Unit tests should run in complete isolation. When you write a unit test make sure it does not depend on any external resource or other unit tests. Examples of external resources can be a database, the file system, a REST or SOAP web service, etc.

When the component under test has a hard dependency on a resource (it cannot be run without it), you can create a “fake” copy of the dependency and define its behavior for each test case. This practice is called Mocking and many libraries and frameworks make it possible to mock dependencies and define their behavior. Powermock, Mockito and Easymock are example of libraries you can use with JUnit/Java .

Example​

Consider the following java class which represents a “simplified” BankAccount.

public class BankAccount {

    private AccountService accountService;

    public BankAccount(AccountService accountService){
        this.accountService = AccountService();
    }

    public BigDecimal getBalance(Integer accountNumber) {
        return accountService.getBalance(accountNumber);
    }
}

In the following test is not isolated. Why ? we want to test that the BankAccount.getBalance() method returns the correct balance. We do not care about the AccountService class here (Remember, Test one thing at a time). However, the test class seems to create a real instance of AccountService. This can cause problems since we depend on a “real” service even if it is “dev” service. Moreover, Running against a real service can corrupt data or create unwanted side effects.

class BankAccountTest {

    private AccountService accountService;
    private BankAccount bankAccountInstance;
    private final Integer ACCOUNT_NUMBER = 123456789;
    

    @BeforeEach
    void init(){
      accountService = AccountService.getInstance(); // Real instance !
      bankAccountInstance = new BankAccount(accountService);
    }

    @Test
    void getBalance_Should_Return_Correct_Balance(){
        final BigDecimal EXPECTED_AMT = 100.00;

        BigDecimal actualBalance = bankAccountInstance.getBalance(ACCOUNT_NUMBER);

        assertEquals(EXPECTED_AMT, actualBalance);
    }

Improved example​

To improve the BankAccountTest class we can mock the AccountService class like in the following example.

class BankAccountTest {

    @Mock
    private AccountService accountServiceMock;

    private BankAccount bankAccountInstance;
    private final Integer ACCOUNT_NUMBER = 123456789;
    

    @BeforeEach
    void init(){
      accountServiceMock = mock(AccountService.class); // create a mock
      bankAccountInstance = new BankAccount(accountServiceMock); // Use the mock
      when(accountServiceMock.getBalance(ACCOUNT_NUMBER)).thenReturn(EXPECTED_AMT); // Define the behavior
    }

    @Test
    void getBalance_Should_Return_Correct_Balance(){
        final BigDecimal EXPECTED_AMT = 100.00;

        BigDecimal actualBalance = bankAccountInstance.getBalance(ACCOUNT_NUMBER);

        assertEquals(EXPECTED_AMT, actualBalance);
    }