How to use mockConstruction method of org.mockito.Mockito class

What is mockconstruction in Mockito?

With the mockConstruction you can mock calls made to the constructor. Let us illustrate this with an example:

1class Car {
2    public Car() {}
3    public String makeSound() {return "Vroom";}
4}
5
6@Test
7void mockingConstructor() {
8
9    // Scope of constructor mocking
10    try (MockedConstruction<Car> mock = mockConstruction(Car.class)) {
11        // creating a mock instance
12        Car car = new Car();
13        when(car.makeSound()).thenReturn("Weee");
14
15        assertThat(car.makeSound()).isEqualTo("Weee");
16
17        // Get a list of all created mocks
18        List<Car> constructed = mock.constructed();
19        assertThat(constructed).hasSize(1);
20    }
21    
22    // Normal Car instance that is not mocked
23    assertThat(new Car().makeSound()).isEqualTo("Vroom");
24
25}

We mock the Car class's constructor, which is called from the test code. Within a try statement, we limit the scope to prevent any further calls to the constructor. When your code calls the constructor inside this try statement, it returns a mock object. Outside that try statement, calls to the class's constructor will not be mocked by Mockito. This example also works when you mock a private constructor. If the Car class had a private constructor, calls to it would still succeed outside of our try-with-resources statement.

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