Baeldung Pro – NPI EA (cat = Baeldung)
announcement - icon

Baeldung Pro comes with both absolutely No-Ads as well as finally with Dark Mode, for a clean learning experience:

>> Explore a clean Baeldung

Once the early-adopter seats are all used, the price will go up and stay at $33/year.

Partner – Microsoft – NPI EA (cat = Baeldung)
announcement - icon

Azure Container Apps is a fully managed serverless container service that enables you to build and deploy modern, cloud-native Java applications and microservices at scale. It offers a simplified developer experience while providing the flexibility and portability of containers.

Of course, Azure Container Apps has really solid support for our ecosystem, from a number of build options, managed Java components, native metrics, dynamic logger, and quite a bit more.

To learn more about Java features on Azure Container Apps, visit the documentation page.

You can also ask questions and leave feedback on the Azure Container Apps GitHub page.

Partner – Microsoft – NPI EA (cat= Spring Boot)
announcement - icon

Azure Container Apps is a fully managed serverless container service that enables you to build and deploy modern, cloud-native Java applications and microservices at scale. It offers a simplified developer experience while providing the flexibility and portability of containers.

Of course, Azure Container Apps has really solid support for our ecosystem, from a number of build options, managed Java components, native metrics, dynamic logger, and quite a bit more.

To learn more about Java features on Azure Container Apps, you can get started over on the documentation page.

And, you can also ask questions and leave feedback on the Azure Container Apps GitHub page.

Partner – Orkes – NPI EA (cat=Spring)
announcement - icon

Modern software architecture is often broken. Slow delivery leads to missed opportunities, innovation is stalled due to architectural complexities, and engineering resources are exceedingly expensive.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

With Orkes Conductor managed through Orkes Cloud, developers can focus on building mission critical applications without worrying about infrastructure maintenance to meet goals and, simply put, taking new products live faster and reducing total cost of ownership.

Try a 14-Day Free Trial of Orkes Conductor today.

Partner – Orkes – NPI EA (tag=Microservices)
announcement - icon

Modern software architecture is often broken. Slow delivery leads to missed opportunities, innovation is stalled due to architectural complexities, and engineering resources are exceedingly expensive.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

With Orkes Conductor managed through Orkes Cloud, developers can focus on building mission critical applications without worrying about infrastructure maintenance to meet goals and, simply put, taking new products live faster and reducing total cost of ownership.

Try a 14-Day Free Trial of Orkes Conductor today.

eBook – Guide Spring Cloud – NPI EA (cat=Spring Cloud)
announcement - icon

Let's get started with a Microservice Architecture with Spring Cloud:

>> Join Pro and download the eBook

eBook – Mockito – NPI EA (tag = Mockito)
announcement - icon

Mocking is an essential part of unit testing, and the Mockito library makes it easy to write clean and intuitive unit tests for your Java code.

Get started with mocking and improve your application tests using our Mockito guide:

Download the eBook

eBook – Java Concurrency – NPI EA (cat=Java Concurrency)
announcement - icon

Handling concurrency in an application can be a tricky process with many potential pitfalls. A solid grasp of the fundamentals will go a long way to help minimize these issues.

Get started with understanding multi-threaded applications with our Java Concurrency guide:

>> Download the eBook

eBook – Reactive – NPI EA (cat=Reactive)
announcement - icon

Spring 5 added support for reactive programming with the Spring WebFlux module, which has been improved upon ever since. Get started with the Reactor project basics and reactive programming in Spring Boot:

>> Join Pro and download the eBook

eBook – Java Streams – NPI EA (cat=Java Streams)
announcement - icon

Since its introduction in Java 8, the Stream API has become a staple of Java development. The basic operations like iterating, filtering, mapping sequences of elements are deceptively simple to use.

But these can also be overused and fall into some common pitfalls.

To get a better understanding on how Streams work and how to combine them with other language features, check out our guide to Java Streams:

>> Join Pro and download the eBook

eBook – Jackson – NPI EA (cat=Jackson)
announcement - icon

Do JSON right with Jackson

Download the E-book

eBook – HTTP Client – NPI EA (cat=Http Client-Side)
announcement - icon

Get the most out of the Apache HTTP Client

Download the E-book

eBook – Maven – NPI EA (cat = Maven)
announcement - icon

Get Started with Apache Maven:

Download the E-book

eBook – Persistence – NPI EA (cat=Persistence)
announcement - icon

Working on getting your persistence layer right with Spring?

Explore the eBook

eBook – RwS – NPI EA (cat=Spring MVC)
announcement - icon

Building a REST API with Spring?

Download the E-book

Course – LS – NPI EA (cat=Jackson)
announcement - icon

Get started with Spring and Spring Boot, through the Learn Spring course:

>> LEARN SPRING
Course – RWSB – NPI EA (cat=REST)
announcement - icon

Explore Spring Boot 3 and Spring 6 in-depth through building a full REST API with the framework:

>> The New “REST With Spring Boot”

Course – LSS – NPI EA (cat=Spring Security)
announcement - icon

Yes, Spring Security can be complex, from the more advanced functionality within the Core to the deep OAuth support in the framework.

I built the security material as two full courses - Core and OAuth, to get practical with these more complex scenarios. We explore when and how to use each feature and code through it on the backing project.

You can explore the course here:

>> Learn Spring Security

Course – All Access – NPI EA (cat= Spring)
announcement - icon

All Access is finally out, with all of my Spring courses. Learn JUnit is out as well, and Learn Maven is coming fast. And, of course, quite a bit more affordable. Finally.

>> GET THE COURSE
Course – LSD – NPI EA (tag=Spring Data JPA)
announcement - icon

Spring Data JPA is a great way to handle the complexity of JPA with the powerful simplicity of Spring Boot.

Get started with Spring Data JPA through the guided reference course:

>> CHECK OUT THE COURSE

Partner – LambdaTest – NPI EA (cat=Testing)
announcement - icon

End-to-end testing is a very useful method to make sure that your application works as intended. This highlights issues in the overall functionality of the software, that the unit and integration test stages may miss.

Playwright is an easy-to-use, but powerful tool that automates end-to-end testing, and supports all modern browsers and platforms.

When coupled with LambdaTest (an AI-powered cloud-based test execution platform) it can be further scaled to run the Playwright scripts in parallel across 3000+ browser and device combinations:

>> Automated End-to-End Testing With Playwright

Course – Spring Sale 2025 – NPI EA (cat= Baeldung)
announcement - icon

Yes, we're now running our Spring Sale. All Courses are 25% off until 26th May, 2025:

>> EXPLORE ACCESS NOW

Course – Spring Sale 2025 – NPI (cat=Baeldung)
announcement - icon

Yes, we're now running our Spring Sale. All Courses are 25% off until 26th May, 2025:

>> EXPLORE ACCESS NOW

1. Overview

In this short tutorial, we focus on mocking void methods with Mockito.

Further reading:

Mockito Support for Optional, Streams, Lambda Expressions

Overview of Java 8 support in Mockito framework, including Streams and default interface methods

Mocking Exception Throwing using Mockito

Learn to configure a method call to throw an exception in Mockito.

As with other articles focused on the Mockito framework (such as Mockito Verify, Mockito When/Then, and Mockito’s Mock Methods), the MyList class shown below will be used as the collaborator in test cases.

We’ll add a new method for this tutorial:

public class MyList extends AbstractList<String> {
 
    @Override
    public void add(int index, String element) {
        // no-op
    }
}

2. Simple Mocking and Verifying

Void methods can be used with Mockito’s doNothing(), doThrow(), and doAnswer() methods, making mocking and verifying intuitive:

@Test
public void whenAddCalled_thenVerified() {
    MyList myList = mock(MyList.class);
    doNothing().when(myList).add(isA(Integer.class), isA(String.class));
    myList.add(0, "");
 
    verify(myList, times(1)).add(0, "");
}

However, doNothing() is Mockito’s default behavior for void methods.

This version of whenAddCalledVerified() accomplishes the same thing as the one above:

@Test
void whenAddCalled_thenVerified() {
    MyList myList = mock(MyList.class);
    myList.add(0, "");
 
    verify(myList, times(1)).add(0, "");
}

doThrow() generates an exception:

@Test
void givenNull_whenAddCalled_thenThrowsException() {
    MyList myList = mock(MyList.class);
    
    assertThrows(Exception.class, () -> {
	doThrow().when(myList).add(isA(Integer.class), isNull());
    });

    myList.add(0, null);
}

We’ll cover doAnswer() below.

3. Argument Capture

One reason to override the default behavior with doNothing() is to capture arguments.

In the example above, we used the verify() method to check the arguments passed to add().

However, we may need to capture the arguments and do something more with them.

In these cases, we use doNothing() just as we did above, but with an ArgumentCaptor:

@Test
void givenArgumentCaptor_whenAddCalled_thenValueCaptured() {
    MyList myList = mock(MyList.class);
    
    ArgumentCaptor<String> valueCapture = ArgumentCaptor.forClass(String.class);
    doNothing().when(myList).add(any(Integer.class), valueCapture.capture());
    
    myList.add(0, "captured");
    
    assertEquals("captured", valueCapture.getValue());
}

4. Answering a Call to Void

A method may perform more complex behavior than merely adding or setting value.

For these situations, we can use Mockito’s Answer to add the behavior we need:

@Test
void givenDoAnswer_whenAddCalled_thenAnswered() {
    MyList myList = mock(MyList.class);
    
    doAnswer(invocation -> {
        Object arg0 = invocation.getArgument(0);
        Object arg1 = invocation.getArgument(1);
        
        assertEquals(3, arg0);
        assertEquals("answer me", arg1);
        return null;
    }).when(myList).add(any(Integer.class), any(String.class));
    
    myList.add(3, "answer me");
}

As explained in Mockito’s Java 8 Features, we use a lambda with Answer to define custom behavior for add().

5. Calling the Original void Method

In some cases, although we mocked an object, we would like to call an original void method during our testing. This is also called partial mocking.

Let’s see an example:

class Greeting {
    private static final Logger LOG = LoggerFactory.getLogger(Greeting.class);

    public void sayHello(String name) {
        LOG.info("Hi {}, how are you?", name);
    }
}

As the code shows, the Greeting class has a void method sayHello(). Next, let’s see how to call this method through a mock.

5.1. Using doCallRealMethod()

As the name implies, Mockito’s doCallRealMethod() can be used for calling the original void methods:

@Test
void whenDoCallRealMethodOnMock_thenRealMethodCalled() {
    Greeting greeting = mock(Greeting.class);
    doCallRealMethod().when(greeting).sayHello(any(String.class));
    greeting.sayHello("Tom");
    verify(greeting, times(1)).sayHello("Tom");
}

If we run the test, it passes with the output:

14:39:21.111 [main] INFO com.baeldung...Greeting -- Hi Tom, how are you?

Therefore, we call the original void method using doCallRealMethod() through a mock and verify() it.

5.2. Mock vs. Spy

Sometimes, we might get unexpected results when we leverage doCallRealMethod() to invoke the original void method through a mock object. Let’s see an example:

class Greeting {
    private static final Logger LOG = //... unchanged code
    private Instant timestamp = Instant.now();
  
    public void sayHello(String name) //... unchanged code

    public void sayHelloWithTs(String name) {
        LOG.info("Hi {}, how are you? [Instance Created at {}]", name, timestamp);
    }
}

As the code shows, we add a new void method, sayHelloWithTs(), to the Greeting class. This method’s implementation references an instance variable, timestamp, which records the object creation time.

Now, let’s call this method through a mock:

@Test
void whenDoCallRealMethodWithPropertyOnMock_thenRealMethodCalled() {
    Greeting greeting = mock(Greeting.class);
    doCallRealMethod().when(greeting).sayHelloWithTs(any(String.class));
    greeting.sayHelloWithTs("Jerry");
    verify(greeting, times(1)).sayHelloWithTs("Jerry");
}

The test passes, so sayHelloWithTs() has been invoked once. Let’s examine the output:

21:33:32.464 [main] INFO com.baeldung...Greeting -- Hi Jerry, how are you? [Instance Created at null] 

The log output shows the creation timestamp as null, although we know it won’t be null. This is because when Mockito creates a mock object using mock(), it doesn’t initialize instance variables. A mock object isn’t a real instance of the class. Instead, it is a proxy or dummy object that simulates the class’s behavior under test without actually invoking its internal logic.

To solve the problem, we can call the void method through a spy instead of a mock since spy() copies the instance state over to create a more realistic interaction:

@Test
void whenDoCallRealMethodOnSpy_thenGetExpectedResult() {
    Greeting greeting = spy(Greeting.class);
    doCallRealMethod().when(greeting).sayHello(any(String.class));
    greeting.sayHello("Tom");
    verify(greeting, times(1)).sayHello("Tom");
 
    doCallRealMethod().when(greeting).sayHelloWithTs(any(String.class));
    greeting.sayHelloWithTs("Jerry");
    verify(greeting, times(1)).sayHelloWithTs("Jerry");
}

In this example, we create a spy on Greeting using spy() and call the two void methods via the spy. Let’s see the output we get after executing this test:

21:52:26.516 [main] INFO com.baeldung...Greeting -- Hi Tom, how are you?
21:52:26.521 [main] INFO com.baeldung...Greeting -- Hi Jerry, how are you? [Instance Created at 2024-09-24T19:52:26.498859Z]

So, we get the expected result.

6. Conclusion

In this brief article, we covered four different ways to approach void methods when testing with Mockito.

The code backing this article is available on GitHub. Once you're logged in as a Baeldung Pro Member, start learning and coding on the project.
Baeldung Pro – NPI EA (cat = Baeldung)
announcement - icon

Baeldung Pro comes with both absolutely No-Ads as well as finally with Dark Mode, for a clean learning experience:

>> Explore a clean Baeldung

Once the early-adopter seats are all used, the price will go up and stay at $33/year.

Partner – Microsoft – NPI EA (cat = Spring Boot)
announcement - icon

Azure Container Apps is a fully managed serverless container service that enables you to build and deploy modern, cloud-native Java applications and microservices at scale. It offers a simplified developer experience while providing the flexibility and portability of containers.

Of course, Azure Container Apps has really solid support for our ecosystem, from a number of build options, managed Java components, native metrics, dynamic logger, and quite a bit more.

To learn more about Java features on Azure Container Apps, visit the documentation page.

You can also ask questions and leave feedback on the Azure Container Apps GitHub page.

Partner – Orkes – NPI EA (cat = Spring)
announcement - icon

Modern software architecture is often broken. Slow delivery leads to missed opportunities, innovation is stalled due to architectural complexities, and engineering resources are exceedingly expensive.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

With Orkes Conductor managed through Orkes Cloud, developers can focus on building mission critical applications without worrying about infrastructure maintenance to meet goals and, simply put, taking new products live faster and reducing total cost of ownership.

Try a 14-Day Free Trial of Orkes Conductor today.

Partner – Orkes – NPI EA (tag = Microservices)
announcement - icon

Modern software architecture is often broken. Slow delivery leads to missed opportunities, innovation is stalled due to architectural complexities, and engineering resources are exceedingly expensive.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

With Orkes Conductor managed through Orkes Cloud, developers can focus on building mission critical applications without worrying about infrastructure maintenance to meet goals and, simply put, taking new products live faster and reducing total cost of ownership.

Try a 14-Day Free Trial of Orkes Conductor today.

eBook – HTTP Client – NPI EA (cat=HTTP Client-Side)
announcement - icon

The Apache HTTP Client is a very robust library, suitable for both simple and advanced use cases when testing HTTP endpoints. Check out our guide covering basic request and response handling, as well as security, cookies, timeouts, and more:

>> Download the eBook

eBook – Java Concurrency – NPI EA (cat=Java Concurrency)
announcement - icon

Handling concurrency in an application can be a tricky process with many potential pitfalls. A solid grasp of the fundamentals will go a long way to help minimize these issues.

Get started with understanding multi-threaded applications with our Java Concurrency guide:

>> Download the eBook

eBook – Java Streams – NPI EA (cat=Java Streams)
announcement - icon

Since its introduction in Java 8, the Stream API has become a staple of Java development. The basic operations like iterating, filtering, mapping sequences of elements are deceptively simple to use.

But these can also be overused and fall into some common pitfalls.

To get a better understanding on how Streams work and how to combine them with other language features, check out our guide to Java Streams:

>> Join Pro and download the eBook

eBook – Persistence – NPI EA (cat=Persistence)
announcement - icon

Working on getting your persistence layer right with Spring?

Explore the eBook

Course – LS – NPI EA (cat=REST)

announcement - icon

Get started with Spring Boot and with core Spring, through the Learn Spring course:

>> CHECK OUT THE COURSE

Course – Spring Sale 2025 – NPI EA (cat= Baeldung)
announcement - icon

Yes, we're now running our Spring Sale. All Courses are 25% off until 26th May, 2025:

>> EXPLORE ACCESS NOW

Course – Spring Sale 2025 – NPI (All)
announcement - icon

Yes, we're now running our Spring Sale. All Courses are 25% off until 26th May, 2025:

>> EXPLORE ACCESS NOW

eBook – Mockito – NPI (tag=Mockito)
announcement - icon

Mocking is an essential part of unit testing, and the Mockito library makes it easy to write clean and intuitive unit tests for your Java code.

Get started with mocking and improve your application tests using our Mockito guide:

>> Download the eBook

eBook Jackson – NPI EA – 3 (cat = Jackson)