Mocking void methods in JUnit - Build an Automation Script

Verify void method invocation using Mockito

Preconditions (2)

Step 1: Create a mock of the class containing the void method

Step 2: Stub the void method to do nothing or throw an exception if needed

Step 3: Call the void method on the mock object

Step 4: Verify that the void method was called exactly once

✅ Expected Result: The void method is successfully mocked and verified to be called once without errors

Automation Requirements - JUnit 5 with Mockito

Assertions Needed:

Verify void method invocation count using Mockito.verify()

Best Practices:

Use Mockito.mock() to create mock objects

Use Mockito.doNothing() or Mockito.doThrow() for stubbing void methods

Use Mockito.verify() to check method calls

Keep test methods small and focused

Automated Solution

JUnit

import static org.mockito.Mockito.*;
import org.junit.jupiter.api.Test;

class Service {
    void performAction() {
        // some action
    }
}

public class ServiceTest {

    @Test
    void testPerformActionCalledOnce() {
        // Create mock
        Service mockService = mock(Service.class);

        // Stub void method to do nothing (optional, default is do nothing)
        doNothing().when(mockService).performAction();

        // Call the void method
        mockService.performAction();

        // Verify the method was called once
        verify(mockService, times(1)).performAction();
    }
}

The test imports Mockito static methods for easy mocking and verification.

We define a simple Service class with a void method performAction().

In the test method, we create a mock of Service using mock().

We stub the void method with doNothing() to explicitly show the approach, though Mockito does nothing by default for void methods.

We call the void method on the mock.

Finally, we verify that performAction() was called exactly once using verify() with times(1).

This ensures the void method was invoked as expected without side effects.

Common Mistakes - 3 Pitfalls

Using when().thenReturn() to stub void methods

Not verifying the void method call

Calling real method on mock instead of stubbed method

Bonus Challenge

Now add data-driven testing to verify a void method is called multiple times with different mock objects

Show Hint

Practice

1. What is the main purpose of mocking void methods in JUnit tests?

easy

A. To convert void methods into methods that return values

B. To control their behavior and verify interactions without executing actual code

C. To automatically generate return values for void methods

D. To disable all tests that call void methods

Solution

1. Step 1: Understand void methods in tests

   Void methods do not return values, so mocking helps control their side effects during tests.

2. Step 2: Purpose of mocking void methods

   Mocking allows skipping actual execution and verifying if the method was called, without running real code.

3. Final Answer:

   To control their behavior and verify interactions without executing actual code -> Option B

4. Quick Check:

   Mock void methods = control behavior + verify calls [OK]

Hint: Mock void methods to control calls without running code [OK]

Common Mistakes:

• Thinking void methods return values
• Assuming mocking changes method return type
• Believing mocking disables all tests

2. Which Mockito syntax correctly mocks a void method sendEmail() to do nothing when called?

easy

A. doNothing().when(mock).sendEmail();

B. verify(mock).doNothing().sendEmail();

C. mock.sendEmail().doNothing();

D. when(mock.sendEmail()).thenReturn(null);

Solution

1. Step 1: Recall Mockito syntax for void methods

   Mockito uses doNothing().when(mock).method() to mock void methods doing nothing.

2. Step 2: Analyze options

   doNothing().when(mock).sendEmail(); matches correct syntax; others misuse when() or verify().

3. Final Answer:

   doNothing().when(mock).sendEmail(); -> Option A

4. Quick Check:

   Mock void method to do nothing = doNothing().when() [OK]

Hint: Use doNothing().when(mock).voidMethod() to mock void methods [OK]

Common Mistakes:

• Using when() with void methods incorrectly
• Trying to chain doNothing() after method call
• Confusing verify() with mocking syntax

3. Given the code snippet:

doThrow(new RuntimeException()).when(mock).deleteFile();
mock.deleteFile();

What happens when mock.deleteFile() is called?

medium

A. The test fails to compile

B. The method executes normally without exception

C. The method returns null

D. A RuntimeException is thrown

Solution

1. Step 1: Understand doThrow() usage

   doThrow() tells Mockito to throw the specified exception when the void method is called.

2. Step 2: Analyze method call effect

   Calling mock.deleteFile() triggers the RuntimeException as mocked.

3. Final Answer:

   A RuntimeException is thrown -> Option D

4. Quick Check:

   doThrow() causes exception on void method call [OK]

Hint: doThrow() makes void method throw exception when called [OK]

Common Mistakes:

• Expecting method to run normally
• Thinking void methods return null
• Confusing doThrow() with doNothing()

4. Identify the error in this Mockito code snippet:

doNothing().when(mock.deleteFile());

medium

A. Incorrect use of when() with method call inside

B. Missing exception type in doNothing()

C. doNothing() cannot be used with void methods

D. No error, code is correct

Solution

1. Step 1: Check Mockito syntax for mocking void methods

   Correct syntax is doNothing().when(mock).method(), not passing method call inside when().

2. Step 2: Identify error in code

   Code incorrectly calls method inside when(), causing syntax error.

3. Final Answer:

   Incorrect use of when() with method call inside -> Option A

4. Quick Check:

   when() expects mock object, not method call [OK]

Hint: Use when(mock).method(), not when(method()) [OK]

Common Mistakes:

• Calling method inside when() instead of after when(mock)
• Thinking doNothing() needs exception type
• Believing doNothing() can't mock void methods

5. You want to verify that a void method logAction() was called exactly twice during a test. Which Mockito code correctly does this?

hard

A. verify(mock).logAction(times(2));

B. doNothing().when(mock).logAction(); verify(mock).logAction();

C. verify(mock, times(2)).logAction();

D. when(mock.logAction()).thenReturn(2);

Solution

1. Step 1: Recall Mockito verify syntax for call count

   Use verify(mock, times(n)).method() to check method called n times.

2. Step 2: Analyze options

   verify(mock, times(2)).logAction(); uses correct syntax; others misuse verify or when with void methods.

3. Final Answer:

   verify(mock, times(2)).logAction(); -> Option C

4. Quick Check:

   Verify call count = verify(mock, times(n)) [OK]

Hint: Use verify(mock, times(n)) to check void method call count [OK]

Common Mistakes:

• Using when() with void methods to check calls
• Misplacing times() inside method call
• Not specifying times() for multiple calls