How do I Mock Private Methods for Testing Using PowerMock?

Mocking Private Methods for Testing with PowerMock

When testing classes with private methods, it is often necessary to mock these methods to ensure the public API is working as expected. However, doing so can be challenging when using frameworks like Mockito.

Problem:

You may have encountered issues while trying to mock private methods using PowerMock, specifically when you want to manipulate the return value of the private method.

Solution:

Follow these steps to mock private methods for testing using PowerMock:

1. Import PowerMock APIs: Include the necessary PowerMock annotations and APIs into your test class.

   <code class="java">import org.powermock.api.mockito.PowerMockito;
   import org.powermock.core.classloader.annotations.PrepareForTest;
   import org.powermock.modules.junit4.PowerMockRunner;
   import static org.mockito.Matchers.anyInt;
   import static org.mockito.Matchers.anyString;
   import static org.powermock.api.mockito.PowerMockito.when;
   import static org.powermock.api.support.membermodification.MemberMatcher.method;</code>

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2. Prepare Class for Testing: Annotate your test class with @PrepareForTest and specify the class containing the private method you want to mock.

   <code class="java">@PrepareForTest(CodeWithPrivateMethod.class)</code>

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3. Create Spy Instance: Using PowerMockito, create a spy instance of the class containing the private method.

   <code class="java">CodeWithPrivateMethod spy = PowerMockito.spy(new CodeWithPrivateMethod());</code>

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4. Mock Private Method: Use PowerMockito's when method to mock the private method. Specify the class, method name, parameter types, and the desired return value.

   <code class="java">when(spy, method(CodeWithPrivateMethod.class, "doTheGamble", String.class, int.class))
    .withArguments(anyString(), anyInt())
    .thenReturn(true);</code>

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5. Test Public Method: Call the public method that invokes the private method and verify its behavior based on the mocked return value.

   <code class="java">spy.meaningfulPublicApi();</code>

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Example:

Consider the following class CodeWithPrivateMethod with a private method doTheGamble:

<code class="java">public class CodeWithPrivateMethod {
    public void meaningfulPublicApi() {
        if (doTheGamble("Whatever", 1 << 3)) {
            throw new RuntimeException("boom");
        }
    }
    private boolean doTheGamble(String whatever, int binary) {
        Random random = new Random(System.nanoTime());
        boolean gamble = random.nextBoolean();
        return gamble;
    }
}</code>

To mock doTheGamble and always return true, you would write the following test:

<code class="java">@RunWith(PowerMockRunner.class)
@PrepareForTest(CodeWithPrivateMethod.class)
public class CodeWithPrivateMethodTest {
    @Test(expected = RuntimeException.class)
    public void when_gambling_is_true_then_always_explode() throws Exception {
        CodeWithPrivateMethod spy = PowerMockito.spy(new CodeWithPrivateMethod());
        when(spy, method(CodeWithPrivateMethod.class, "doTheGamble", String.class, int.class))
            .withArguments(anyString(), anyInt())
            .thenReturn(true);
        spy.meaningfulPublicApi();
    }
}</code>

This way, you can effectively mock private methods for testing, ensuring that your public API functions correctly even when private method implementation remains unchanged.

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