When you develop software, you most likely have to think in two target domains at the same time. One domain will be the world of your stakeholder. He might talk about business rules and business processes and business everything, so lets call it the business domain. The other domain is the world you own exclusively with your colleagues, it’s the world of computers, programming languages and coding standards. Lets call it the technical domain. It’s the world where your stakeholders will never follow you.
Mixing the domains
Whenever you create source code, you probably try to solve problems in the business domain with the means of your technical domain – e.g. the programming language you’ve chosen on the hardware platform you anticipate the software to run on. Inevitably, you’ll mix parts of the business domain with parts of the technical domain. The main question is – will it blend? Most of the time, the answer is yes. Like milk in coffee, the parts of two domains will blend into an inseparable mixture. Which isn’t necessarily a bad thing – your solution works just fine.
The hard part comes when you want to reuse your code. It’s like reusing the milk in your coffee, but without the coffee. You’ve probably done it, too (reusing domain-blended code, not extracting the milk from your coffee) and it wasn’t the easy “just copy it over here and everything’s fine” reusability you’ve dreamt of.
Separating the domains
One solution for this task begins by realizing which code belongs to which domain. There isn’t a clear set of rules that you can just check and be sure, but we’ve found two rules of thumb helpful for this decision:
- If you have a strong business domain data type model in your code (that is, you’ve modelled many classes to directly represent concepts and items from your stakeholder’s world), you can look at a line of code and scan for words from the business domain. If there aren’t any, chances are that you’ve found a line belonging to the technical domain. If you prefer to model your data structures with lists and hashmaps containing strings and integers, you’re mostly out of luck here. Hopefully, you’ve chosen explicit names for your variables, so you don’t end with a line stating map.get(key), when in fact, you’re looking up orders.getFor(orderNumber).
- For every line of code, you can ask yourself “do I want to write it or do I have to write it?”. This question assumes that you really want to solve the problems of the business domain. Every line of code you just have to write because otherwise, the compiler, the QA department, your colleagues or, ultimately, your coder idol of choice would be disappointed is a line from the technical domain. Every line of code that would only disappoint your stakeholder if it would be missing is a line from the business domain. Most likely, everything that your business-driven tests assert is code from the business domain.
Once you categorized your lines of code into their associated domain, you can increase the reusability of your code by separating these lines of code. Effectively, you try to avoid the blending of the parts, much like in a good latte macchiato. If you achieve a clear separation of the different code parts, chances are that you have come a long way to the anticipated “copy and paste” reusability.
Example one: Local separation
Well, all theory is nice and shiny, but what about the real (coding) life? Here are two examples that show the mechanics of the separation process.
In the first example, we’re given a compressed zip file archive as an InputStream. Our task is to write the archive entries to disk, given that certain rules apply:
public void extractEntriesFrom(InputStream in) {
ZipInputStream zipStream = new ZipInputStream(in);
try {
ZipEntry entry = null;
while ((entry = zipStream.getNextEntry()) != null) {
if (rulesApplyFor(entry)) {
File newFile = new File(entry.getName());
writeEntry(zipStream,
getOutputStream(basePath(), newFile));
}
zipStream.closeEntry();
}
} catch (IOException e) {
e.printStackTrace();
} finally {
IOHandler.close(zipStream);
}
}
This is fairly common code, nothing to be proud of (we can argue that the method signature isn’t as explicit as it should be, the exceptions are poorly handled, etc.), but that’s not the point of this example. Try to focus your attention to the domain of each code line. Is it from the business or the technical domain? Let me refactor the example to a form where the code from both domains is separated, without changing the additional flaws of the code:
public void extractEntriesFrom(InputStream in) {
ZipInputStream zipStream = new ZipInputStream(in);
try {
ZipEntry entry = null;
while ((entry = zipStream.getNextEntry()) != null) {
handleEntry(entry, zipStream);
zipStream.closeEntry();
}
} catch (IOException e) {
e.printStackTrace();
} finally {
IOHandler.close(zipStream);
}
}
protected void handleEntry(ZipEntry entry,
ZipInputStream zipStream) throws IOException {
if (rulesApplyFor(entry)) {
File newFile = new File(entry.getName());
writeEntry(zipStream,
getOutputStream(basePath(), newFile));
}
}
In this version of the same code, the method extractEntriesFrom(…) doesn’t know anything about rules or how to write an entry to the disk. Everything that’s left in the method is part of the technical domain – code you have to write in order to perform something useful within the business domain. The new method handleEntry(…) is nearly free of technical domain stuff. Every line in this method depends on the specific use case, given by your business domain.
Example two: Full separation
Technically, the first example only consisted of a simple refactoring (Extract Method). But by separating the code domains, we’ve done the first step of a journey towards code reusability. It begins with a simple refactoring and ends with separated classes in separated packages from two separated project parts, named something like “application” and “framework”. Even if you only find a class named “Tools” or “Utils” in your project, you’ve done intermediate steps towards the goal: Separating your technical domain code from your business domain code in order to reuse the former (because no two businesses are alike).
The next example shows a full separation in action:
WriteTo.file(target).using(new Writing() {
@Override
public void writeTo(PrintWriter writer) {
writer.println("Hello world!");
writer.println("Hello second line.");
// more business domain code here
}
});
Everything other than the first line (and the necessary java boilerplate) is business domain code. In the first line, only the specified target file isn’t technical. Everything related to opening the file output stream, handling exceptions, closing all resources and all the other fancy stuff you want to do when writing to a file is encapsulated in the WriteTo class. The equivalent to the handleEntry(…) method from the first example is the writeTo(…) method of the Writing interface. Everything within this method is purely business domain related code. The best thing is: you can nearly forget about the technical domain when filling out the method, as it is embedded in a reusable “code clamp” providing the proper context.
Conclusion
If you want to write reusable code, consider separating your two major code domains: the technical domain and the business domain. The first step is to be aware of the domains and distinguish between them. Your separation process then can start with simple extractions and finally lead to a purely technical framework where you “only” have to fill in the business domain code. By the way, it’s a variation of the classic “separation of concerns” principle, if you want to read more.
Schneide Blog 