Need to mock creation of a object inside a constructor of the class to be tested

Question

I am trying to test a class which is creating a new object inside the constructor. I am using PowerMock with Mockito.

public ClassNeedToTest() throws Exception {

    String targetCategory = "somevalue";
    String targetService = "somevalue";
    invoker = new ServiceInvoker(targetCategory, targetService); // throws Exception
}

For the above given code, I am trying to create a instance of ClassNeedToTest to test different method of that class. I am not able to create the object because ServiceInvoker creation is throwing an Exception. The ServiceInvoker class is a third party class. Is there any way to mock ServiceInvoker so that when test class is trying to create ClassNeedToTest I can get the mocking object of ServiceInvoker instead of really calling constructor of ServiceInvoker.

In my test class is am just creating a new instance of ClassNeedToTest:

ClassNeedToTest obj = new ClassNeedToTest();

Answer 1

I found the answer for the same. If you follow the steps given below properly, you can mock the objects.

Step 1. - Add annotation to prepare the test class.

@PrepareForTest({ ServiceInvoker.class, ClassNeedToTest.class})

Step 2. - Mock the class.

serviceInvokerMck = Mockito.mock(ServiceInvoker.class);

Step 3. Use the below method to mock the object when new operator is called

PowerMockito.whenNew(ServiceInvoker.class).withAnyArguments().thenReturn(serviceInvokerMck);

What I was not doing was adding the class ClassNeedToTest in PrepareForTest annotation thinking that the only class need to be mocked should be added there.

Answer 2

Creating objects within a class' constructor is bad practice (Dependency inversion principle). A common way to get such code under test is adding a new constructor that is package private.

public ClassNeedToTest() throws Exception {
  this(new ServiceInvoker("somevalue", "somevalue"));
}

ClassNeedToTest(ServiceInvoker invoker) throws Exception {
  this.invoker = invoker;
}

Your tests can use the new constructor with a ServiceInvoker mock. Afterwards you can make the new constructor public and refactor your code to using only the new constructor.