How to Mock Private Method for Testing Using Powermock

How to mock private method for testing using PowerMock?

I don't see a problem here. With the following code using the Mockito API, I managed to do just that :

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;
    }
}

And here's the JUnit test :

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();
    }
}

How to mock private method using powermock?

PowerMock annotations required for mock working:

@RunWith(PowerMockRunner.class)
@PrepareForTest(LuckyNumberGenerator.class)

Example of working test:

import org.junit.Assert;
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.powermock.api.mockito.PowerMockito.*;

@RunWith(PowerMockRunner.class)
@PrepareForTest(LuckyNumberGenerator.class)
public class ServiceTest {
    @Test
    public void test() throws Exception {
        LuckyNumberGenerator mock = spy(new LuckyNumberGenerator());
        PowerMockito.when(mock, "getDefaultLuckyNumber").thenReturn(300);
        Assert.assertEquals(300, mock.getLuckyNumber("name"));
    }
}

public class LuckyNumberGenerator {
    public int getLuckyNumber(String name) {
        return getDefaultLuckyNumber();
    }
    private int getDefaultLuckyNumber() {
        return 1;
    }
}

PowerMock version:

        <dependency>
            <groupId>org.powermock</groupId>
            <artifactId>powermock-module-junit4</artifactId>
            <version>2.0.9</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.powermock</groupId>
            <artifactId>powermock-api-mockito2</artifactId>
            <version>2.0.9</version>
            <scope>test</scope>
        </dependency>

Mocking private method with PowerMock

I finally found the answer:

@Test
public void methodTest() {
    ClassToTest objectToTest = PowerMockito.spy(new ClassToTest());
    try {
        when(objectToTest, method(ClassToTest.class, "privateMethod", boolean.class)).withArguments(false).thenReturn(mockAsyncTask);
    } catch (Exception e) {
        e.printStackTrace();
    }
}

And also just before the class declaration:

@PrepareForTest(ClassToTest.class)

Testing Private method using mockito

You can't do that with Mockito but you can use Powermock to extend Mockito and mock private methods. Powermock supports Mockito. Here's an example.

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