Groovy Threads and MetaClass example

* Update - the code has been updated. The original test was incorrect and was producing a false positive. What you see now is the correct way.

It's been awhile since I have been able to play around with grails/groovy. Now, instead of using a pretend app to learn grails/groovy, I have teamed up with Jeff Black, Chad Gallemore, and Sam Jones (fellow office co-workers here in Joplin, MO) to rewrite an existing small internal java webapp using grails. It's a perfect application for grails and so far we are loving it.

Early on, we needed to figure out 2 things:
1) Groovy way of creating Threads
2) Writing an integration test for a Groovy Service

Groovy Threads
Below is a summary of our service that shows how to start new threads in groovy.

class ProjectService {
  def discover() {
      Project.list().each {
          Thread.startDaemon {
              jobService.update(it)
          }
      }
  }
}

Service Integration test with MetaClass
Here is the integration test I wrote that tests the above Service.

void testEmptyProject() {
   def called = false

   Project.metaClass.static.list = {[]}
   Thread.metaClass.static.startDaemon = {Closure c -> c.call()}
   JobService.metaClass.update = {called = true}
   new ProjectService(jobService: new JobService()).discover()

   assertFalse('updateJobs should not have been called', called)
}

The first missing key for me was the static keyword on metaClass since in the service I am calling Project.list() and Thread.startDaemon(). The second mystery was how to mock out Thread.startDaemon() since there could be, and was, a race condition between the update closure setting called = true and my assertFalse.

Thanks Chad for the suggestion of using metaClass. I also got a lot of help from Glenn Smith's blog about testing controllers and Dustin's groovy thread example.