Stubbing server responses with wiremock for functional testing a web app

If, like me, you want to run functional tests against a stub server, or fake server as some call it, the easiest way I’ve found is to simply start a wiremock server in standalone mode before running your tests.

• Download the wiremock standalone jar from Maven
• In whatever build tool you’re using, ensure you start wiremock before your functional tests run. I’m using NPM as a build tool so my package.json looks a bit like this:

{
  "scripts": {
    "prefunc-web": "npm run fakes",
    "func-web": "intern-runner config=functionaltests/intern-web",
    "postfunc-web": "npm run killfakes",

    "fakes": "nohup java -jar ./functionaltests/wiremock-standalone-2.3.1.jar --port 1234 --root-dir wiremock > wiremock.log &",
    "killfakes": "curl -X POST http://localhost:1234/__admin/shutdown"
  },
  "devDependencies": {
    ...
  }
}

NPM is often overlooked as a build tool, and the pre and post hooks here are very useful. They allow tasks to be dependent on other tasks or arbitrary commands, simply by adding the right prefix. So pretest will always run before the test script. Similarly posttest will always run after. I’m using these hooks here to start the wiremock fake server before executing my tests.

Canned wiremock responses

The --root-dir wiremock flag is being used here to point the fake server to some pre-canned responses that are setup in that directory. This can be useful for local testing, but could also be used in functional tests, although I wouldn’t recommend this as its usually best to set up your state in the test, rather than rely on global expected state such as these canned responses.

For example, in /wiremock/mappings/default.json

{
  "priority": 10,
  "request": {
    "method": "ANY",
    "urlPattern": ".*"
  },
  "response": {
    "status": 200,
    "headers": {
      "Content-Type": "application/json",
      "Access-Control-Allow-Origin": "*"
    },
    "jsonBody": {
      "someResponseProperty": "some response value",
      "someListOfThings": [{
        "ohLook": "a thing"
      }]
    },
    "fixedDelayMilliseconds": 2000
  }
}

Here we are defining a default response that wiremock will return in response to any request. You can change the urlPattern property to make it specific to a single endpoint in your stubbed out API. For example "urlPattern": "/resource/thing" would mean wiremock would only respond with the defined response when http://localhost:1234/resource/thing is called.