Marcin Gryszko

If you wrote multithreaded code in Groovy/Grails, I’m sure you stumbled upon the GPars library. It simplifies parallel processing – sometimes it’s as easy as wrapping a code fragment with a GPars closure, changing the iteration method (in our example from each to eachParallel) and passing the closure to a GPars pool. The library implementation handles the executor pool creation and adds new methods to collections.

Whenever I deliberately add multithreading, I always separate responsibilities (processing logic and concurrency handling). I wrap the single-threaded class with a new one, handing the parallel execution (creating a pool, managing threads, handling cancellation, etc.):

class MultithreadedProcessorService {
    ProcessorService processorService
    def threadPoolSize = ConfigurationHolder.config.threadPool.size

    void processMultithreaded(batches) {
        GParsPool.withPool(threadPoolSize) {
            batches.eachParallel { batch ->
                processorService.process(batch)
            }
        }
    }
}

class ProcessorService {
    void process(batch) {
        // some processing
    }
}

To test the multithreaded class you can you use Spock interactions. As opposed to Groovy and Grails mocks, they are thread-safe and you won’t get any weird and unstable results:

class MultithreadedProcessorServiceSpec extends UnitSpec {
    private MultithreadedProcessorService multithreadedProcessorService
    // Using Spock mocks because they are thread-safe
    private ProcessorService processorService = Mock(ProcessorService)

    def setup() {
        mockConfig('threadPool.size=2')
        multithreadedProcessorService = new MultithreadedProcessorService(
            processorService: processorService
        )
    }

    def 'processes batches using multiple threads'() {
        given:
        def batches = [[0, 1], [2, 3, 4], [5]]
        def processedBatches = markedAsNotProcessed(batches)

        when:
        multithreadedProcessorService.processMultithreaded(batches)

        then:
        batches.size() * processorService.process({ batch ->
            processedBatches[batch] = true
            batch in batches
        })
        assertAllProcessed(processedBatches)

    }

    private markedAsNotProcessed(batches) {
        batches.inject([:]) { processed, batch ->
            processed[batch] = false
            processed
        }
    }

    private void assertAllProcessed(batches) {
        assert batches*.value == [true] * batches.size()
    }
}

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This post series present the best of Osoco tests – tests that were tricky or we are just proud of. You can find a runnable source code for this test and more in the Grails Test Gallery project shared on GitHub.

In our projects we include at least one integration test per domain class. We want to assure us that the GORM mapping is correct and the class is persistable in the target database.

The test plan is simple:

• create a valid persistable sample of the domain object
• save it
• clear Hibernate first level cache (i.e. flush and clear current session)
• retrieve the object once again and compare it with the previously saved object. They must be equal as by equals() but be different object references

We put the steps above in a base abstract specification. In concrete specifications we implement merely the factory method spawning a persistable object instance.

abstract class PersistenceSpec extends IntegrationSpec {
    SessionFactory sessionFactory

    protected abstract createPersistableDomainObject()
    
    def 'persistable domain object should be able to be saved and retrieved'() {
        given:
        def persistableDomainObject = createPersistableDomainObject()

        when:
        def savedDomainObject = persistableDomainObject.save()

        then:
        savedDomainObject.id != null

        when:
        clearFirstLevelCache()

        def retrievedDomainObject = persistableDomainObject.class.get(savedDomainObject.id)

        then:
        savedDomainObject == retrievedDomainObject
        !savedDomainObject.is(retrievedDomainObject)
    }

    private clearFirstLevelCache() {
        sessionFactory.currentSession.flush()
        sessionFactory.currentSession.clear()
    }
}

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This post series present the best of Osoco tests – tests that were tricky or we are just proud of. You can find a runnable source code for this test and more in the Grails Test Gallery project shared on GitHub.

If you work with GORM and your objects are held in collections, you should implement equals and hashCode methods. As everything, they should be thouroughly tested if they obey equals-hashCode contract. At Osoco we created a equals-hashcode-test Grails plugin that provides a base Spock specification for that.

In your equals and hashCode tests you simply have to extend the EqualsHashCodeSpec and provide at least two methods:

• a factory method createDomainObjectToCompare that will create a fresh object for the comparison
• a map of modified properties included in equals and hashCode
• optionally a map of modified properties ignored in both methods

Additionally in case of inheritance, we check whether the equals method complies with the symmetry rule. We do it in an extra feature method added to the spec:

def 'equals relation must be symmetric with inheritance'() {
    given:
    def content = new Content()

    and:
    def book = new Book()

    expect:
    (content == book) == (book == content)
}

You may say, in Grails 2.0 and Groovy 1.8 there are @Equals and @HashCode AST transformations and you don’t need to write tests of them. I agree to some point. I think you can trust Groovy developers and you don’t have to test the methods for reflexivity, symmetry, transivity, consistency, and ‘non-nullity’. Anyway, you have to test the annotation configuration if it includes the properties that actually account for the logical equality between objects.

In the future releases of the plugin we will surely simplify the base specification and remove some checks but we will surely continue testing both methods (with our plugin it’s dirt-cheap!)

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This post series present the best of Osoco tests – tests that were tricky or we are just proud of. You can find a runnable source code for this test and more in the Grails Test Gallery project shared on GitHub.