Introduction to WireMock

1. Overview

WireMock is a library for stubbing and mocking web services. It constructs a HTTP server that we could connect to as we would to an actual web service.

When a WireMock server is in action, we can set up expectations, call the service, and then verify its behaviors.

2. Maven Dependencies

In order to be able to take advantage of the WireMock library, we need to include the following dependency in the POM:

<dependency>
    <groupId>com.github.tomakehurst</groupId>
    <artifactId>wiremock</artifactId>
    <version>1.58</version>
    <scope>test</scope>
</dependency>

3. Programmatically Managed Server

This section will cover the way to manually configure a WireMock server. i.e. without the support of JUnit auto-configuration. The usage is demonstrated by a very simple stub.

3.1. Server Setting-Up

A WireMock server can be instantiated like this:

WireMockServer wireMockServer = new WireMockServer(String host, int port);

In case no arguments are provided, the server host defaults to localhost and the server port to 8080.

The server may then be started and stopped using two simple methods:

wireMockServer.start();

And:

wireMockServer.stop();

3.2. Basic Usage

The WireMock library will be firstly demonstrated by a basic usage, where a stub for an exact URL without any further configuration is provided. Let’s create a server instance:

WireMockServer wireMockServer = new WireMockServer();

WireMock server must be running before the client connects to it:

wireMockServer.start();

The web service is then stubbed:

configureFor("localhost", 8080);
stubFor(get(urlEqualTo("/baeldung")).willReturn(aResponse().withBody("Welcome to Baeldung!")));

This tutorial makes use of the Apache HttpClient API to represent a client connecting to the server:

CloseableHttpClient httpClient = HttpClients.createDefault();

A request is executed and a response is returned, respectively, afterwards:

HttpGet request = new HttpGet("http://localhost:8080/baeldung");
HttpResponse httpResponse = httpClient.execute(request);

We will convert the httpResponse variable to a String using a helper method:

String responseString = convertResponseToString(httpResponse);

Here is the implementation of that conversion helper method:

private String convertResponseToString(HttpResponse response) throws IOException {
    InputStream responseStream = response.getEntity().getContent();
    Scanner scanner = new Scanner(responseStream, "UTF-8");
    String responseString = scanner.useDelimiter("\\Z").next();
    scanner.close();
    return responseString;
}

The following code verifies that the server has got a request to the expected URL and the response arriving at the client is exactly what was sent:

verify(getRequestedFor(urlEqualTo("/baeldung")));
assertEquals("Welcome to Baeldung!", stringResponse);

Finally, the WireMock server should be stopped to release system resources:

wireMockServer.stop();

4. JUnit Managed Server

In contrast to section 3, this section illustrates the usage of a WireMock server with the help of JUnit Rule.

4.1. Server Setting-Up

A WireMock server can be integrated into JUnit test cases by using the @Rule annotation. This allows JUnit to manage the lifecycle, starting the server prior to each test method and stopping it after the method returns.

Similar to the programmatically managed server, a JUnit managed WireMock server can be created as a Java object with the given port number:

@Rule
public WireMockRule wireMockRule = new WireMockRule(int port);

If no arguments are supplied, server port will take the default value, 8080. Server host, defaulting to localhost, and other configurations may be specified using the Options interface.

4.2. URL Matching

After setting up a WireMockRule instance, the next step is to configure a stub. In this subsection, we will provide a REST stub for a service endpoint using regular expression:

stubFor(get(urlPathMatching("/baeldung/.*"))
  .willReturn(aResponse()
  .withStatus(200)
  .withHeader("Content-Type", "application/json")
  .withBody("\"testing-library\": \"WireMock\"")));

Let’s move on to creating an HTTP client, executing a request and receive a response:

CloseableHttpClient httpClient = HttpClients.createDefault();
HttpGet request = new HttpGet("http://localhost:8080/baeldung/wiremock");
HttpResponse httpResponse = httpClient.execute(request);
String stringResponse = convertHttpResponseToString(httpResponse);

The above code snippet takes advantage of a conversion helper method:

private String convertHttpResponseToString(HttpResponse httpResponse) throws IOException {
    InputStream inputStream = httpResponse.getEntity().getContent();
    return convertInputStreamToString(inputStream);
}

This in turn makes use of another private method:

private String convertInputStreamToString(InputStream inputStream) {
    Scanner scanner = new Scanner(inputStream, "UTF-8");
    String string = scanner.useDelimiter("\\Z").next();
    scanner.close();
    return string;
}

The stub’s operations are verified by the testing code below:

verify(getRequestedFor(urlEqualTo("/baeldung/wiremock")));
assertEquals(200, httpResponse.getStatusLine().getStatusCode());
assertEquals("application/json", httpResponse.getFirstHeader("Content-Type").getValue());
assertEquals("\"testing-library\": \"WireMock\"", stringResponse);

4.3. Request Header Matching

Now we will demonstrate how to stub a REST API with the matching of headers. Let’s start with the stub configuration:

stubFor(get(urlPathEqualTo("/baeldung/wiremock"))
  .withHeader("Accept", matching("text/.*"))
  .willReturn(aResponse()
  .withStatus(503)
  .withHeader("Content-Type", "text/html")
  .withBody("!!! Service Unavailable !!!")));

Similar to the preceding subsection, we illustrate HTTP interaction using the HttpClient API, with help of the same helper methods:

CloseableHttpClient httpClient = HttpClients.createDefault();
HttpGet request = new HttpGet("http://localhost:8080/baeldung/wiremock");
request.addHeader("Accept", "text/html");
HttpResponse httpResponse = httpClient.execute(request);
String stringResponse = convertHttpResponseToString(httpResponse);

The following verification and assertions confirm functions of the stub we created before:

verify(getRequestedFor(urlEqualTo("/baeldung/wiremock")));
assertEquals(503, httpResponse.getStatusLine().getStatusCode());
assertEquals("text/html", httpResponse.getFirstHeader("Content-Type").getValue());
assertEquals("!!! Service Unavailable !!!", stringResponse);

4.4. Request Body Matching

The WireMock library can also be used to stub a REST API with body matching. Here is the configuration for a stub of this kind:

stubFor(post(urlEqualTo("/baeldung/wiremock"))
  .withHeader("Content-Type", equalTo("application/json"))
  .withRequestBody(containing("\"testing-library\": \"WireMock\""))
  .withRequestBody(containing("\"creator\": \"Tom Akehurst\""))
  .withRequestBody(containing("\"website\": \"wiremock.org\""))
  .willReturn(aResponse()
  .withStatus(200)));

Now, it is time to create a StringEntity object that will be used as the body of a request:

InputStream jsonInputStream 
  = this.getClass().getClassLoader().getResourceAsStream("wiremock_intro.json");
String jsonString = convertInputStreamToString(jsonInputStream);
StringEntity entity = new StringEntity(jsonString);

The code above uses one of the conversion helper methods define before, convertInputStreamToString.

Here is content of the wiremock_intro.json file on the classpath:

{
    "testing-library": "WireMock",
    "creator": "Tom Akehurst",
    "website": "wiremock.org"
}

HTTP requests and responses can be configured and executed as follows:

CloseableHttpClient httpClient = HttpClients.createDefault();
HttpPost request = new HttpPost("http://localhost:8080/baeldung/wiremock");
request.addHeader("Content-Type", "application/json");
request.setEntity(entity);
HttpResponse response = httpClient.execute(request);

This is the testing code used to validate the stub:

verify(postRequestedFor(urlEqualTo("/baeldung/wiremock"))
  .withHeader("Content-Type", equalTo("application/json")));
assertEquals(200, response.getStatusLine().getStatusCode());

4.5. Stub Priority

The previous subsections deal with situations where an HTTP request matches only a single stub. It would be more complicated if there is more than a match for a request. By default, the most recently added stub will take precedence in such a case. However, users are allowed to customize that behavior to take more control of WireMock stubs.

We will demonstrate operations of a WireMock server when a coming request matches two different stubs, with and without setting the priority level, at the same time. Both scenarios will use the following private helper method:

private HttpResponse generateClientAndReceiveResponseForPriorityTests() throws IOException {
    CloseableHttpClient httpClient = HttpClients.createDefault();
    HttpGet request = new HttpGet("http://localhost:8080/baeldung/wiremock");
    request.addHeader("Accept", "text/xml");
    return httpClient.execute(request);
}

Firstly, configure two stubs without consideration of the priority level:

stubFor(get(urlPathMatching("/baeldung/.*"))
  .willReturn(aResponse()
  .withStatus(200)));
stubFor(get(urlPathEqualTo("/baeldung/wiremock"))
  .withHeader("Accept", matching("text/.*"))
  .willReturn(aResponse()
  .withStatus(503)));

Next, create an HTTP client and execute a request using the helper method described right above:

HttpResponse httpResponse = generateClientAndReceiveResponseForPriorityTests();

The following code snippet verifies that the last configured stub is applied regardless of the one defined before when a request matches both of them:

verify(getRequestedFor(urlEqualTo("/baeldung/wiremock")));
assertEquals(503, httpResponse.getStatusLine().getStatusCode());

Let’s move on to stubs with priority levels being set, where a lower number represents a higher priority:

stubFor(get(urlPathMatching("/baeldung/.*"))
  .atPriority(1)
  .willReturn(aResponse()
  .withStatus(200)));
stubFor(get(urlPathEqualTo("/baeldung/wiremock"))
  .atPriority(2)
  .withHeader("Accept", matching("text/.*"))
  .willReturn(aResponse()
  .withStatus(503)));

Creation and execution of an HTTP request:

HttpResponse httpResponse = generateClientAndReceiveResponseForPriorityTests();

The following code validates the effect of priority levels, where the first configured stub is applied instead of the last:

verify(getRequestedFor(urlEqualTo("/baeldung/wiremock")));
assertEquals(200, httpResponse.getStatusLine().getStatusCode());

5. Conclusion

This tutorial introduced WireMock and how to set up as well as configure this library for testing of REST APIs using various techniques, including matching of URL, request headers and body.

The implementation of all the examples and code snippets can be found in a GitHub project.