Testing legacy code with Mockito spies

When we're working with legacy code, before making any changes we should write unit tests that will prevent us from breaking existing functionality. However most of the time, writing tests for legacy code is not easy.

Let's take a look at a simple example, that will try to show the challenges that we may face:

	public class HardToTestClass {
	public Double calculateDiscount(String customerName, Integer amount) {
	Double discount = 0.0;
	if (amount > 10 || CustomerService.isImportantCustomer(customerName)) {
	discount = 25.0;
	}
	return discount;
	}
	}

view raw HardToTestClass1.java hosted with ❤ by GitHub

We have a class that calculates a discount for a customer based on his name and the amount of product that he's buying.

The problem hides in a static method call CustomerService.isImportantCustomer(). In the old code we can see a lot of static methods. Let's say that the one in our example is calling the database.
We need to mock it in order to write a proper unit test.

First of all, we will extract the static call to a separate method:

	public class HardToTestClass {
	public Double calculateDiscount(String customerName, Integer amount) {
	Double discount = 0.0;
	if (amount > 10 || isImportantCustomer(customerName)) {
	discount = 25.0;
	}
	return discount;
	}
	boolean isImportantCustomer(String customerName) {
	return CustomerService.isImportantCustomer(customerName);
	}
	}

view raw HardToTestClass.java hosted with ❤ by GitHub

Refactoring using IDE (extracting methods, renaming etc.) is considered to be safe. Once we did that we can use a nice feature of Mockito - spies. 
If we declare our tested class as a spy, we can mock its method the same way as with standard mocks. The test for our class could look like:

	@RunWith(MockitoJUnitRunner.class)
	public class HardToTestClassTest {
	private static final double ZERO_DISCOUNT = 0.0;
	@Spy
	private HardToTestClass hardToTestClass;
	private String customerName = "Neal Stephenson";
	private Integer lowAmount = 1;
	@Test
	public void shouldReturnZeroDiscountIfAmountIsLowAndCustomerIsNotImportantOne() {
	// given
	given(hardToTestClass.isImportantCustomer(customerName)).willReturn(false);
	// when
	Double result = hardToTestClass.calculateDiscount(customerName, lowAmount);
	// then
	assertThat(result, is(ZERO_DISCOUNT));
	}
	}

view raw HardToTestClassTest.java hosted with ❤ by GitHub

The Mockito API for spies is similar to the one for mocks (the syntax is different for overriding void methods).