Extending WireMock

WireMock can be customised via a variety of extension points.

Each extension point is defined by an interface that extends from Extension and extension implementations are loaded at startup time.

At present, the following extension interfaces are available:

  • RequestFilterV2/AdminRequestFilterV2/StubRequestFilterV2: Intercept requests, modifying them or taking alternative actions based on their content.
  • ResponseDefinitionTransformerV2: Modify the response definition used to generate a response. See Transforming responses.
  • ResponseTransformerV2: Modify the response served to the client. See Transforming responses.
  • ServeEventListener: Listen for events at various points in the request processing lifecycle. See Listening for Serve Events.
  • AdminApiExtension: Add admin API functions. See Admin API Extensions.
  • RequestMatcherExtension: Implement custom request matching logic. See Custom matching.
  • GlobalSettingsListener: Listen for changes to the settings object. See Listening for Settings Changes.
  • StubLifecycleListener: Listen for changes to the stub mappings. See Listening for Stub Changes.
  • TemplateHelperProviderExtension: Provide custom Handlebars helpers to the template engine. See Adding Template Helpers.
  • TemplateModelDataProviderExtension: Provide additional data to the model passed to response templates. See Adding Template Model Data.
  • MappingsLoaderExtension: Provide additional source to load the stub mappings. See Adding Mappings Loader.

The interfaces in this list ending with V2 supercede deprecated equivalents with an older, more restrictive interface. Additionally ServeEventListener deprecates PostServeAction.

As of WireMock version 3.6.0, the Extension interface has two new lifecycle methods called start() and stop(). The start() method is called on each extension when the WireMock server first starts (just before the stub mappings are loaded) and the stop() method is called when the server is stopped. This allows extensions to perform any initialisation or cleanup tasks.

Registering Extensions #

You can directly register the extension programmatically via its class name, class or an instance:

new WireMockServer(wireMockConfig()
  .extensions("com.mycorp.BodyContentTransformer", "com.mycorp.HeaderMangler"));

new WireMockServer(wireMockConfig()
  .extensions(BodyContentTransformer.class, HeaderMangler.class));

new WireMockServer(wireMockConfig()
  .extensions(new BodyContentTransformer(), new HeaderMangler()));

See Running as a Standalone Process for details on running with extensions from the command line.

Factories #

You can also register an extension factory, which allows an extension to be created with various core WireMock services passed to the constructor:

new WireMockServer(wireMockConfig()
  .extensions(services ->
                        new MiscInfoApi(

Services currently available to extension factories are:

  • Admin: the main WireMock functional interface for stubbing, verification and configuration tasks.
  • Options: the configuration object built at startup.
  • Stores: the root interface for gaining access to the various stores of WireMock’s state and creating/using custom stores.
  • FileSource: the __files directory where larger response body files are often kept.
  • Extensions: the service for creating and providing extension implementations.
  • TemplateEngine: the Handlebars template engine.

Extension registration via service loading #

Extensions that are packaged with the relevant Java service loader framework metadata will be loaded automatically if they are placed on the classpath.

See https://github.com/wiremock/wiremock/tree/master/test-extension for an example of such an extension.

Attaching sub-events during request processing #

Sub-events are a used to report interesting/useful information during request processing. WireMock attaches the diff report generated when a request is not matched as a sub-event, and custom extension can exploit this approach to surface e.g. diagnostic and validation data in the serve event log, where it can be retrieved later via the API or exported to monitoring/observability tools via listeners.

Several types of extension act on WireMock’s request processing: RequestFilterV2 (and its stub/admin sub-interfaces), ResponseDefinitionTransformer, ResponseTransformer and ServeEventListener.

The primary method in each of these takes the current ServeEvent as a parameter and sub-events can be attached to this:

  Map.of("message", "Single quotes are not permitted")

The second parameter to appendSubEvent() can be a Map or object containing any data required.