Answer is used when you need to do additional actions when a mocked method is invoked, e.g. when you need to compute the return value based on the parameters of this method call. Use doAnswer() when you want to stub a void method with generic Answer .
One thing that when/thenReturn gives you, that doReturn/when doesn't, is type-checking of the value that you're returning, at compile time. However, I believe this is of almost no value - if you've got the type wrong, you'll find out as soon as you run your test. I strongly recommend only using doReturn/when .
The thenReturn() methods lets you define the return value when a particular method of the mocked object is been called.
Mockito provides following methods that can be used to mock void methods. doAnswer() : We can use this to perform some operations when a mocked object method is called that is returning void. doThrow() : We can use doThrow() when we want to stub a void method that throws exception.
You should use thenReturn
or doReturn
when you know the return value at the time you mock a method call. This defined value is returned when you invoke the mocked method.
thenReturn(T value)
Sets a return value to be returned when the method is called.
@Test
public void test_return() throws Exception {
Dummy dummy = mock(Dummy.class);
int returnValue = 5;
// choose your preferred way
when(dummy.stringLength("dummy")).thenReturn(returnValue);
doReturn(returnValue).when(dummy).stringLength("dummy");
}
Answer
is used when you need to do additional actions when a mocked method is invoked, e.g. when you need to compute the return value based on the parameters of this method call.
Use
doAnswer()
when you want to stub a void method with genericAnswer
.Answer specifies an action that is executed and a return value that is returned when you interact with the mock.
@Test
public void test_answer() throws Exception {
Dummy dummy = mock(Dummy.class);
Answer<Integer> answer = new Answer<Integer>() {
public Integer answer(InvocationOnMock invocation) throws Throwable {
String string = invocation.getArgumentAt(0, String.class);
return string.length() * 2;
}
};
// choose your preferred way
when(dummy.stringLength("dummy")).thenAnswer(answer);
doAnswer(answer).when(dummy).stringLength("dummy");
}
doAnswer
and thenReturn
do the same thing if:
Let's mock this BookService
public interface BookService {
String getAuthor();
void queryBookTitle(BookServiceCallback callback);
}
You can stub getAuthor() using doAnswer
and thenReturn
.
BookService service = mock(BookService.class);
when(service.getAuthor()).thenReturn("Joshua");
// or..
doAnswer(new Answer() {
@Override
public Object answer(InvocationOnMock invocation) throws Throwable {
return "Joshua";
}
}).when(service).getAuthor();
Note that when using doAnswer
, you can't pass a method on when
.
// Will throw UnfinishedStubbingException
doAnswer(invocation -> "Joshua").when(service.getAuthor());
So, when would you use doAnswer
instead of thenReturn
? I can think of two use cases:
Using doAnswer you can do some additionals actions upon method invocation. For example, trigger a callback on queryBookTitle.
BookServiceCallback callback = new BookServiceCallback() {
@Override
public void onSuccess(String bookTitle) {
assertEquals("Effective Java", bookTitle);
}
};
doAnswer(new Answer() {
@Override
public Object answer(InvocationOnMock invocation) throws Throwable {
BookServiceCallback callback = (BookServiceCallback) invocation.getArguments()[0];
callback.onSuccess("Effective Java");
// return null because queryBookTitle is void
return null;
}
}).when(service).queryBookTitle(callback);
service.queryBookTitle(callback);
When using when-thenReturn on Spy Mockito will call real method and then stub your answer. This can cause a problem if you don't want to call real method, like in this sample:
List list = new LinkedList();
List spy = spy(list);
// Will throw java.lang.IndexOutOfBoundsException: Index: 0, Size: 0
when(spy.get(0)).thenReturn("java");
assertEquals("java", spy.get(0));
Using doAnswer we can stub it safely.
List list = new LinkedList();
List spy = spy(list);
doAnswer(invocation -> "java").when(spy).get(0);
assertEquals("java", spy.get(0));
Actually, if you don't want to do additional actions upon method invocation, you can just use doReturn
.
List list = new LinkedList();
List spy = spy(list);
doReturn("java").when(spy).get(0);
assertEquals("java", spy.get(0));
The simplest answer is:
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