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May 22, 2022

How to write Spring JUnit tests using @InjectMocks and @Mock

Steps

1. Create the test file
2. Initialize an instance of the class to test using @InjectMocks and @Mock
3. Write the JUnit test method

Create the test file

Consider a new class Store.java and we want to test the getMaleCashiers() method:

/* Store.java */
public class Store() {

  private String id;

  @Autowired
  private CashierServiceImpl cashierService;

  public Store(String id) {
    this.id = id;
  }

  public List<Cashier> getMaleCashiers() throws CacheException {
    // Get list of cashiers by store id from cache or database
    List<Cashier> cashierList = cashierService.getByStoreId(this.id);

    // Filter and return list of male cashiers
    return cashierList
      .stream()
      .filter(c -> c.getGender().equalsIgnoreCase('male'))
      .collect(Collectors.toList()));
  }
}

Note that Store has a private member variable CashierServiceImpl which is initialized through Spring’s dependency injection framework using @Autowired.

We start by creating StoreTest.java which will contain our JUnit test. The test class is annotated with @RunWith(SpringJUnit4ClassRunner.class):

/* StoreTest.java */
@RunWith(SpringJUnit4ClassRunner.class)
public class StoreTest() {
  // TODO: Initialize objects
  // TODO: Write test cases
}

Initialize an instance of the class to test using @InjectMocks and @Mock

We declare the member variables store and cashierServiceImpl in the test class.

By annotating store with @InjectMocks and cashierServiceImpl with @Mock, store will have a mock instance of cashierServiceImpl instead of null.

Finally, we also need to call MockitoAnnotations.initMocks(this) to initialize the mock variables before the test is run. One way to do this is by calling it in a method annotated with @Before:

/* StoreTest.java */
@RunWith(SpringJUnit4ClassRunner.class)
public class StoreTest() {

  @InjectMocks
  Store store;

  @Mock
  CashierServiceImpl cashierService;

  @Before
  public void initMocks() {
    // Initialize mock variables
    MockitoAnnotations.initMocks(this);
  }

}

Write the JUnit test

Let’s now write the test for the store.getMaleCashiers(). To recap:

/* Store.java */
public class Store {
  // ...

  public List<Cashier> getMaleCashiers() throws CacheException {
    // Get list of cashiers by store id from cache or database
    List<Cashier> cashierList = cashierService.getByStoreId(this.id);

    // Return male cashiers
    return cashierList
      .stream()
      .filter(c -> c.getGender().equalsIgnoreCase('male'))
      .collect(Collectors.toList()));
  }

}

cashierService.getByStoreId(this.id) retrieves the list of cashiers associated with the given store id from either the cache or database.

For our test on store.getMaleCashers():

1. we are not concerned with the underlying implementation of cashierService.getByStoreId(this.id)
2. cashierService.getByStoreId(this.id) does not need to retrieve the result from an actual cache or database when we run the test

As such, we can use Mockito’s when and thenReturn methods to mock the result when cashierService.getByStoreId(this.id) is being called during testing.

What we are concerned with is the following line that filters cashierList by male cashiers and returns the correct result.

As such, here we have the complete code for our test:

/* StoreTest.java */
@RunWith(SpringJUnit4ClassRunner.class)
public class StoreTest() {

  @InjectMocks
  Store store;

  @Mock
  CashierServiceImpl cashierService;

  @Before
  public void initMocks() {
    // Initialize mock variables
    MockitoAnnotations.initMocks(this);
  }

  @Test
  public void getMaleCashiersTest_shouldReturnOne() throws CacheException {
    // CashierList
    List<Cashier> cashierList = new ArrayList<>();

    Cashier maleCashier = new Cashier();
    maleCashier.setGender("male");
    cashierList.add(maleCashier);

    Cashier femaleCashier = new Cashier();
    femaleCashier.setGender("female");
    cashierList.add(femaleCashier);

    // Mock getByStoreId(this.id) to return cashierList that we have created
    when(cashierService.getByStoreId(any())).thenReturn(cashierList);

    // Test and assert
    List<Cashier> result = store.getMaleCashiers();
    assertEquals(1, result.size());
  }

}

Other ways to initialize the Store instance

In the above example, we have seen how we can initialize Store using @InjectMocks and @Mocks.

There are also other ways to initialize Store in the test class:

1. Manually initialize a Store instance
2. Use @Autowired on store member variable

Manually initialize a Store instance

@RunWith(SpringJUnit4ClassRunner.class)
public void StoreTest() {

  Store store;

  public StoreTest() {
    this.store = new Store("Cheers");
  }

  // @Test
  // ...

}

The issue with manually initializing is that you may not be able to initialize the private member variables of Store:

1. Store does not take in an instance of CashierServiceImpl in its constructor
2. CashierServiceImpl is a private member variable which is initialized by Spring’s dependency injection framework using @Autowired

Use @Autowired on store member variable

Well, you may then consider to annotate Store with Autowired instead and let Spring do its own dependency injection magic thingy. However it works because we don’t really care…do we?

@RunWith(SpringJUnit4ClassRunner.class)
public void StoreTest() {

  @Autowired
  Store store;

  // @Test
  // ...

}

Unfortunately, you see errors in your console log when you try to run StoreTest.

Resolving these errors may or may not be straightforward, depending on how complex the member variables are. You will need to declare @ComponentScan so that the autowired variables can be initialized when the test is run.

On top of that, component scanning can result in longer compile time when running the test file, especially for a large codebase.