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

These days, we expect to call REST APIs in most of our services. Spring provides a few options for building a REST client, and WebClient is recommended.

In this quick tutorial, we’ll learn how to unit test services that use WebClient to call APIs.

2. Mocking

We have two main options for mocking in our tests:

3. Using Mockito

Mockito is the most common mocking library for Java. It’s good at providing pre-defined responses to method calls, but things get challenging when mocking fluent APIs. This is because in a fluent API, a lot of objects pass between the calling code and the mock.

For example, let’s have an EmployeeService class with a getEmployeeById method fetch data via HTTP using WebClient:

public class EmployeeService {

    public EmployeeService(String baseUrl) {
        this.webClient = WebClient.create(baseUrl);
    }
    public Mono<Employee> getEmployeeById(Integer employeeId) {
        return webClient
                .get()
                .uri("http://localhost:8080/employee/{id}", employeeId)
                .retrieve()
                .bodyToMono(Employee.class);
    }
}

We can use Mockito to mock this:

@ExtendWith(MockitoExtension.class)
public class EmployeeServiceUnitTest {
   
    @Test
    void givenEmployeeId_whenGetEmployeeById_thenReturnEmployee() {

        Integer employeeId = 100;
        Employee mockEmployee = new Employee(100, "Adam", "Sandler", 
          32, Role.LEAD_ENGINEER);
        when(webClientMock.get())
          .thenReturn(requestHeadersUriSpecMock);
        when(requestHeadersUriMock.uri("/employee/{id}", employeeId))
          .thenReturn(requestHeadersSpecMock);
        when(requestHeadersMock.retrieve())
          .thenReturn(responseSpecMock);
        when(responseMock.bodyToMono(Employee.class))
          .thenReturn(Mono.just(mockEmployee));

        Mono<Employee> employeeMono = employeeService.getEmployeeById(employeeId);

        StepVerifier.create(employeeMono)
          .expectNextMatches(employee -> employee.getRole()
            .equals(Role.LEAD_ENGINEER))
          .verifyComplete();
    }

}

As we can see, we need to provide a different mock object for each call in the chain, with four different when/thenReturn calls required. This is verbose and cumbersome. It also requires us to know the implementation details of how exactly our service uses WebClient, making this a brittle way of testing.

So how can we write better tests for WebClient?

4. Using MockWebServer

MockWebServer, built by the Square team, is a small web server that can receive and respond to HTTP requests.

Interacting with MockWebServer from our test cases allows our code to use real HTTP calls to a local endpoint. We get the benefit of testing the intended HTTP interactions, and none of the challenges of mocking a complex fluent client.

Using MockWebServer is recommended by the Spring Team for writing integration tests.

4.1. MockWebServer Dependencies

To use MockWebServer, we need to add the Maven dependencies for both okhttp and mockwebserver to our pom.xml:

<dependency>
    <groupId>com.squareup.okhttp3</groupId>
    <artifactId>okhttp</artifactId>
    <version>5.0.0-alpha.12</version>
    <scope>test</scope>
</dependency>
<dependency>
    <groupId>com.squareup.okhttp3</groupId>
    <artifactId>mockwebserver</artifactId>
    <version>5.0.0-alpha.12</version>
    <scope>test</scope>
</dependency>

4.2. Adding MockWebServer to Our Test

Let’s test our EmployeeService with MockWebServer:

public class EmployeeServiceIntegrationTest {

    public static MockWebServer mockBackEnd;

    @BeforeAll
    static void setUp() throws IOException {
        mockBackEnd = new MockWebServer();
        mockBackEnd.start();
    }

    @AfterAll
    static void tearDown() throws IOException {
        mockBackEnd.shutdown();
    }
}

In the above JUnit Test class, the setUp and tearDown method takes care of creating and shutting down the MockWebServer.

The next step is to map the port of the actual REST service call to the MockWebServer’s port:

@BeforeEach
void initialize() {
    String baseUrl = String.format("http://localhost:%s", 
      mockBackEnd.getPort());
    employeeService = new EmployeeService(baseUrl);
}

Now it’s time to create a stub so that the MockWebServer can respond to an HttpRequest.

4.3. Stubbing a Response

Let’s use MockWebServer’s handy enqueue method to queue a test response on the webserver:

@Test
void getEmployeeById() throws Exception {
    Employee mockEmployee = new Employee(100, "Adam", "Sandler", 
      32, Role.LEAD_ENGINEER);
    mockBackEnd.enqueue(new MockResponse()
      .setBody(objectMapper.writeValueAsString(mockEmployee))
      .addHeader("Content-Type", "application/json"));

    Mono<Employee> employeeMono = employeeService.getEmployeeById(100);

    StepVerifier.create(employeeMono)
      .expectNextMatches(employee -> employee.getRole()
        .equals(Role.LEAD_ENGINEER))
      .verifyComplete();
}

When the actual API call is made from the getEmployeeById(Integer employeeId) method in our EmployeeService class, MockWebServer will respond with the queued stub.

4.4. Checking a Request

We may also want to make sure that the MockWebServer was sent the correct HttpRequest.

MockWebServer has a handy method named takeRequest that returns an instance of RecordedRequest:

RecordedRequest recordedRequest = mockBackEnd.takeRequest();
 
assertEquals("GET", recordedRequest.getMethod());
assertEquals("/employee/100", recordedRequest.getPath());

With RecordedRequest, we can verify the HttpRequest that was received to make sure our WebClient sent it correctly.

5. Conclusion

In this article, we demonstrated the two main options available to mock WebClient based REST client code.

While Mockito worked, and may be a good option for simple examples, the recommended approach is to use MockWebServer.

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

Partner – Microsoft – NPI (cat=Spring)
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.

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