Starting with Mockito version 3.5.0, we can now mock Java constructors with Mockito. This allows us to return a mock from every object construction for testing purposes. Similar to mocking static method calls with Mockito, we can define the scope of when to return a mock from a Java constructor for a particular Java class. While the practical (and reasonable) use cases for this feature might be limited, at least it's the next step towards replacing PowerMock with Mockito entirely. Let's take a look at this feature in action.
Configuration of the Mockito InlineMockMaker
This new feature of Mockito is only available if we use an InlineMockMaker. The InlineMockMaker is an interface that extends the MockMaker interface that defines Mockito's internal API to create mocks. There are multiple implementations for this interface, and the default one is the SubclassByteBuddyMockMaker.
To override the default MockMaker and use a InlineMockMaker instead, we can replace our mockito-core dependency with mockito-inline :
| 1 2 3 4 5 6 7 | <dependency> <groupId>org.mockito</groupId> <artifactId>mockito-inline</artifactId> <!-- version needs to be >= 3.5.0 --> <version>${mockito.version}</version> <scope>test</scope> </dependency> |
If, for some reason, we are not able to add this dependency to our project and only rely on mockito-core, we can still configure the InMockMaker with an extension file.
Therefore, we can put a org.mockito.plugins.MockMaker file inside src/test/resources/mockito-extensions with the following content:
| 1 | mock-maker-inline |
For those using the Spring Boot Starter Test to manage the versions of the basic testing libraries, refer to the Mockito setup section of this article.
Mocking a Java Constructor With Mockito
Let's see how we can mock the construction of an object with Mockito. As an example, we'll use the following Java class:
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 | public class PaymentProcessor { private boolean allowForeignCurrencies; private String fallbackOption; private BigDecimal taxRate; public PaymentProcessor() { this(false, "DEBIT", new BigDecimal("19.00")); } public PaymentProcessor(String fallbackOption, BigDecimal taxRate) { this(false, fallbackOption, taxRate); } public PaymentProcessor(boolean allowForeignCurrencies, String fallbackOption, BigDecimal taxRate) { this.allowForeignCurrencies = allowForeignCurrencies; this.fallbackOption = fallbackOption; this.taxRate = taxRate; } public BigDecimal chargeCustomer(String customerId, BigDecimal netPrice) { // any arbitrary implementation System.out.println("About to charge customer: " + customerId); return BigDecimal.ZERO; } } |
Furthermore, let's assume an instance of this PaymentProcess is created manually inside the CheckoutService using the new operator.
| 1 2 3 4 5 6 7 8 9 | public class CheckoutService { public BigDecimal purchaseProduct(String productName, String customerId) { // any arbitrary implementation PaymentProcessor paymentProcessor = new PaymentProcessor(); return paymentProcessor.chargeCustomer(customerId, BigDecimal.TEN); } } |
If we used constructor/field/setter injection instead, this Mockito feature wouldn't be relevant. In such cases, we can pass the mocked version of a collaborator and don't have to mock the constructor.
When writing a unit test for the purchaseProduct method, we want to mock collaborators of our class under test. For this purpose, we can now use the new feature of Mockito and mock the object construction of the PaymentProcessor inside purchaseProduct.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 | class CheckoutServiceTest { @Test void mockObjectConstruction() { try (MockedConstruction<PaymentProcessor> mocked = Mockito.mockConstruction(PaymentProcessor.class, (mock, context) -> { // further stubbings ... when(mock.chargeCustomer(anyString(), any(BigDecimal.class))).thenReturn(BigDecimal.TEN); })) { CheckoutService cut = new CheckoutService(); BigDecimal result = cut.purchaseProduct("MacBook Pro", "42"); assertEquals(BigDecimal.TEN, result); } } } |
The new method that makes mocking object constructions possible is Mockito.mockConstruction(). This method takes a non-abstract Java class that constructions we're about to mock as a first argument. In the example above, we use an overloaded version of mockConstruction() to pass a MockInitializer as a second argument. This MockInitializer is a callback that we use to stub the behavior of the mock.
We can define the scope of mocking any object creation for our PaymentProcessor by using Java's try-with-resources construct, as the MockedConstruction is extending the AutoClosable interface. This allows mocking the object construction only for a temporary and user-defined purpose.
If you've already mocked a static method with Mockito, this setup might seem familiar.
Let's take a look at further examples of how we can use this new feature of Mockito.
Further Examples to Mock Java Constructors with Mockito
First, let's come back to the scope of mocking the object construction. Whenever we use the try-with-resources (highly recommended for this use case) construct, every constructor call inside this block is mocked. Any object creation before or after returns a real instance:
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 | class PaymentProcessorTest { @Test void mockObjectConstruction() { // a real object of the PaymentProcessor is returned System.out.println(new PaymentProcessor().chargeCustomer("42", BigDecimal.valueOf(42))); try (MockedConstruction<PaymentProcessor> mocked = mockConstruction(PaymentProcessor.class)) { // every object creation is returning a mock from now on PaymentProcessor paymentProcessor = new PaymentProcessor(); when(paymentProcessor.chargeCustomer(anyString(), any(BigDecimal.class))).thenReturn(BigDecimal.TEN); assertEquals(BigDecimal.TEN, paymentProcessor.chargeCustomer("42", BigDecimal.valueOf(42))); } // a real object of the PaymentProcessor is returned System.out.println(new PaymentProcessor().chargeCustomer("42", BigDecimal.valueOf(42))); } } |
Furthermore, as all object constructions for this class are mocked, it doesn't matter which public constructor we use:
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 | @Test void mockDifferentObjectConstruction() { try (MockedConstruction<PaymentProcessor> mocked = Mockito.mockConstruction(PaymentProcessor.class)) { PaymentProcessor firstInstance = new PaymentProcessor("PayPal", BigDecimal.TEN); PaymentProcessor secondInstance = new PaymentProcessor(true, "PayPal", BigDecimal.TEN); when(firstInstance.chargeCustomer(anyString(), any(BigDecimal.class))).thenReturn(BigDecimal.TEN); when(secondInstance.chargeCustomer(anyString(), any(BigDecimal.class))).thenReturn(BigDecimal.TEN); assertEquals(BigDecimal.TEN, firstInstance.chargeCustomer("42", BigDecimal.valueOf(42))); assertEquals(BigDecimal.TEN, secondInstance.chargeCustomer("42", BigDecimal.valueOf(42))); assertEquals(2, mocked.constructed().size()); } } |
In the test above, all object creations are mocked, independent of which constructor of PaymentProcessor we use. We can even inspect how many mock objects were created during a test using the mock controller that is returned from mockConstruction().
As the last example, let's take a look at mockConstructionWithAnswer. Using this method, we can add a default Answer and additional Answers that define our mock's behavior.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 | @Test void mockDifferentObjectConstructionWithAnswer() { try (MockedConstruction<PaymentProcessor> mocked = Mockito.mockConstructionWithAnswer(PaymentProcessor.class, // default answer for the first mock invocation -> new BigDecimal("500.00"), // additional answer for all further mocks invocation -> new BigDecimal("42.00"))) { PaymentProcessor firstInstance = new PaymentProcessor(); PaymentProcessor secondInstance = new PaymentProcessor(); PaymentProcessor thirdInstance = new PaymentProcessor(); assertEquals(new BigDecimal("500.00"), firstInstance.chargeCustomer("42", BigDecimal.ZERO)); assertEquals(new BigDecimal("42.00"), secondInstance.chargeCustomer("42", BigDecimal.ZERO)); assertEquals(new BigDecimal("42.00"), thirdInstance.chargeCustomer("42", BigDecimal.ZERO)); } } |
In our example, we can always return a BigDecimal as our PaymentProcessor only has one public method. For classes with multiple methods and different return types, either use the passed invocation to determine what to return or use the mockConstruction() that takes a MockInitializer for type-safe stubbings.
Summary of Mocking Java Constructors
As we can now mock Java constructors and static method calls with Mockito, the need for PowerMock becomes less important. With this feature, you can get rid of PowerMock if you've only used it for those two purposes and rely solely on Mockito.
However, we shouldn't jump into using this feature from now on for every test. Mocking the object construction, e.g., of internally used value objects, is an anti-pattern that we should avoid.
There shouldn't be many valid use cases for this feature whenever we favor constructor injection for our Java classes. With constructor injection, we can pass the collaborators of a class via the constructor during tests. This way, we can prepare a mock of the collaborator before instantiating the class under test and don't have to mock any constructor.
For further practical Mockito advice, consider enrolling in the Hands-On Mocking With Mockito Online Course to learn the ins and outs of the most popular mocking library for JVM applications.
You can find related Mockito articles here:
- Mocking Static Methods with Mockito (Java & Kotlin)
- @Mock vs. @MockBean When Testing Spring Boot Applications
- Maven Setup For Testing Java Applications
The source code for this blog post is available on GitHub.
Have fun mocking your Java constructors,
Philip
Mocking the constructors is the first step, is there a way to use verify on the mocked constructors?
Hi Daniel,
you can use the
.constructed()methodMockedConstruction mocked to retrieve a list of mocked objects that were created using the constructor. You can check the length of this list to verify that the constructor returned a mock (once or multiple times).I’m not aware that you can somehow use Mockito’s
verify()for advanced verifications for the constructor.Kind regards,
Philip
Hello,
That’s a very good guide.
do you know how to use mockConstruction() to mock construction of File.
see:
https://stackoverflow.com/questions/68097251/can-the-mockito3-mockconstruction-make-stub-on-new-file
Thanks.
Hi Pop Shi,
I’ll take a look into the question on Stack Overflow.
Kind regards,
Philip
I tried to mock FileInputStream that’s called internally from another class but I got
org.mockito.exceptions.base.MockitoException: Could not initialize mocked construction
at java.io.FileInputStream.(FileInputStream.java)
I posted full details on StackOverflow
https://stackoverflow.com/q/68908833/3972599
Can you please check it?
Hi Amgad,
thanks for providing the link to the question on Stack Overflow – I’ll take a look at it.
Kind regards,
Philip
Hi Philip Riecks
for a specific object verification, is this the right way
try (MockedConstruction spwMock = Mockito.mockConstruction(TestClass1.class)) {
final TestClass1 testClass1 = actualClassToTest.get();
List obl =
spwMock.constructed().stream().filter(obj -> obj instanceof TestClass1)
.collect(Collectors.toList());
Assert.assertEquals(obl.get(0), testClass1);
}
Comparing it with PowerMock:
TestClass1 expected = Mockito.mock(TestClass1.class);PowerMockito.whenNew(TestClass1.class)
.withAnyArguments().thenReturn(expected);
final TestClass1 response = actualClassToTest.get();
Assert.assertEquals(expected, response);
You can do it that way but I would wonder why you want to verify that Mockito was able to construct the object creation. You’re close to testing the Mockito framework itself with this test.
Checking the size of
spwMock.constructed()can also be shorter.Is there a way to make the constructor call throw an exception? Similar to PowerMock’s whenNew(clazz).thenThrow(exceptionInstance)?
I have class that creates an instance of RandomAccessFile which can throw an IOException from its constructor. For one particular testcase I need to force the constructor of RandomAccessFile to throw a FileNotFoundException. I can’t seem to find where in the documentation it tells how to do this.
Hi David,
I’m not aware of a similar feature within Mockito. Is there any chance you can refactor your code to make your constructor side-effect free and introduce, e.g., an initialize() method?
Kind regards,
Philip
It’s great to find content like this.
Thank you for this useful information.
Some really excellent info, I look forward to the continuation.