Baeldung Pro – NPI EA (cat = Baeldung)
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Baeldung Pro comes with both absolutely No-Ads as well as finally with Dark Mode, for a clean learning experience:

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

Partner – Microsoft – NPI EA (cat = Baeldung)
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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)
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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)
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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)
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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)
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Let's get started with a Microservice Architecture with Spring Cloud:

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eBook – Mockito – NPI EA (tag = Mockito)
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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:

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eBook – Java Concurrency – NPI EA (cat=Java Concurrency)
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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:

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eBook – Reactive – NPI EA (cat=Reactive)
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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:

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eBook – Java Streams – NPI EA (cat=Java Streams)
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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:

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eBook – Jackson – NPI EA (cat=Jackson)
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Do JSON right with Jackson

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eBook – HTTP Client – NPI EA (cat=Http Client-Side)
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Get the most out of the Apache HTTP Client

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Get Started with Apache Maven:

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eBook – Persistence – NPI EA (cat=Persistence)
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Working on getting your persistence layer right with Spring?

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eBook – RwS – NPI EA (cat=Spring MVC)
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Building a REST API with Spring?

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Course – LS – NPI EA (cat=Jackson)
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Get started with Spring and Spring Boot, through the Learn Spring course:

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Course – RWSB – NPI EA (cat=REST)
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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)
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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)
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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.

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Course – LSD – NPI EA (tag=Spring Data JPA)
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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:

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Partner – LambdaTest – NPI EA (cat=Testing)
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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)
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Yes, we're now running our Spring Sale. All Courses are 25% off until 26th May, 2025:

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Course – Spring Sale 2025 – NPI (cat=Baeldung)
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Yes, we're now running our Spring Sale. All Courses are 25% off until 26th May, 2025:

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1. Overview

In this tutorial, we’ll illustrate how to make the most out of spies in Mockito.

We’ll talk about the @Spy annotation and how to stub a spy. Finally, we’ll go into the difference between Mock and Spy.

Of course, for more Mockito goodness, have a look at the series here.

Further reading:

Mockito Verify Cookbook

<strong>Mockito Verify</strong> examples, usage and best practices.

Injecting Mockito Mocks into Spring Beans

This article will show how to use dependency injection to insert Mockito mocks into Spring Beans for unit testing.

Mockito's Mock Methods

This tutorial illustrates various uses of the standard static mock methods of the Mockito API.

2. Simple Spy Example

Let’s start with a simple example of how to use a spy.

Simply put, the API is Mockito.spy() to spy on a real object.

This will allow us to call all the normal methods of the object while still tracking every interaction, just as we would with a mock.

Now let’s do a quick example where we’ll spy on an existing ArrayList object:

@Test
void givenUsingSpyMethod_whenSpyingOnList_thenCorrect() {
    List<String> list = new ArrayList<String>();
    List<String> spyList = spy(list);

    spyList.add("one");
    spyList.add("two");

    verify(spyList).add("one");
    verify(spyList).add("two");

    assertThat(spyList).hasSize(2);
}

Note how the real method add() is actually called and how the size of spyList becomes 2.

3. The @Spy Annotation

Next, let’s see how to use the @Spy annotation. We can use the @Spy annotation instead of spy():

@Spy
List<String> spyList = new ArrayList<String>();

@Test
void givenUsingSpyAnnotation_whenSpyingOnList_thenCorrect() {
    spyList.add("one");
    spyList.add("two");

    verify(spyList).add("one");
    verify(spyList).add("two");

    assertThat(aSpyList).hasSize(2);
}

To enable Mockito annotations (such as @Spy, @Mock, … ), we need to use @ExtendWith(MockitoExtension.class) that initializes mocks and handles strict stubbings.

4. Stubbing a Spy

Now let’s see how to stub a Spy. We can configure/override the behavior of a method using the same syntax we would use with a mock.

Here we’ll use doReturn() to override the size() method:

@Test
void givenASpy_whenStubbingTheBehaviour_thenCorrect() {
    List<String> list = new ArrayList<String>();
    List<String> spyList = spy(list);

    assertEquals(0, spyList.size());

    doReturn(100).when(spyList).size();
    assertThat(spyList).hasSize(100);
}

5. Mock vs Spy in Mockito

Let’s discuss the difference between Mock and Spy in Mockito. We won’t examine the theoretical differences between the two concepts, just how they differ within Mockito itself.

When Mockito creates a mock, it does so from the Class of a Type, not from an actual instance. The mock simply creates a bare-bones shell instance of the Class, entirely instrumented to track interactions with it.

On the other hand, the spy will wrap an existing instance. It will still behave in the same way as the normal instance; the only difference is that it will also be instrumented to track all the interactions with it.

Here we’ll create a mock of the ArrayList class:

@Test
void whenCreateMock_thenCreated() {
    List mockedList = mock(ArrayList.class);

    mockedList.add("one");
    verify(mockedList).add("one");

    assertThat(mockedList).hasSize(0);
}

As we can see, adding an element into the mocked list doesn’t actually add anything; it just calls the method with no other side effects.

A spy, on the other hand, will behave differently; it will actually call the real implementation of the add method and add the element to the underlying list:

@Test
void whenCreateSpy_thenCreate() {
    List spyList = Mockito.spy(new ArrayList());

    spyList.add("one");
    Mockito.verify(spyList).add("one");

    assertThat(spyList).hasSize(1);
}

6. Understanding the Mockito NotAMockException

In this final section, we’ll learn about the Mockito NotAMockException. This exception is one of the common exceptions we will likely encounter when misusing mocks or spies.

Let’s start by understanding the circumstances in which this exception can occur:

List<String> list = new ArrayList<String>();
doReturn(100).when(list).size();

When we run this code snippet, we’ll get the following error:

org.mockito.exceptions.misusing.NotAMockException: 
Argument passed to when() is not a mock!
Example of correct stubbing:
    doThrow(new RuntimeException()).when(mock).someMethod();

Thankfully, it is quite clear from the Mockito error message what the problem is here. In our example, the list object is not a mock. The Mockito when() method expects a mock or spy object as the argument.

As we can also see, the Exception message even describes what a correct invocation should look like. Now that we have a better understanding of what the problem is, let’s fix it by following the recommendation:

final List<String> spyList = spy(new ArrayList<>());
assertThatNoException().isThrownBy(() -> doReturn(100).when(spyList).size());

Our example now behaves as expected, and we no longer see the Mockito NotAMockException.

7. Conclusion

In this brief article, we discussed the most useful examples of using Mockito spies.

We learned how to create a spy, use the @Spy annotation, stub a spy, and finally, the difference between Mock and Spy.

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.

This is a Maven project, so it should be easy to import and run as it is.

Finally, for more Mockito goodness, have a look at the series here.

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)
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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)
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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)
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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)
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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)
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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)
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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)
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Working on getting your persistence layer right with Spring?

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Course – LS – NPI EA (cat=REST)

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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)
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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)
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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)
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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)