Mocking a Private Variable that is Assumed to Exist

Question

How can you get a mock object in at runtime when it is not created/initialized in the class you are testing, it is not static (singleton pattern), or you don't have some sort of test constructor to hook into?

In a class that I am writing some unit testing for, I have come across a scenario I haven't encountered/solved yet. I have a JMS resource (a QueueConnectionFactory for reference, but it shouldn't matter), that is a private variable of the class I am testing. Since it has the javax.annotation.Resource annotation, at runtime it is assumed to be available. During testing, it is not, which creates the need for mocking this object.

It is not a static class and is not being used in a static way, if it was I could easily mock using the various static mocking methods I have run into. Since the resource is never created locally (in a constructor or even in a test constructor), I have no way of passing in a Mock object so that at runtime of the test, the mock is used instead of the actual object. How can I mock this Resource so that when the test executes, it will be used in place of the private @Resource object in the class I am testing?

For reference, the code is calling createConnection() on the QueueConnectionFactory which is throwing a null pointer exception since the Factory has not been initialized/mocked.

@Stateless
public class Example{
  @Resource(name = "jms/exampleQCF")
  private QueueConnectionFactory queueFactory;

  ...

  public void testMe(){
    Connection connection = queueFactory.createConnection();
    ...
  }
}

score 41 · Accepted Answer · answered Oct 22 '13 at 19:49

After a lot more hunting around and looking at all the options Mockito/Powermock had to offer, I found the solution (which I will share in case others run into this same issue).

When you have private member variables that are never initialized (and just assumed created in other places), you can use the @InjectMocks annotation to "inject" Mocks you want into your class you are testing.

Add a variable in your test class for the class you are testing, and give it the annotation @InjectMocks (org.Mockito.InjectMocks).
Use @Mock annotations to setup the mocks you want to inject. Use the @Mock (name = "privateVariableNameHere") name property to map the Mock object to the private variable inside your class you are testing.
In either a setup function or before you call your class, initialize the mocks. The easiest way I have found is to use a "setup" method with the @Before annotation. Then inside there call MockitoAnnotations.initMocks(this); to quickly initialize anything with the @Mock annotation.
Define your Mock functionality in your test method (before calling the method you are testing).
Using the @InjectMock object, call your method you are testing... the mocks SHOULD be hooked in and working as defined in the earlier steps.

So for the example class I use above, the code to test/mock would have Connection returned as a mock which you can do whatever with. Based on the example above in my question, this is what the code would look like:

@RunWith(PowerMockRunner.class)
@PrepareForTest({/* Static Classes I am Mocking */})
public class ExampleTest {
  @Mock (name = "queueFactory") //same name as private var.
  QueueConnectionFactory queueFactoryMock;
  @Mock
  Connection connectionMock; //the object we want returned
  @InjectMocks
  Example exampleTester; //the class to test

  @Before
  public void setup(){
    MockitoAnnotations.initMocks(this); // initialize all the @Mock objects
    // Setup other Static Mocks
  }

  @Test
  public void testTestMe(){
    //Mock your objects like other "normally" mocked objects
    PowerMockito.when(queueFactoryMock.createConnection()).thenReturn(connectionMock);
    //...Mock ConnectionMock functionality...
    exampleTester.testMe();
  }
}

This one answer helped me 2 issues – Sankalp Dec 04 '15 at 11:02 — Sankalp, Dec 04 '15 at 11:02

WesternGun · Answer 2 · 2019-10-03T07:44:40.410

Several approaches here:

ReflectionTestUtils of Spring Testing framework: ReflectionTestUtils.setField(objectToTest, "privateFieldName", mockObjectToInject);. With this you don't introduce another dependency.
org.mockito.internal.util.reflection.FieldSetter.
PowerMock.Whitebox.setInternalState() to mock a private field.

If you need to mock internal local variable creation, use PowerMockito.whenNew(Foo.class).withNoArguments().thenReturn(foo);. Very, very useful. Cannot find other ways to do the same.

With only Mockito you cannot mock local variable creation, because when(any(Foo.class) does not work; will return null. It compiles but does not work.

References: Mockito: Mock private field initialization

Mocking a Private Variable that is Assumed to Exist

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