Powemock is a very powerful tool in any Java Developer's arsenal, it allows a developer to change his code at the bytecode level. This is especially beneficial when we want to modify some functionality of source (System Under Test).
Whey we need whenNew() ?
whenNew() allows us to inject our fake or mock instead of actually creating a new object of said class.
This works great, we just need to add our class in @PrepareForTest annotation and PowerMock will inject our mock. Just like that.
Here is an example class:
public class ExampleClass {
public int getHeight() {
final CustomClass customClassInstance = new CustomClass();
return customClass.getHeight();
}
}
and here is a test case using whenNew() injection.
@RunWith(PowerMockRunner.class)
@PrepareForTest({
ExampleClass.class
})
public class ExampleClassTest {
private ExampleClass classUnderTest;
@Before
public void setUp() throws Exception {
classUnderTest = new ExampleClass();
}
@Test
public void getHeight_shoudReturnHeightOfView() throws NoSuchMethodException {
final int expectedHeight = 100;
final CustomClass mckCustomClass = Mockito.mock(CustomClass.class);
Mockito.when(mckCustomClass.getHeight()).theReturn(expectedHeight);
// Make sure you match exact parameters while mocking (same as we do for Mockito.mock statements).
PowerMockito.whenNew(CustomClass.class).withNoArguments().thenReturn(mckCustomClass);
final int actualResult = classUnderTest.getHeight();
Assert.assertEquals(expectedHeight, actualResult);
}
}
Now let's come to the problem...
Let's say you have the following class to test:
public class ClassToTest {
private void doInBackground() {
new Thread() {
@Override
public void run() {
final CustomClass customClassInstance = new CustomClass();
int height = customClass.getHeight();
}
}.start();
}
}
Now, in the above example, a new instance of CustomClass is created inside a nested scope (Thread), hence modifying bytecode of ClassToTest will no longer work as new CustomClass() is not inside its scope.
What to do?
Powermock has other tools to deal with such situations, firstly, you need to add CustomClass in @PrepareForTest() and suppress() constructors and stub() getHeight() method to modify its behaviour according to your test requirements.
Here is example code:
@RunWith(PowerMockRunner.class)
@PrepareForTest({
ExampleClass.class,
CustomClass.class
})
public class ExampleClassTest {
private ExampleClass classUnderTest;
@Before
public void setUp() throws Exception {
classUnderTest = new ExampleClass();
}
@Test
public void getHeight_shoudReturnHeightOfView() throws NoSuchMethodException {
final int expectedHeight = 100;
PowerMockito.suppress(CustomClass.class.getConstructors());
PowerMockito.stub(CustomClass.class.getMethod("getHeight")).toReturn(expectedHeight);
}
}
P.S. PowerMock has suppress() , sub() and replace() methods to modify individual elements of any class. Please read more about methods declared in PowerMockito class and see if you find an alternate solution to your use case.
