Mocking static methods with Mockito

From Mockito 3.4.0, it is possible to mock static method even in Junit 5 thus reducing the dependency on using PowerMock.

To use this, add dependency with mockito-inline >= 3.4.0 and remove dependency on mockito-core if it exists.

Let’s consider the usecase where we need to write unit Test for the Lambda class created in this blog where we used Guice.createInjector for dependency injection.

1. Mock Injector

@Mock
private Injector injector;

1. Create a @BeforeEach method which mocks the Guice class and initializes the Lambda class

@BeforeEach
public void setup() {

    try(MockedStatic<Guice> guiceMockedStatic = mockStatic(Guice.class)) {
        guiceMockedStatic.when(() -> Guice.createInjector(any(ApplicationModule.class))).thenReturn(injector);
        Mockito.doReturn(bookService).when(injector).getInstance(Mockito.eq(UserService.class));
        testLambda = new TestLambda(); // or perform any assert operation if testing the static method directly
    }
}

The above example provides mocking a static method with arguments.

To mock a static method without arguments

try(MockedStatic<TestStatic> mockedstatic = mockStatic(TestStatic.class)) {
    mockedstatic.when(TestStatic::getTestData).thenReturn("TestData");
    assetEquals("TestData", TestStatic.getTestData());
}

Note that if you want to test a static method you have created, it is worth investigating if it can be refactored and make it more testable as it hints a code smell.

We need to mock static methods sometimes especially when we are working with the methods in dependent library eg. Guice in this post.