Tuesday 26 August 2014

Fun with Scala - Post-Patterns Patterns, Part 1 - Loan Star

Are the original Software Design Patterns dead?

Seriously, aside from perhaps Builder, the dreaded Singleton, Model-View-Controller or its hipster cousin Model-View-ViewModel, when was the last time you saw one of the Gang Of Four's patterns used in a new project? Even the direct use of an Iterator is borderline bad-practice nowadays!

I'm thinking that in these days of maximal code-avoidance (and these are great days - less code is always better code in my opinion), the just amount of overhead required to implement most of these patterns is a big turn-off. It's not quite "boilerplate", that word that implies so much burden these days, but it is definitely Not Fun to churn out all those interfaces and abstract classes that do very little aside from give you that apparently-vital level of indirection which so often ends up being nothing more than a level of annoyance.

But I'm in no doubt that a new generation of post-Patterns design patterns have started to arrive, as more powerful, expressive languages enable formations of code that Gamma et al could only have dreamt of. Over the next little bit I'm going to explore a couple of nice ones that I've come across:

The Loan Pattern

... is actually the Strategy pattern but without the dreaded inheritance requirement - to refresh, here's a micro-Strategy example:
abstract class StrategySuperclass<T> {
  public T doSomethingIntricateInThreePartsWherePartTwoVaries() {
    T part1Result = doFirstPart();
    T part2Result = doSecondPart(part1Result);
    return doThirdPart(part2Result);

  protected abstract T doSecondPart(T firstPartResult);

public class ConcreteStrategyClass<T> extends StrategySuperclass<T> {
  protected T doSecondPart(T firstPartResult) {
    // Do stuff here
The principal idea is to shield concrete classes from the complexity or intricate orchestration of resources required to do some "large" task, by allowing them to just "slot in" the specialisation or detail that they need for their solution.

The Loan Pattern does not mandate any inheritance structure at all - the two parts of the solution could be within the same file, mixed in as traits, inherited, or composed together. It is particularly excellent at protecting limited/valuable/scarce resources that have some kind of lifecycle where they should be closed/returned/de-allocated after use. Here's an example that I gave as an answer to a Stack Overflow problem related to closing resources:

Here's the loan "provider" for want of a better term:
def withPrintWriter(dir:String, name:String)(f: (PrintWriter) => Any) {
  val file = new File(dir, name)
  val writer = new FileWriter(file)
  val printWriter = new PrintWriter(writer)
  try {
  finally {
Which you use like this, as a "consumer":
withPrintWriter("/tmp", "myFile") { printWriter =>
  printWriter.write("all good")
Scala makes this kind of anonymous-function goodness really easy to both write and use. I've been using something similar in Specs2 tests recently for things like:
  • Database connections. Borrow one, give it back at the end, no matter what happened
  • Working directories. The provider makes sure the dir is empty, gives to the consumer, and then empties it out again at the end, just to be sure
  • System properties This is a really nice pattern for this hard-to-unit-test situation. Set it, call the test function, then clear it out again. Just make sure your tests are both isolated and sequential to avoid unpleasant inter-test interference

Wednesday 6 August 2014

Scala by Stealth part 2: Scala-powered tests

Testing from Scala

Now for the fun part - we get to write some Scala!

Now if may turn out that this ends up being the end of the Scala road at your shop, due to restrictive policies about production code. That's a shame, and it could take a very long time to change. I know of one large company where "Java for production code, Scala for tests" has been the standard now for several years. Sure it's not perfect, but it's better than nothing, and developers who haven't yet caught the Scala bug can learn it in their day job.

The tests you write may eventually be the only unit tests for this code, so I would strive for complete coverage rather than merely a copy of the "legacy" Java-based tests. For the purposes of measuring this coverage I can highly recommend the jacoco4sbt plugin which is simple to get going, well-documented and produces excellent output that makes sense in Scala terms (some other Java-based coverage tools seem to struggle with some of the constructs the Scala compiler emits).

In addition to (possibly) getting introduced to Scala and also learning the basics of writing a specs2 test, you might even discover that your code under test is a little tricky to test from this new perspective. This is a good thing, and if it encourages a bit of mild refactoring (while keeping both Java- and Scala-based unit tests passing of course) then so much the better.

Once you've got some solid, measurable coverage from the Scala side (I like to shoot for 90+% line coverage), it's time to commit those changes again, and push them to your CI build. If you haven't already, install the JaCoCo Plugin for Jenkins so you can get pretty coverage graphs on the project page, and even automatically fail the build if coverage drops below your nominated threshold(s).

Switching your Jenkins build project to SBT

Speaking of which, you'll be wanting to adjust your Jenkins job (or equivalent) to push your new, somewhat-Scala-ish artifact to your Nexus (or equivalent). Firstly, for safety, I would actually be duplicating the existing job and disabling it rather than getting all gung-ho with something that can potentially be a very carefully-configured, nay curated Jenkins project configuration.

Luckily this should be pretty straightforward if you employ the Jenkins SBT Plugin - set the Actions to something like clean jacoco:cover publish to get the optimal blend of cleanliness, test-coverage visualisation, speed, and build traceability.

If for any reason you can't use the plugin, I'd recommend using your CI tool's Run script functionality, and including a dead-simple shell script in a suitable place in your repository; e.g.:

echo "Running `which sbt`"
sbt -Dsbt.log.noformat=true -Dbuild.version=$BUILD_NUMBER clean jacoco:cover publish

Once you've got everything sorted out and artifacts uploading, you'll notice that your Nexus now has a new set of artifacts alongside your old Java ones, with a _2.10 (or whatever Scala version you're running) suffix. Scala in your corporate repo! Progress!