Testing Mastery: JUnit, Mockito & Integration Tests · Lesson

Mocking Static Methods and Constructors

Explore advanced techniques to mock static methods, final classes, and constructors using Mockito extensions like `mockito-inline`.

Mocking Static Methods and Constructors is a free Testing Mastery: JUnit, Mockito & Integration Tests lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Testing Mastery: JUnit, Mockito & Integration Tests learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Beyond Regular Mocks

Welcome! So far, you've learned to mock interfaces and regular classes using Mockito. But what about those tricky bits of code that seem resistant to testing?

Sometimes, you encounter static methods, final classes, or direct constructor calls (new MyObject()) within the code you want to test. Standard Mockito can't handle these directly.

When Traditional Mocks Fail

Why do we need to mock these?

Static utility methods: Often used for common tasks, but hard to isolate if they have side effects or external dependencies.
Final classes/methods: Cannot be subclassed or overridden, making traditional proxy-based mocking impossible.
Constructor calls (new): Direct object creation within a method makes it hard to inject a mock instead.

Mocking these helps you isolate the code under test, even in complex or legacy systems.

Powering Up Mockito

To overcome these limitations, Mockito offers advanced capabilities through its inline mock maker, often referred to as mockito-inline.

This feature uses byte-code manipulation to "rewrite" classes at runtime, allowing Mockito to mock things it normally can't, such as static methods, final classes, and even constructors.

It's a powerful tool, but use it thoughtfully!

Setting Up Your Project

For Mockito versions 3.4.0 and newer, the inline mock maker is often enabled by default or available simply by using the standard mockito-core dependency.

If you're using an older version or encountering issues, you might need to explicitly declare mockito-inline. For most modern setups, just ensure you have a recent mockito-core version.

Here's how your build.gradle might look:

dependencies {
    testImplementation 'org.junit.jupiter:junit-jupiter-api:5.10.0'
    testRuntimeOnly 'org.junit.jupiter:junit-jupiter-engine:5.10.0'
    testImplementation 'org.mockito:mockito-core:5.8.0' // Includes inline mock maker
    // For older Mockito versions, you might need:
    // testImplementation 'org.mockito:mockito-inline:5.8.0'
}

Mocking Static Helpers

To mock static methods, Mockito provides the MockedStatic interface. You create a mock context using Mockito.mockStatic() within a try-with-resources block. This ensures the mock is active only for the duration of that block.

Let's see an example with a utility class:

import org.mockito.Mockito;
import org.mockito.MockedStatic;

// Utility class with a static method
class MyStaticService {
    public static String getGreeting() {
        return "Hello from real static service!";
    }
    public static int add(int a, int b) {
        return a + b;
    }
}

public class Main {
    public static void main(String[] args) {
        System.out.println("Before mock: " + MyStaticService.getGreeting());

        // Mock the static method within a try-with-resources
        try (MockedStatic<MyStaticService> mockedStatic = Mockito.mockStatic(MyStaticService.class)) {
            mockedStatic.when(MyStaticService::getGreeting).thenReturn("Hello from mocked static!");
            mockedStatic.when(() -> MyStaticService.add(1, 2)).thenReturn(100);

            System.out.println("During mock (greeting): " + MyStaticService.getGreeting());
            System.out.println("During mock (add): " + MyStaticService.add(1, 2));
            System.out.println("During mock (add other): " + MyStaticService.add(5, 5)); // Not mocked, returns 0 by default

            // Verify calls (typically in a test assertion)
            mockedStatic.verify(MyStaticService::getGreeting);
            mockedStatic.verify(() -> MyStaticService.add(1, 2), Mockito.times(1));

        } // Mock is closed here

        System.out.println("After mock: " + MyStaticService.getGreeting());
    }
}

Checking Static Interactions

Just like with regular mocks, you can verify interactions with static methods. Use the verify() method on your MockedStatic object.

This ensures that your code under test called the static method as expected, with the correct arguments and number of times.

Notice the Mockito.times(1) in the previous example to check invocation count.

import org.mockito.Mockito;
import org.mockito.MockedStatic;

// Class from previous scene
class MyStaticService {
    public static String getGreeting() { return "Hello from real static!"; }
    public static int multiply(int a, int b) { return a * b; }
}

public class Main {
    public static void main(String[] args) {
        // Simulate a scenario where a static method is called
        System.out.println("Initial call: " + MyStaticService.multiply(2, 3));

        try (MockedStatic<MyStaticService> mockedStatic = Mockito.mockStatic(MyStaticService.class)) {
            mockedStatic.when(() -> MyStaticService.multiply(2, 3)).thenReturn(777);
            mockedStatic.when(() -> MyStaticService.multiply(5, 5)).thenReturn(123);

            // Call the static method through the mock
            System.out.println("Mocked call 1: " + MyStaticService.multiply(2, 3));
            System.out.println("Mocked call 2: " + MyStaticService.multiply(5, 5));

            // Verify specific calls
            mockedStatic.verify(() -> MyStaticService.multiply(2, 3), Mockito.times(1));
            mockedStatic.verify(() -> MyStaticService.multiply(5, 5), Mockito.atLeastOnce());
            // Verify that a different call was NOT made
            mockedStatic.verify(() -> MyStaticService.multiply(10, 10), Mockito.never());

            System.out.println("Verifications completed.");

        } // Mock is closed here
    }
}

Taming Final Classes

The mockito-inline mock maker also allows you to mock final classes and final methods. This is particularly useful when dealing with third-party libraries or legacy code where you can't easily change the class design.

You mock final classes just like regular classes: Mockito.mock(FinalClass.class).

import org.mockito.Mockito;

// A final class that cannot be extended normally
final class FinalProcessor {
    public final String process(String input) {
        return "Real processed: " + input.toUpperCase();
    }
    public int getVersion() {
        return 1;
    }
}

public class Main {
    public static void main(String[] args) {
        FinalProcessor realProcessor = new FinalProcessor();
        System.out.println("Real output: " + realProcessor.process("data"));
        System.out.println("Real version: " + realProcessor.getVersion());

        // Mocking a final class
        FinalProcessor mockProcessor = Mockito.mock(FinalProcessor.class);

        // Stubbing a final method
        Mockito.when(mockProcessor.process("data")).thenReturn("Mocked processed: data");
        Mockito.when(mockProcessor.getVersion()).thenReturn(99);

        System.out.println("Mocked output: " + mockProcessor.process("data"));
        System.out.println("Mocked version: " + mockProcessor.getVersion());

        // Verify interaction with the mocked final class
        Mockito.verify(mockProcessor).process("data");
    }
}

Intercepting New Objects

What if your code creates new objects directly using new MyObject()? MockedConstruction lets you intercept these calls and return mock instances instead of real ones.

This is crucial for testing classes that internally manage their dependencies rather than receiving them via dependency injection.

import org.mockito.Mockito;
import org.mockito.MockedConstruction;

// A class that creates another object internally
class DependentService {
    private final Helper helper;

    public DependentService() {
        this.helper = new Helper(); // Direct constructor call
    }

    public String doWork() {
        return "Service working with: " + helper.getGreeting();
    }
}

// The class whose constructor we want to mock
class Helper {
    public String getGreeting() {
        return "Real Helper greeting";
    }
}

public class Main {
    public static void main(String[] args) {
        System.out.println("Without mock:");
        DependentService realService = new DependentService();
        System.out.println(realService.doWork());

        System.out.println("\nWith mocked construction:");
        // Mock the constructor of Helper
        try (MockedConstruction<Helper> mockedConstruction = Mockito.mockConstruction(Helper.class,
                (mock, context) -> {
                    // This block runs every time new Helper() is called
                    Mockito.when(mock.getGreeting()).thenReturn("Mocked Helper greeting!");
                    System.out.println("Helper constructor intercepted!");
                })) {

            // When DependentService() is called, it calls new Helper(),
            // which now returns our stubbed mock.
            DependentService serviceWithMockedHelper = new DependentService();
            System.out.println(serviceWithMockedHelper.doWork());

            // You can get all constructed mocks
            Helper constructedMock = mockedConstruction.constructed().get(0);
            Mockito.verify(constructedMock).getGreeting(); // Verify interaction with the mock
            System.out.println("Verified mock interaction.");

        } // MockedConstruction is closed here
    }
}

Use With Care

While powerful, mocking statics and constructors can be an indicator of a design that's hard to test.

Design Smell: Heavily relying on these mocks might suggest your classes are too tightly coupled or not following dependency injection principles.
Readability: Tests using these advanced mocks can be harder to understand and maintain.
When to use: Best for legacy code, third-party libraries, or when refactoring isn't immediately feasible. Prioritize refactoring for testability when possible!

Advanced Mocking Check

Which of the following statements about mockito-inline and advanced mocking techniques are TRUE?

Advanced Mocking Recap

Great job! You've explored advanced Mockito techniques:

The mockito-inline mock maker enables mocking of statics, final classes, and constructors.
MockedStatic allows you to stub and verify static method calls within a defined scope.
MockedConstruction helps intercept and replace objects created via the new keyword.

These tools are powerful for testing challenging code, but remember to consider underlying design patterns. Next, you'll learn about Mockito best practices!

Frequently asked questions

Is the “Mocking Static Methods and Constructors” lesson free?

Yes — the full text of “Mocking Static Methods and Constructors” is free to read here on the web, and the Testing Mastery: JUnit, Mockito & Integration Tests course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Testing Mastery: JUnit, Mockito & Integration Tests course, upgrade to CoddyKit PRO.

What will I learn in “Mocking Static Methods and Constructors”?

Explore advanced techniques to mock static methods, final classes, and constructors using Mockito extensions like `mockito-inline`. You practise Testing Mastery: JUnit, Mockito & Integration Tests with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

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No prior experience is required. Testing Mastery: JUnit, Mockito & Integration Tests on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Mocking Static Methods and Constructors” lesson take?

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