Tuesday, August 10, 2010

Testing with Lift's TestFramework

Getting started



HTTP testing is sufficiently tedious that some folks don't do it. Even if we do it, in Java it doesn't look as pretty as it could be.

The Scala Lift web framework, apart from the other advantages it has (which are a topic for many a blog post) offers some syntax-sugar wrappers so that testing of our REST APIs can be concise and to the point.

Combined with Jetty, this leads to some seriously short and readable code.

First, we need to:

  • use the trait "with TestKit" in our test

  • override the baseUrl property

  • start our Jetty server





Then if we want to output our own failure message later, we need to provide an implicit class of type ReportFailure, where we only need to implement the fail method, which predictably takes a String. For example, regardless of whether we use ScalaTest or Specs, we can fall back to the fail method which is inherited by our test class:



This is all we need so far.

Using it



Now we're ready to GET some action.



There's already a lot going on here. First of all, the get and post methods return a TestResponse (you don't see it, because it's inferred). And the nice thing about TestResponse (and Scala) is that since it implements the foreach method, it can be used in for expressions together with the "<-" symbol.

Whatever is extracted by the for expression can be used to issue get and post requests, but in the context of the previous request. This means that cookies are preserved, so you can use the same HTTP session. In this contrived example we use an httpClient as an argument to the get method (which is supposed to use some HTTP credentials and log us in). In the following call, we don't need to use the authenticating http client, because we can have our cookies (and eat them, too).

We can also use http parameters as a sequence of tuples, which can be delimited using the "->" operator.

For any TestResponse, we can use the xml property, which is the XML as a scala.xml.Elem wrapped in the (ubiquitous for Lift) Box.

Finally, there's concise syntax to assert certain properties about the response. For example the "!@" operator checks if the response code is 200, otherwise it fails with the error message specified after the operator.

If we expect a specific return code other than 200, we can also specify it explicitly:



Customizing



If you want to define a HTTP basic authentication client to be used by default by your requests, you can override the method theHttpClient. Lift's TestFramework provides the buildBasicAuthClient method, which can be reused to quickly create an HTTP client with a set user and password.



If you're like me, you might lose hours trying to find out why the request doesn't work when the server doesn't explicitly request authentication. Then setting the preemptive flag on the client would definitely save some time.

I know some of you Scala geeks are already bored because everything so far is just so simple. There wasn't even any mention of implicits! But rest assured, I will find some use for implicits.

Pimping



The usage snippet in the for expression above is not quite right. If the response contains no valid xml, then the specs matcher will not be executed and the test will not fail, although it should. Here we can use the fact that TestResponse also implements the filter method, which allows us to have if expressions (also known as filters). The post snippet could then be rewritten like this:



We are again using the Specs matcher to test if the response contains xml and fail otherwise. Notice that this expression does not return true- it need not be boolean. This is due to the fact that TestResponse's filter method intentionally returns Unit, not Boolean.

However, that's too much boilerplate, which we have to do for every time we want to check the contents of an xml response:


  1. Check if the response is valid and has a response code of 200

  2. Check if the xml is not empty

  3. Extract the xml

  4. Only then check the contents of the XML



If only we could make TestResponse do what Specs does with XML... But since this is Scala, we can create a wrapper and then transparently substitute it using our implicit conversion:



An important thing to remember about implicit conversions is that if we want them to feel seamless, the wrapper class' methods should return the original class type (the one before the conversion). Otherwise we could get unexpected object classes similarly to what occurred with the Scala 2.7 RichString, for instance with "aaa" reverse == "aaa"

Since we have chosen to use the same operator as Specs uses, we had to disambiguate with the XmlBaseMatchers class name, which is the specs trait containing all the XML matching goodness.

Now we can use our fancy TestResponse operators. Notice that since we return the same class type that we already had, we can chain:



This looks more compact than the original version so you can focus on the API differences and not be distracted by boilerplate preconditions.

3 comments:

Ittay Dror said...

Note that Option is also good for when null is not an option ( ;-) ). For example, if your method returns one of the value classes (e.g., Int). In Java, you might try to return an unreasonable value, or box the value, both are ugly

Ittay Dror said...

How did my previous comment end up on the wrong post???

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