How to Use doNothing in Mockito with JUnit Tests

In Mockito with JUnit, use doNothing() to stub void methods that you want to ignore during testing. It tells Mockito to do nothing when the method is called, avoiding side effects or exceptions. Use it with doNothing().when(mock).voidMethod() syntax.

Syntax

The doNothing() method is used to stub void methods in Mockito. It is combined with when() to specify which method call should do nothing.

• doNothing(): Tells Mockito to do nothing when the stubbed method is called.
• when(mock).voidMethod(): Specifies the void method on the mock object to stub.

java

doNothing().when(mockObject).voidMethod();

Example

This example shows how to use doNothing() to stub a void method sendEmail() in a mocked EmailService. The test verifies that the method is called without throwing exceptions or performing real actions.

java

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

class EmailService {
    void sendEmail(String recipient) {
        // Actual sending logic
        System.out.println("Sending email to " + recipient);
    }
}

public class EmailServiceTest {

    @Test
    void testSendEmailDoesNothing() {
        EmailService mockEmailService = mock(EmailService.class);

        // Stub sendEmail to do nothing
        doNothing().when(mockEmailService).sendEmail(anyString());

        // Call the method
        mockEmailService.sendEmail("[e-mail]");

        // Verify it was called once
        verify(mockEmailService, times(1)).sendEmail("[e-mail]");
    }
}

Common Pitfalls

Common mistakes when using doNothing() include:

• Trying to use when(mock.voidMethod()).thenReturn() which does not work for void methods.
• Not using doNothing() with when() syntax for void methods.
• Forgetting to verify the method call if you want to check interaction.

Correct usage requires doNothing().when(mock).voidMethod() instead of when(mock.voidMethod()).thenReturn().

java

/* Wrong way - causes compilation error or runtime exception */
// when(mockEmailService.sendEmail(anyString())).thenReturn(null); // invalid for void

/* Right way */
doNothing().when(mockEmailService).sendEmail(anyString());

Quick Reference

Use doNothing() to stub void methods that should not perform any action during tests. Combine it with when() and the mock method call. Always verify calls if needed.

Usage	Description
doNothing().when(mock).voidMethod();	Stub void method to do nothing when called
verify(mock).voidMethod();	Check that the void method was called
doThrow(exception).when(mock).voidMethod();	Stub void method to throw exception

Key Takeaways

Use doNothing() with when() to stub void methods in Mockito.

doNothing() prevents side effects or exceptions from void methods during tests.

Never use thenReturn() with void methods; it causes errors.

Always verify void method calls if you want to check interactions.

doNothing() is useful for ignoring void methods that have no test impact.