In part 3, we applied the SeparateAbstractions pattern so that we could independently manage the two different AuditFacade interface implementations. The result was the module relationships shown at right (click to enlarge). The code can for this version of the system can be found in the SeparateAbstractions project. Also, be sure to check out Part 1, where we introduced our initial version of the system, and Part 2 where we applied a couple of the modularity patterns. Now, we’re going to focus on the financial.jar module and try to decouple bill.jar from financial.jar so the two are completely independent of each other.
I think you’ll really like this one…But first, some additional housekeeping.
Before we decouple the bill.jar module from the financial.jar module, we’re going to talk about exceptions. In general, we need to answer the question.
Where do exceptions belong?
In the CollocateExceptions project, we introduce an AuditException that is thrown if an error is encountered. The CollocateExceptions pattern says that exceptions should be close to the classes (or interfaces) that throw them. Since the AuditFacade interface throws the exception (shown below), we should put the AuditException in the same module as the AuditFacade interface, which is the auditspec.jar module. Note that if we put the exception anywhere else, we’d create an unwanted dependency between modules.
Ok, the previous refactoring was pretty simple, and it’s time to move on. Here’s the need, and I think you’ll find the solution quite interesting.
A new requirement has emerged, and we need to use the bill.jar module in another system. In this other system, we don’t need the financial.jar module to make the payment. We just want to use the bill.jar functionality. So, what do we do?
First, recall our initial version of the class structure of the system. In the third refactoring in Part 2, we introduced a Payable interface, and then created the financial.jar module. After we finished Part 3, we were left with the module structure shown at right (click to enlarge). Again, our goal here is to eliminate the dependency between the bill.jar and financial.jar modules so that we can deploy the bill.jar module without the financial.jar module. We’re going to apply a little trick called escalation. Essentially, we’re going to escalate the dependency to a higher level module. How? First, let’s take a look at the new class structure that’s going to allow us to remove the module dependency.
As shown at right (click to enlarge), we see the new class structure that’s going to allow us to decouple the bill.jar and financial.jar modules. Things are starting to get a bit tricky here, so before we allocate these classes to their respective modules, let’s walk through the steps to illustrate how these classes will callaborate to give us what we need.
We start by passing a BillPayAdapter instance to the Bill as a BillPayer type. The BillPayAdapter is going to manage the relationship between Bill and Payment. When the pay method on Bill is called, it will turn around and invoke the generateDraft method on BillPayAdapter. The BillPayAdapter invokes the generateDraft method on Payment, passing itself in as a Payable. This allows the Payment to callback on the Payable and invoke the appropriate method (either getAmount or getAuditedAmount) on BillPayAdapter. The BillPayAdapter can now query the Bill to obtain the audited amount and pass it back to the payment. Once the Payment has this amount, it can make the payment. Whew! A fairly complex callaboration, but enough to ensure we can decouple the bill.jar module from the financial.jar module. The code for the new BillPayAdapter class, which mediates (hmm..possibly naming it a mediator would have been better, huh?) the relationship between Bill and Payment, is shown below.
Now it’s time to allocate these classes to the appropriate modules. The key decision here is where we place the BillPayAdapter. Because this controls the Bill and Payment callaboration, we can’t put it in either of those two modules. In fact, we’re going to create a new module that contains the BillPayAdapter. The new module, which we’ll call billpay.jar, will depend on both the bill.jar and financial.jar modules, as shown at right (click to enlarge). The code for this version can be found in the IndependentDeployment project.
For the most part, our system is complete now. But we do have one nasty little problem that we haven’t solved yet. We have two different AuditFacade implementations, but the current system hardcodes creation of AuditFacade1 within the AuditAction class. All this flexibility compromised by a single line of code. Unnerving how that happens! We really do have to architect all the way down. Anyway, let’s fix this problem.
The problem of creating instances is pretty common, so the solution is relative easy. There are a lot of different ways to do this, but we’re going to take the easiest route (and not the most flexibile mind you) by creating an AuditFacadeFactory class, which can be found in the ImplementationFactory project. Now, instead of the AuditAction creating the AuditFacade implementation, it invokes the factory and is returned an instance, as shown below.
Wrapping Up and Getting Ready for the Postmortem
We’ve made considerable progress. Amazing really. From a single monolithic application to a fully modularized architecture with considerable flexibility. But there are a lot of very interesting takeaways that we haven’t talked about yet. A number of positive side affects have resulted that aren’t immediately obvious. In our final post in this series, we’ll take a more in-depth look at the impact of modularity.