Mockito: `doReturn()` vs. `when()`: When Should You Use Which with Spied Objects?

Mockito: Understanding the Subtle Difference between doReturn() and when()

When utilizing the mocking framework Mockito to enhance testing capabilities, developers often encounter the doReturn() and when() methods. While both methods serve the purpose of stubbing method invocations, a subtle distinction lies between them when working with spied objects (annotated with @Spy).

when(...).thenReturn(...) vs doReturn(...).when(...)

when(...).thenReturn(...):

• Makes a real method call before returning the specified value.
• If the called method throws an exception, it must be handled or mocked separately.

doReturn(...).when(...):

• Avoids the actual method call entirely.

Practical Example

Consider the following MyClass:

public class MyClass {
    protected String methodToBeTested() {
        return anotherMethodInClass();
    }

    protected String anotherMethodInClass() {
        throw new NullPointerException();
    }
}

Testing with Spy

doReturn(...).when(...):

@Spy
private MyClass myClass;

// Works as expected
doReturn("test").when(myClass).anotherMethodInClass();

when(...).thenReturn(...):

// Throws a NullPointerException
when(myClass.anotherMethodInClass()).thenReturn("test");

In this scenario, doReturn() ensures that the exception in anotherMethodInClass() is avoided while still returning the desired value. In contrast, when() triggers the actual method call, resulting in the NullPointerException being thrown.

Therefore, when working with spied objects, the choice between doReturn() and when() depends on whether you want to invoke the actual method or bypass it altogether.

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