Refactoring Service Layer classes_问答_开发者_运维开发者技术经验分享

My company is on a Unit Testing kick, and I'm having a little trouble with refactoring Service Layer code. Here is an example of some code I wrote:

public class InvoiceCalculator:IInvoiceCalculator
{
   public CalculateInvoice(Invoice invoice)
   {
      foreach (InvoiceLine il in invoice.Lines)
      {
          UpdateLine(il);
      }
      //do a ton of other stuff here
   }

   private UpdateLine(InvoiceLine line)
   {
      line.Amount = line.Qty * line.Rate;
      //do a bunch of other stuff, including calls to other private methods
   }
}

In this simplified case (it is reduced from a 1,000 line class that has 1 public method and ~30 private ones), my boss says I should be able to test my CalculateInvoice and UpdateLine separately (UpdateLine actually calls 3 other private methods, and performs database calls as well). But how would I do this? His suggested refactoring seemed a little convoluted to me:

//Tiny part of original code
public class InvoiceCalculator:IInvoiceCalculator
{
   public ILineUpdater _lineUpdater;

   public InvoiceCalculator (ILineUpdater lineUpdater)
   {
      _lineUpdater = lineUpdater;
   }

   public CalculateInvoice(Invoice invoice)
   {
      foreach (InvoiceLine il in invoice.Lines)
      {
          _lineUpdater.UpdateLine(il);
      }
      //do a ton of other stuff here
   }
}

public class LineUpdater:ILineUpdater
{
   public UpdateLine(Invoice开发者_运维技巧Line line)
   {
      line.Amount = line.Qty * line.Rate;
      //do a bunch of other stuff
   }
}

I can see how the dependency is now broken, and I can test both pieces, but this would also create 20-30 extra classes from my original class. We only calculate invoices in one place, so these pieces wouldn't really be reusable. Is this the right way to go about making this change, or would you suggest I do something different?

Thank you!

Jess

This is an example of Feature Envy:

line.Amount = line.Qty * line.Rate;

It should probably look more like:

  var amount = line.CalculateAmount();

There isn't anything wrong with lots of little classes, it's not about re-usability as much as it's about adaptability. When you have many single responsibility classes, it's easier to see the behavior of your system and change it when your requirements change. Big classes have intertwinded responsibilities which make it very difficult to change.

IMO this all depends on how 'significant' that UpdateLine() method really is. If it's just an implementation detail (e.g. it could easily be inlined inside CalculateInvoice() method and they only thing that would hurt is readability), then you probably don't need to unit test it separately from the master class.

On the other hand, if UpdateLine() method has some value to the business logic, if you can imagine situation when you would need to change this method independently from the rest of the class (and therefore test it separately), then you should go on with refactoring it to a separate LineUpdater class.

You probably won't end up with 20-30 classes this way, because most of those private methods are really just implementation details and do not deserve to be tested separately.

Well, your boss goes more correct way in terms of unit-testing:
He is now able to test CalculateInvoice() without testing UpdateLine() function. He can pass mock object instead of real LineUpdater object and test only CalculateInvoice(), not a whole bunch of code.
Is it right? It depends. Your boss wants to make real unit-tests. And testing in your first example would not be unit-testing, it would be integration testing.

What are advantages of unit-tests before integration tests?
1) Unit-tests allow you to test only one method or property, without it being affected by other methods/database and so on.
2) Second advantage - unit tests execute faster (for example, you said UpdateLine uses database) because they don't test all the nested methods. Nested methods can be database calls so if you have thousand of tests your tests can run slow (several minutes).
3) Third advantage: if your methods make database calls then sometimes you need to setup database (fill it with data which is necessary for test) and it can be not easy - maybe you will have to write a couple of pages of code just to prepare database for a test. With unit tests, you separate database calls from the methods being tested (using mock objects).

But! I am not saying that unit tests a better. They are just different. As I said, unit tests allow you to test a unit in isolation and quickly. Integration tests are easier and allow you to test results of a joint work of different methods and layers. Honestly, I prefer integration tests more :)

Also, I have a couple of suggestions for you:
1) I don't think having Amount field is a good idea. It seems that Amount field is extra because it's value can be calculated based on 2 other public fields. If you want to do it anyway, I would do it as a read only property which returns Qty * Rate.
2) Usually, having a class which consists of 1000 rows may mean that it's badly designed and should be refactored.

Now, I hope you better understand the situation and can decide. Also, if you understand the situation you can talk to your boss and you can decide together.

yeah, nice one. I'm not sure wether the InvoiceLine object also has some logic included, otherwise then you would probably need a IInvoiceLine also. I sometimes have the same questions. On one hand you want to do things right and unit test your code, but when database calls and maybe filewriting is involved it causes a lot of extra work to setup the first test with all the testobjects which step in when filewriting and database io is about to happen, interfaces, asserts and you also want to test that the datalayer doesn't contain any errors. So a test which is more 'process' then 'unit' is often easier to build. If you have a project that will be changed a lot (in the future) and lots of dependencies of this code (maybe other programs read the file or database data) it can be nice to have a solid unit test for all parts of your code, and the investment time is worthwhile. But if the project is, like my latest client says 'let's get it live and maybe we'll tweak a bit next year and next year there will be something new', than i wouldn't be so hard on myself to get all unit tests up and running.

Michel

Your boss' example looks reasonable to me.

Some of the key considerations I try to keep in mind when designing for any scenario are:

Single Responsibility Principle

A class should only change for one reason.
Does each new class justify its existence

Have classes been created just for the sake of it, or do they encapsulate a meaningful portion of logic?
Are you able to test each piece of code in isolation?

In your scenario, just looking at the names, it would appear that you were wandering away from Single Responsibility - You have an IInvoiceCalculator, yet this class is then also responsible for updating InvoiceLines. Not only have you made it very difficult to test the update behaviour, but you now need to change your InvoiceCalculator class both when calculation business rules change and when the rules around updating change.

Then there is the question about the updating logic - does the logic justify a seperate class? That really depends and it is hard to say without seeing the code, but certainly the fact that your boss wants that logic under test would suggest that it is more than a simple on liner call off to a datalayer.

You say that this refactoring creates a great number of extra classes, (I'm taking that you mean across all your business entities, since I only see a couple of new classes and their interfaces in your example) but you have to consider what you get from this. It looks like you gain full testability of your code, the ability to introduce new calculation and new update logic in isolation, and a more clear encapsulation of what are seperate pieces of business logic.

The gains above are of course subject to a cost benefit analysis, but since your boos is asking for them, it sounds like he is happy that they will pay off, against the extra work to implement the code this way.

The final, third point about testing in isolation is also a key benefit of the way your boss has designed this - the closer your public methods are to the code that does that actualy work, the easier it is to inject stubs or mocks for parts of your system that are not under test. For example, if you are testing an update method that calls off the a datalayer, you do not want to test the datalayer, so you would usually inject a mock. If you need to pass that mocked datalayer through all your calculator logic first, your test setup is going to be far more complicated since the mock now needs to meet many other potential requirements, not related to the actual test in question.

While this approach is extra work initially, I'd say that the majority of the work is the time to think through the design, after that, and after you get up to speed on the more injection based style of code, the raw implementation time of software that is structured in that way is actually comparible.

Your hoss' approach is a great example of dependency injection and how doing so allows you to use a mock ILineUpdater to conduct your tests efficiently.