PowerMockito throws NullPointerException when trying to stub private overloaded method

问题

I'm (still) trying to check if bar(Alpha, Baz) called bar(Xray, Baz) using PowerMockito (as bar(Xray, Baz) is private) - without actually calling the later, given my MCVE class Foo below. (I went through the same class earlier, with all methods in Foo being public - in case you've got a déjà vu...)

public class Foo {
    private String bar(Xray xray, Baz baz) {
        return "Xray";
    }

    private String bar(Zulu zulu, Baz baz) {
        return "Zulu";
    }

    public String bar(Alpha alpha, Baz baz) {
        if(alpha.get() instanceof Xray) {
            return bar((Xray)alpha.get(), baz);
        } else if(alpha.get() instanceof Zulu) {
            return bar((Zulu)alpha.get(), baz);
        } else {
            return null;
        }
    }
}

When I try to run the test below, I get a NPE from PowerMock:

@RunWith(PowerMockRunner.class)
// @PrepareOnlyThisForTest(Foo.class) // we aren't looking at the byte code, I think
public class FooTest {

    @Test
    public void testBar_callsBarWithXray() throws Exception {
        Baz baz = new Baz(); //POJOs
        Alpha alpha = new Alpha();
        alpha.set(new Xray());

        Foo foo = new Foo();
        Foo stub = spy(foo); // using Mockito, as it's neither final nor "not spyable"

        // NPE at org.powermock.api.mockito.internal.expectation.PowerMockitoStubberImpl.addAnswersForStubbing(PowerMockitoStubberImpl.java:67)
        PowerMockito.doReturn("ok").when(stub, "bar", Xray.class, Baz.class);

        stub.bar(alpha, baz);
        // Testing if bar(Xray, Baz) was called by bar(Alpha, Baz)
        PowerMockito.verifyPrivate(foo).invoke("bar", Xray.class, Baz.class);
        // Mockito's equivalent for a public method: verify(stub, times(1)).bar(any(Xray.class), any(Baz.class));
    }
}

If I make the stub a PowerMockito.spy(foo), I get an IllegalArgumentException: argument type mismatch at org.powermock.reflect.internal.WhiteboxImpl.performMethodInvocation(WhiteboxImpl.java:2014) instead. (It's bubbling up on the same line as the NPE.)

I'm using Mockito-core 1.9.5, PowerMock 1.5.4 (module-junit4 and api-mockito) and JUnit 4.11.

What do I need to change to stop the exceptions from being thrown? How can I make this test work? (Other than testing that my class works, instead of how... ;-) )

回答1:

When setting the expectations, we have to use the exact argument matcher. In your case, it is Matchers.any(Xray.class), Matchers.any(Baz.class)

I have modified your code as below and also added assert statement on output object of your test method.

@RunWith(PowerMockRunner.class)
//@PrepareOnlyThisForTest(Foo.class) // we aren't looking at the byte code, I think
 public class FooTest {

   @Test
   public void testBar_callsBarWithXray() throws Exception {
       Baz baz = new Baz(); //POJOs
       Alpha alpha = new Alpha();
       alpha.set(new Xray());

       Foo foo = new Foo();
       Foo stub = PowerMockito.spy(foo); // using Mockito, as it's neither final nor "not spyable"

      // NPE at org.powermock.api.mockito.internal.expectation.PowerMockitoStubberImpl.addAnswersForStubbing(PowerMockitoStubberImpl.java:67)
      PowerMockito.doReturn("ok").when(stub, "bar", Matchers.any(Xray.class), Matchers.any(Baz.class));

      String res = stub.bar(alpha, baz);
      Assert.assertEquals("ok", res);

     //Testing if bar(Xray, Baz) was called by bar(Alpha, Baz)
     PowerMockito.verifyPrivate(stub).invoke("bar", Matchers.any(Xray.class), Matchers.any(Baz.class));
     // Mockito's equivalent for a public method: verify(stub, times(1)).bar(any(Xray.class), any(Baz.class));
   }
}

Observation : when calling verify methods, we have to pass stub object not actual object because we set expectations on stub object. As I added assert statements to test the method, you don't have to verify on the stub whether it is working properly or not.

ADDED: I added sysout statements in both public & private 'bar' methods and when I tested again, I see that sysout statement of public bar method is not printed. It means the above code mocked public method only but not private method.

To mock private 'bar' method, I tried with another way of mocking using MemberMatcher.method which worked perfectly.

import org.powermock.api.support.membermodification.MemberMatcher;

@RunWith(PowerMockRunner.class)
@PrepareForTest(Foo.class) // we need this
public class FooTest {

@Test
 public void testBar_callsBarWithXray() throws Exception {
     Baz baz = new Baz(); //POJOs
     Alpha alpha = new Alpha();
     alpha.set(new Xray());

     Foo stub = PowerMockito.spy(new Foo());

     PowerMockito.doReturn("ok")
        .when(stub,
                MemberMatcher.method(Foo.class,
                        "bar",
                        Xray.class, Baz.class))
        .withArguments(Matchers.any(Xray.class), Matchers.any(Baz.class));

     String res = stub.bar(alpha, baz);

     Assert.assertEquals("ok", res);

     //Testing if bar(Xray, Baz) was called by bar(Alpha, Baz)
     PowerMockito.verifyPrivate(stub).invoke("bar", Matchers.any(Xray.class), Matchers.any(Baz.class));
     // Mockito's equivalent for a public method: verify(stub, times(1)).bar(any(Xray.class), any(Baz.class));
 }

 output : public bar

Test method is also passed. Below are foo methods having sysouts.

private String bar(Xray xray, Baz baz) {
    System.out.println("private bar");
    return "Xray";
}

public String bar(Alpha alpha, Baz baz) {

    System.out.println("public bar");

    if(alpha.get() instanceof Xray) {
        return bar((Xray)alpha.get(), baz);
    } else if(alpha.get() instanceof Zulu) {
        return bar((Zulu)alpha.get(), baz);
    } else {
        return null;
    }
}

来源：https://stackoverflow.com/questions/32208501/powermockito-throws-nullpointerexception-when-trying-to-stub-private-overloaded

标签

java

powermock

overloading

private-members

powermockito