Introduction to WireMock

1. Overview

WireMock is a library for stubbing and mocking web services. It constructs an HTTP server that we can 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 verify its behaviors.

2. Maven Dependencies

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

<dependency>
    <groupId>org.wiremock</groupId>
    <artifactId>wiremock</artifactId>
    <version>3.9.1</version>
    <scope>test</scope>
</dependency>

3. Programmatically Managed Server

This section will cover how to manually configure a WireMock server, i.e., without the support of JUnit auto-configuration. We demonstrate the usage with a very simple stub.

3.1. Server Setup

First, we instantiate a WireMock server:

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.

Then we can start and stop the server using two simple methods:

wireMockServer.start();

and:

wireMockServer.stop();

3.2. Basic Usage

We’ll first demonstrate the WireMock library with 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();

The 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 afterward:

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

We’ll 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 that the response arriving at the client is exactly what was sent:

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

Finally, we should stop the WireMock server to release system resources:

wireMockServer.stop();

4. JUnit Managed Server

This section illustrates the usage of a WireMock server with the help of JUnit 5. In JUnit 5, the @Rule annotation used in JUnit 4 is replaced by lifecycle methods, such as @BeforeEach and @AfterEach, or custom extensions.

4.1. Server Setup

Using lifecycle methods, we can manage the WireMock server’s lifecycle in JUnit test cases, ensuring the server starts before each test and stops after completion.

Similar to the programmatically managed server, we create a WireMock server as a Java object:

private WireMockServer wireMockServer;

@BeforeEach
void setup() {
    wireMockServer = new WireMockServer(WireMockConfiguration.options().port(port));
    wireMockServer.start();
    WireMock.configureFor("localhost", port);
}

@AfterEach
void teardown() {
    if (wireMockServer != null) {
        wireMockServer.stop();
    }
}

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

4.2. URL Matching

After setting up a WireMock server instance, the next step is to configure a stub.

In this subsection, we’ll provide a REST stub for a service endpoint using a regular expression:

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

Next, we create an HTTP client, execute a request, and capture the 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 uses a helper method to convert the HttpResponse to a String:

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;
}

Finally, we verify the stub’s behavior using the following assertions:

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 by matching 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 from 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

We can also use the WireMock library 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’s 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 defined before, convertInputStreamToString.

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

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

And we can configure and run HTTP requests and responses:

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’s 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 can customize that behavior to take more control of WireMock stubs.

We’ll demonstrate the 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);
}

First, we 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, we create an HTTP client and execute a request using the helper method:

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)));

Now we’ll carry out the 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 article 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.