState = tested.getFormattedState();

                // PowerMock.verify(..) must be used. 
		verify(stateHolderMock);

		assertEquals(expectedState, actualState);
	}

Note that you need to tell PowerMock which classes you like to mock. This is done by adding the @PrepareForTest annotation to the class-level of test case, in this case @PrepareForTest(StateHolder.class). You also need to tell JUnit to execute the test using PowerMock which is done by using the @RunWith(PowerMockRunner.class) annotation. Without these two annotations the test will fail.

See the partial mocking section for information on how to mock only specific methods of a class.

References

• StateFormatterTest
• StateFormatter
• StateHolder