领域驱动设计 - 可测试性和“新”关键字

Question

I have been trying to follow a domain driven design approach in my new project. I have always generally used Spring for dependency injection, which nicely separates my application code from the construction code, however, with DDD I always seem to have one domain object wanting to create another domain object, both of which have state and behaviour.

For example, given a media file, we want to encode it to a different format - the media asset calls on a transcode service and receives a callback:

class MediaAsset implements TranscodingResultListener {

    private NetworkLocation permanentStorage;
    private Transcoder transcoder;

    public void transcodeTo(Format format){
        transcoder.transcode(this,format);
    }

    public void onSuccessfulTranscode(TranscodeResult result){
        Rendition rendition = new Rendition(this, result.getPath(), result.getFormat());
        rendition.moveTo(permanentStorage);
    }

}

Which throws two problems:

• If the rendition needs some dependencies (like the MediaAsset requires a "Transcoder") and I want to use something like Spring to inject them, then I have to use AOP in order for my program to run, which I don't like.
• If I want a unit test for MediaAsset that tests that a new format is moved to temporary storage, then how do I do that? I cannot mock the rendition class to verify that it had its method called... the real Rendition class will be created.

Having a factory to create this class is something that I've considered, but it is a lot of code overhead just to contain the "new" keyword which causes the problems.

Is there an approach here that I am missing, or am I just doing it all wrong?

Answer 1

I think that the injection of a RenditionFactory is the right approach in this case. I know it requires extra work, but you also remove a SRP violation from your class. It is often tempting to construct objects inside business logic, but my experience is that injection of the object or a objectfactory pays off 99 out of 100 times. Especially if the mentioned object is complex, and/or if it interacts with system resources.

Answer 2

I assume your approach for unit testing is to test the MediaAsset in isolation. Doing this, I think a factory is the common solution.

Another approach is to test the whole system (or almost the whole system). Let your test access the outer interface[1] (user interface, web service interface, etc) and create test doubles for all external systems that the system accesses (database, file system, external services, etc). Then let the test inject these external dependencies.

Doing this, you can let the tests be all about behaviour. The tests become decoupled from implementation details. For instance, you can use dependency injection for Rendition , or not: the tests don't care. Also, you might discover that MediaAsset and Rendition are not the correct concepts[2], and you might need to split MediaAsset in two and merge half of it with Rendition . Again, you can do it without worrying about the tests.

(Disclaimer: Testing on the outer level does not always work. Sometimes you need to test common concepts, which requires you to write micro tests. And then you might run into this problem again.)

[1] The best level might actually be a "domain interface", a level below the user interface where you can use the domain language instead of strings and integers, and where you can talk domain actions instead of button clicks and focus events.

[2] Perhaps this is actually your problem: Are MediaAsset and Rendition the correct concepts? If you ask your domain expert, does he know what these are? If not, are you really doing DDD?