Yet More Spock Magic: Mocks

Spock's built-in mock object capabilities are just a dream to use ... unlike other systems I've used, it doesn't get in your way, or force you to think backwards or inside out. Once again, some listings. These are for tests of Tapestry IoC's AspectDecorator service, which is used to create a wrapper interceptor around some other object. The test below shows how a supplied MethodAdvice callback object is invoked by the interceptor, if the advice is associated with the invoked method.

TestNG with EasyMock (Java)

public class AspectInterceptorBuilderImplTest extends IOCInternalTestCase
{
    private AspectDecorator decorator;

    @BeforeClass
    public void setup()
    {
        decorator = getService(AspectDecorator.class);
    }

    public interface Subject
    {
        void advised();

        void notAdvised();
    }

    @Test
    public void some_methods_not_intercepted() throws Exception
    {
        Subject delegate = newMock(Subject.class)

        MethodAdvice advice = new MethodAdvice()
        {
            public void advise(MethodInvocation invocation)
            {
                assertEquals(invocation.getMethod().getName(), "advised");

                invocation.proceed();
            }
        };

        delegate.advised();
        delegate.notAdvised();

        replay();

        AspectInterceptorBuilder<Subject> builder = decorator.createBuilder(Subject.class, delegate, "<Subject>");

        builder.adviseMethod(Subject.class.getMethod("advised"), advice);

        Subject interceptor = builder.build();

        interceptor.advised();
        interceptor.notAdvised();

        verify();
    }
}

Even this example is a bit streamlined, as some of the mock object capabilities, such as methods newMock(), replay() and verify() are being derived from the TestBase base class.

Spock

interface InterceptorSubject {

  void advised()

  void notAdvised()
}

class AspectInterceptorBuilderImplSpec extends AbstractSharedRegistrySpecification {

  @Shared
  private AspectDecorator decorator

  def setupSpec() {
    decorator = getService AspectDecorator
  }

  def "ensure that non-advised methods are not passed through the MethodAdvice object"() {
    InterceptorSubject delegate = Mock()
    MethodAdvice advice = Mock()

    def builder = decorator.createBuilder(InterceptorSubject, delegate, "<InterceptorSubject>")

    builder.adviseMethod(InterceptorSubject.getMethod("advised"), advice)

    InterceptorSubject interceptor = builder.build()

    when:

    interceptor.advised()

    then:

    1 * advice.advise(_) >> { MethodInvocation mi ->
      assert mi.method.name == "advised"
      mi.proceed()
    }
    1 * delegate.advised()
    0 * _

    when:

    interceptor.notAdvised()

    then:

    1 * delegate.notAdvised()
    0 * _
  }

}

Spock's wonderful when: / then: blocks organize the behavior into a stimulus followed by a response; using EasyMock, you have to train the mock objects for the response before introducing the stimulus (the method invocation). Further, with EasyMock there's one API for methods that return a fixed response, a second API for methods that throw an exception, and a third API for methods where the result must be calculated; in Spock the value after the >> operator is either a literal value, or a closure that can do what it likes, such as the one attached to MethodAdvice.advice() that checks for the expected method name, and then proceed()s to the delegate mock object's method.

I think that a reasonable developer, even without a thorough understanding of Spock, would get the gist of what this test does (perhaps with a little side note about the interaction system inside the then: block). On the other hand, I've seen when training people with TestNG and EasyMock that it very rarely sinks in immediately, if at all.