NullPointerException when trying to call method on mocked Eclipse Paho MqttClient

Question

I have the following, heavily simplified class:

public class MyClass
{
    private MqttClient field;

    public void test() throws MqttException
    {
        field = new MqttClient(null, null);
        field.connect();
    }
}

I am using Eclipse Paho's MqttClient class.

I want to test the test method with a UnitTest. In order to have control over the MqttClient instance I want to mock the object. My test-class looks as follows:

public class TestMyClass
{
    // Make sure we are running with the PowerMockAgent initialized
    static
    {
        PowerMockAgent.initializeIfNeeded();
    }

    // Make sure we are using PowerMock
    @Rule
    public PowerMockRule    rule    = new PowerMockRule();

    // Mocked MqttClient
    @Mock
    MqttClient              field;

    // MyClass-Object injected with mocks.
    @InjectMocks
    MyClass                 myClass = new MyClass();

    @PrepareForTest(MyClass.class)
    @Test
    public void test() throws Exception
    {
        // Initialize our mocks from annotations
        MockitoAnnotations.initMocks(this);

        // Catch the "new MqttClient(null, null);"
        PowerMockito.whenNew(MqttClient.class).withAnyArguments().thenReturn(field);


        myClass.test();

    }
}

When I execute my test I am met with the following error:

java.lang.NullPointerException
    at test.java.test.MyClass.test(MyClass.java:13)
    at test.java.test.TestMyClass.test(TestMyClass.java:45)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:47)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:44)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
    at org.powermock.modules.junit4.rule.PowerMockStatement.evaluate(PowerMockRule.java:73)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:271)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:70)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:238)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:63)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:236)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:53)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:229)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:309)
    at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:86)
    at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:538)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:760)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:460)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:206)

I can call any method on field it is always a null-reference. My problem is I have no idea why this is the case.

I changed the type of field to several other classes, including java.awt.point and SerializationInstantiatorHelper (in order to test if it has to do with MqttClient being an external dependency) as well as several classes in my project, calling an appropriate method on those objects. In all those cases the problem did not exist and the test succeeded without any issues. The method on the mocked object is called and does nothing - just as I want it to.

Only when using the MqttClient class this problem occurs. Or at least I did not find any other class yet to reproduce this problem.

I am in all honesty totally lost at this point. What makes the MqttClient class special? Why can I seemingly not mock it like any other class? What did I miss?

//EDIT

I narrowed down the problem by creating a local version of MqttClient, copying over the source code. In all honesty, I am even more confused now though. The class has three constructors with the following signatures:

1.)

public MqttClient(  String serverURI,
                    String clientId) throws MqttException

2.)

public MqttClient(  String serverURI,
                    String clientId,
                    MqttClientPersistence persistence) throws MqttException

3.)

public MqttClient(  String serverURI,
                    String clientId,
                    MqttClientPersistence persistence,
                    ScheduledExecutorService executorService) throws MqttException

If one of those three constructors is removed, everything works. Any combination of the class with two of those three constructors is working just fine. As soon as all three constructors exist, the beforementioned error message appears. This leads me to the assumption it could have something to do with PowerMockito not finding the correct constructor? If so, how would that be avoidable?

Answer 1

The mocked target appears to have not been configured correctly so it is not behaving as expected when exercising the test.

Sometimes keeping things simple (KISS) is the way to go.

@RunWith(PowerMockRunner.class)
@PrepareForTest(MyClass.class)
public class TestMyClass {

    @Test
    public void test() throws Exception {
        //Arrange

        // mock MqttClient
        MqttClient field = mock(MqttClient.class);
        // setup the "new MqttClient(null, null);"
        PowerMockito.whenNew(MqttClient.class).withArguments(isNull(), isNull()).thenReturn(field);

        MyClass subject = new MyClass();

        //Act
        subject.test();

        //Assert
        PowerMockito.verifyNew(MqttClient.class).withArguments(isNull(), isNull());
        verify(field).connect();
    }
}

By using .withArguments with the appropriate arguments or matchers, it helps narrow down specifically which constructor is to be mocked for the test, avoiding any conflicts when multiple constructors exists.

Based on the simplified class under test shown in the example, there are no dependencies being injected as it initialized the dependency locally (i.e: tight coupling), so there really was no need to inject mocks into the class under test.

Also PrepareForTest is usually done on the test class and not the test method in this case, as according to documentation

All usages require @RunWith(PowerMockRunner.class) and @PrepareForTest annotated at class level.

emphasis mine

Answer 2

I think you're running to the same trouble as this reported in this issue

Mocking whenNew using withAnyArguments does not match null arguments #891

The problem is if multiple constructors are declared:

The first constructor's matchers are generated properly (using the form or(isA(Class), isNull())). But the remaining constructors are not (they only have any(Class) which causes them to not match against nullvalues).

So till the issue is addressed, you can use the work around:

MqttClient field = mock(MqttClient.class);

instead of @Mock MqttClient field;