Transforming Responses

Sometimes, returning wholly static responses to stub requests isn’t practical e.g. when there are transaction IDs being passed between request/responses, dates that must be current. Via WireMock’s extension mechanism it is possible to dynamically modify responses, allowing header re-writing and templated responses amongst other things.

There are two ways to dynamically transform output from WireMock. WireMock stub mappings consist in part of a ResponseDefinition. This is essentially a description that WireMock (sometimes) combines with other information when producing the final response. A basic ResponseDefinition closely resembles an actual response in that it has status, headers and body fields, but it can also indicate to WireMock that the actual response should be the result of a proxy request to another system or a fault of some kind.

ResponseDefinition objects are “rendered” by WireMock into a Response, and it is possible to interject either before or after this process when writing an extension, meaning you can either transform the ResponseDefinition prior to rendering, or the rendered Response afterwards.

Parameters #

Transformer extensions can have parameters passed to them on a per-stub basis via a Parameters fetched by calling serveEvent.getTransformerParameters(). Parameters derives from Java’s Map and can be therefore arbitrarily deeply nested. Only types compatible with JSON (strings, numbers, booleans, maps and lists) can be used.

Response definition transformation #

To transform ResponseDefinition implement the ResponseDefinitionTransformerV2 interface:

public static class ExampleTransformer implements ResponseDefinitionTransformerV2 {

        @Override
        public ResponseDefinition transform(ServeEvent serveEvent) {
            return new ResponseDefinitionBuilder()
                    .withHeader("MyHeader", "Transformed")
                    .withStatus(200)
                    .withBody("Transformed body")
                    .build();
        }

        @Override
        public String getName() {
            return "example";
        }
    }

Transformer classes must have a no-args constructor unless you only intend to register them via an instance as described below.

Supplying parameters #

Parameters are supplied on a per stub mapping basis:

stubFor(get(urlEqualTo("/transform")).willReturn(
        aResponse()
                .withTransformerParameter("newValue", 66)
                .withTransformerParameter("inner", ImmutableMap.of("thing", "value")))); // ImmutableMap is from Guava, but any Map will do

{
    "request": {
        "url": "/transform",
        "method": "GET"
    },
    "response": {
        "status": 200,
        "transformerParameters": {
            "newValue": 66,
            "inner": {
                "thing": "value"
            }
        }
    }
}

Non-global transformations #

By default transformations will be applied globally. If you only want them to apply in certain cases you can refer to make them non-global by adding this to your transformer class:

@Override
public boolean applyGlobally() {
    return false;
}

Then you add the transformation to specific stubs via its name:

stubFor(get(urlEqualTo("/local-transform")).willReturn(aResponse()
        .withStatus(200)
        .withBody("Original body")
        .withTransformers("my-transformer", "other-transformer")));

{
    "request": {
        "method": "GET",
        "url": "/local-transform"
    },
    "response": {
        "status": 200,
        "body": "Original body",
        "transformers": ["my-transformer", "other-transformer"]
    }
}

The Java API also has a convenience method for adding transformers and parameters in one call:

stubFor(get(urlEqualTo("/transform")).willReturn(
        aResponse()
                .withTransformer("body-transformer", "newValue", 66)));

Response transformation #

A response transformer extension class is identical to ResponseDefinitionTransformerV2 with the exception that it takes a Response in its transform method’s parameter list and returns a Response.

This transformer is the best option if you want to transform the response from a proxy call.

public static class StubResponseTransformerWithParams implements ResponseTransformerV2 {

        @Override
        public Response transform(Response response, ServeEvent serveEvent) {
            Parameters parameters = serveEvent.getTransformerParameters();
            return Response.Builder.like(response)
                    .but().body(parameters.getString("name") + ", "
                            + parameters.getInt("number") + ", "
                            + parameters.getBoolean("flag"))
                    .build();
        }

        @Override
        public String getName() {
            return "stub-transformer-with-params";
        }
}