Mock an exception object (JUnit/Mockito)

Question

I am facing an issue testing a legacy code using JUnit/Mockito, my method throws an exception (HandlerException) which is derived from (BaseException) which is part of our infrastructure.

public class HandlerException extends BaseException

My system-under-test is super simple:

public static void parse(JsonElement record) throws HandlerException
{   
    JsonElement element = record.getAsJsonObject().get(ID_TAG); 
    if(element == null) {
        throw new HandlerException("Failed to find Id ...");
    }
    ...
}

And also the test itself

@Test (expected=HandlerException.class)
public void testParseg() throws HandlerException {
    JsonElement jsonElement = new JsonParser().parse("{}");
    Parser.parse(jsonElement);
}

The problem is with BaseException. It is a complex class and depends on an initialization. Without initializing it BaseException throws an exception in its constructor :( which in turn causes StackOverflowException.

Is it possible to Mock BaseException or HandlerException in any way to avoid this initialization keeping my test simple?

Answer 1

Looks like the answer to my issue was using PowerMockito

First I use @PrepareForTest on my system-under-test (Parser.class)

@RunWith(PowerMockRunner.class)
@PrepareForTest({Parser.class})

Then I mocked the HandlerExeption class using PowerMockito.whenNew

@Mock
HandlerException handlerExceptionMock;

@Before 
public void setup() throws Exception {
    PowerMockito.whenNew(HandlerException.class)
        .withAnyArguments().thenReturn(handlerExceptionMock);
}

@Test (expected=HandlerException.class)
public void testParseg() throws HandlerException {
    JsonElement jsonElement = new JsonParser().parse("{}");
    Parser.parse(jsonElement);
}

This way BaseException was not constructed and my test passed without the need to initialize it.

Note: I am aware that this is not the best practice, however, in my case, I had to since I cannot edit BaseException.