How to Mock Private Methods in Java with PowerMock?

Testing Private Methods with PowerMock

During software development, it is inevitable to encounter situations where you need to test public methods that rely on private ones. In such instances, assuming the correct functionality of private methods can simplify the testing process. PowerMock is a Java testing framework that provides mechanisms to mock private methods for testing purposes.

Mocking Private Methods Using PowerMock

To mock a private method using PowerMock, follow these steps:

1. Spy on the Class: Create a spy instance of the class containing the private method you wish to mock. This can be achieved using the PowerMockito.spy() method.
2. Specify the Expected Method Behavior: Utilize the when() method to define the expected behavior of the private method. This includes specifying the arguments and the return value. Use method matchers, such as anyString() and anyInt(), to handle different values.
3. Execute the Test: Invoke the public method that calls the private method and assert the expected behavior.

Example:

Consider the following class with a public method (meaningfulPublicApi()) that invokes 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>

The corresponding JUnit test using PowerMock would look like:

<code class="java">import org.junit.Test;
import org.junit.runner.RunWith;
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;

@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>

In this example:

• The @PrepareForTest annotation marks the CodeWithPrivateMethod class for preparation, allowing PowerMock to modify its private method.
• The when() method defines the expected return value of the doTheGamble() method for any input arguments.
• The test verifies that an exception is thrown when the public method is executed.

This simple example demonstrates how to mock private methods for testing using PowerMock. By leveraging this technique, you can effectively isolate and test the logic of your public methods, ensuring the reliability of your software.

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