How to Partially Mock Methods in Mockito: Mocking Specific Methods While Preserving Others?

Partial Mocking: Mocking Specific Methods in Mockito

When testing a class, it might be necessary to mock only certain methods while preserving the behavior of others. In Mockito, this is achieved through "partial mocking," where a selection of methods is mocked while the unmocked ones execute as intended.

Consider the following Stock class with getPrice(), getQuantity(), and getValue() methods:

public class Stock {
    ...
    public double getValue() {
        return getPrice() * getQuantity();
    }
}

In a test case, if we wish to mock the getPrice() and getQuantity() methods but leave getValue() to execute normally, we can define a partial mock as follows:

Stock stock = mock(Stock.class);
when(stock.getPrice()).thenReturn(100.00);
when(stock.getQuantity()).thenReturn(200);
when(stock.getValue()).thenCallRealMethod();

Here, the getPrice() and getQuantity() methods have mocked implementations, while getValue() executes as in the original Stock class.

Alternatively, using the "spy" API:

Stock stock = spy(Stock.class);
when(stock.getPrice()).thenReturn(100.00);
when(stock.getQuantity()).thenReturn(200);

All other method calls, in this case getValue(), use the real implementation.

To ensure the real method is not called and that mocking logic is consistently applied, consider the following alternative notation:

doReturn(100.00).when(stock).getPrice();
doReturn(200).when(stock).getQuantity();

However, it's important to note that in this example, the getValue() method relies on price and quantity directly, not their getters. As a result, mocking getPrice() and getQuantity() may still not yield the desired behavior.

An alternative approach would be to avoid mocking entirely and instead rely on a direct instantiation of the Stock class:

Stock stock = new Stock(100.00, 200);
double value = stock.getValue();

By creating a real instance, we can ensure that getValue() executes as intended, relying on the actual price and quantity values.

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