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javamockingmockito
Elliot Chance
Elliot Chance

Reputation: 5736

How does when() work?

In the following example 

when(myMethod("abc", 123)).thenReturn(456);

How does the when() method catch the method name and arguments without actually invoking myMethod()?

Can I write a method to do the same thing as when() so that I get a method pointer and an array of Objects as arguments to invoke later?

Upvotes: 4

Views: 210

Answers (2)

Jeff Bowman
Jeff Bowman

Reputation: 95614

In your above example, myMethod is a method on a mock object. Without any expectations, Mockito will return null, 0, or false depending on the data type, which when will silently discard.

However, You may also use when on an object which is not a mock, but rather an object created using Mockito.spy(). In this case, the method would actually be called in the when method, which is often not what you want to do. Mockito provides another method called doReturn (or possibly doAnswer or doThrow) which provides you a replacement object so the original is never called (docs):

doReturn(1).when(mySpiedObject).getSomeInteger(anyString(), eq("seven"));

Note that the Mockito docs recommend using when rather than doReturn because the latter is not type-safe.

Upvotes: 0

Ray Toal
Ray Toal

Reputation: 88378

The method myMethod is invoked. But it is being invoked on a mock object -- that's the "trick".

Of course you can write code that accepts a "method pointer" (in Java, it would be an object of class Method) and some arguments, and use invoke, but doing so does not actually buy you anything over calling the mock object's myMethod directly.

It is more common to see when called as follows:

MyObject myObject = mock(MyObject.class);
when(myObject.myMethod("abc", 123)).thenReturn(456);

Try printing (or logging) the expression

myObject.getClass().getName()

here. You will see that the class of the mock object is not actually MyObject. But it is of a class that has the same interface. The calls on this object update some internal state. This allows Mockito to keep track of how it is used, and allows you to assert various things.

Upvotes: 2

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