How to mock private method for testing using PowerMock?

Question

I have a class which I would like to test with a public method that calls private one. I'd like to assume that private method works correctly. For example, I'd like something like doReturn....when.... I found that there is possible solution using PowerMock, but this solution doesn't work for me. How It can be done? Did anybody have this problem?

Answer 1

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();
    }
}