What is the difference between mocking and spying when using Mockito?

java testing mocking mockito
Victor Grazi picture Victor Grazi · Feb 24, 2013 · Viewed 108.2k times · Source

What would be a use case for a use of a Mockito spy?

It seems to me that every spy use case can be handled with a mock, using callRealMethod.

One difference I can see is if you want most method calls to be real, it saves some lines of code to use a mock vs. a spy. Is that it or am I missing the bigger picture?

Answer

JB Nizet picture JB Nizet · Feb 24, 2013

The answer is in the documentation:

Real partial mocks (Since 1.8.0)

Finally, after many internal debates & discussions on the mailing list, partial mock support was added to Mockito. Previously we considered partial mocks as code smells. However, we found a legitimate use case for partial mocks.

Before release 1.8 spy() was not producing real partial mocks and it was confusing for some users. Read more about spying: here or in javadoc for spy(Object) method.

callRealMethod() was introduced after spy(), but spy() was left there of course, to ensure backward compatibility.

Otherwise, you're right: all the methods of a spy are real unless stubbed. All the methods of a mock are stubbed unless callRealMethod() is called. In general, I would prefer using callRealMethod(), because it doesn't force me to use the doXxx().when() idiom instead of the traditional when().thenXxx()

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