I'm relatively unskilled in Dependency Injection, and I'd like to learn some best practices and anti-patterns to use and avoid respectively when using DI.
I really enjoyed this article regarding DI, as it's targeted towards people who don't have a ton of DI experience, or don't even know what it is.
https://mtaulty.com/2009/08/10/m_11554/
What’s Unity?
It’s a “dependency injection container”.
Now, at that point a bunch of folks
reading this will say “Yes, we know
and we’re already using it for reasons
A, B, C or we’ve elected not to use it
for reasons X,Y,Z ” and I imagine a
bunch of other folks might say;
“Huh? What’s a dependency injection container?”
This post is for the latter people –
it’s not meant to be exhaustive but
hopefully it’s not completely
unhelpful either :-)
In my opinion, Dhanji Prasanna's book Dependency Injection is a must read for software designers, both beginners and experts. It deals directly with your DI questions.
There's a best practices section in Guice's user's guide.
I've found that when I see a violation of the Law of Demeter that is a hint that I might want dependency injection.
For example:
void doit()
{
i += object.anotherobject.addvalue; //violation of Law of Demeter
}
Sometimes hints that I might want to dependency inject anotherobject.
My basic rule about when to use DI is that I will inject between layers, so between my controller and the dao would be a layer, so I can inject, so that if I want to mock out a layer I can.
I think using DI within the same layer is not a good idea mainly because the layer should be tightly coupled, as they are related, unless you have a user story that makes it useful.
For example, if your DAO is may be on separate computers then you may need to be able to pretend that they are one layer, but use DI to actually switch between all on one machine and separate machines. Then the developer can do everything on one machine and it should work on separate machines.
But, unless there is some pressing need, I think DI within the same layer is an unnecessary complication.
Here's a dependency injection anti-pattern: Multiple Constructors.
Related
I find more and more aspects where Smalltalk was the innovator, i.e. created the technique or at least the overall concept for the first time. I can think of the following:
xunit approach
IDE concepts
VM optimizations
fluent interfaces
several design patterns (e.g. model-view-controller)
the class-free prototype paradigm.
Are all of these correct? Which further innovations did Smalltalk bring?
I'm sure there are more (e.g. in the field of language design?)
The mouse
Unit Testing
Refactoring
Scavenging GC
image concept (snapshot)
It is the first language that was a clear improvement on a large majority of its successors (with the possible exceptions of self and newspeak). If you want to see the future of java and c#, look no further than smalltalk.
Also, Dan Ingalls is usually given credit for inventing BitBLT as part of Smalltalk 72.
I would also add "IDE" to the list, but I have no citation to back that up.
You forgot one BIG thing: object-oriented programming
I read somewhere that smalltalk implemented the first window based GUI. Hard to beat that ;)
Domain-Driven Design: Trygve Renskaug's papers on the MVC pattern discuss heavily the importance of representing the domain of the system in the object model and separating it from the conceptual view.
Obviously, "Hello World" doesn't require a separated, modular front-end and back-end. But any sort of Enterprise-grade project does.
Assuming some sort of spectrum between these points, at which stage should an application be (conceptually, or at a design level) multi-layered? When a database, or some external resource is introduced? When you find that the you're anticipating spaghetti code in your methods/functions?
when a database, or some external resource is introduced.
but also:
always (except for the most trivial of apps) separate AT LEAST presentation tier and application tier
see:
http://en.wikipedia.org/wiki/Multitier_architecture
Layers are a mean to keep a design loosely coupled and highly cohesive.
When you start to have a few classes (either implemented or just sketched with UML), they can be grouped logically, into layers - or more generally packages, or modules. This is called the art of separating the concerns.
The sooner the better: if you do not start layering early enough, then you risk to have never do it as the effort can be too important.
Here are some criteria of when to...
Any time you anticipate the need to
replace one part of it with a
different part.
Any time you find
yourself need to divide work amongst
parallel team.
There is no real answer to this question. It depends largely on your application's needs, and numerous other factors. I'd suggest reading some books on design patterns and enterprise application architecture. These two are invaluable:
Design Patterns: Elements of Reusable Object-Oriented Software
Patterns of Enterprise Application Architecture
Some other books that I highly recommend are:
The Pragmatic Programmer: From Journeyman to Master
Refactoring: Improving the Design of Existing Code
No matter your skill level, reading these will really open your eyes to a world of possibilities.
I'd say in most cases dealing with multiple distinct levels of abstraction in the concepts your code deals with would be a strong signal to mirror this with levels of abstraction in your implementation.
This does not override the scenarios that others have highlighted already though.
I think once you ask yourself "hmm should I layer this" the answer is yes.
I've worked on too many projects that probably started off as proof of concept/prototype that ended up being full projects used in production, which are horribly written and just wreak of "get it done quick, we'll fix it later." Trust me, you wont fix it later.
The Pragmatic Programmer lists this as the Broken Window Theory.
Try and always do it right from the start. Separate your concerns. Build it with modularity in mind.
And of course try and think of the poor maintenance programmer who might take over when you're done!
Thinking of it in terms of layers is a little limiting. It's what you see in whitepapers about a product, but it's not how products really work. They have "boxes" that depend on each other in various ways, and you can make it look like they fit into layers but you can do this in several different configurations, depending on what information you're leaving out of the diagram.
And in a really well-designed application, the boxes get very small. They are down to the level of individual interfaces and classes.
This is important because whenever you change a line of code, you need to have some understanding of the impact your change will have, which means you have to understand exactly what the code currently does, what its responsibilities are, which means it has to be a small chunk that has a single responsibility, implementing an interface that doesn't cause clients to be dependent on things they don't need (the S and the I of SOLID).
You may find that your application can look like it has two or three simple layers, if you narrow your eyes, but it may not. That isn't really a problem. Of course, a disastrously badly designed application can look like it has layers tiers if you squint as hard as you can. So those "high level" diagrams of an "architecture" can hide a multitude of sins.
My generic rule of thumb is to at least to separate the problem into a model and view layer, and throw in a controller if there is a possibility of more than one ways of handling the model or piping data to the view.
(Or as the first answer, at least the presentation tier and the application tier).
Loose coupling is all about minimising dependencies, so I would say 'layer' when a dependency is introduced. i.e. a database, third party application, etc.
Although 'layer' is probably the wrong term these days. Most of the time I use Dependency Injection (DI) through an Inversion of Control container such as Castle Windsor. This means that I can code on one part of my system without worrying about the rest. It has the side effect of ensuring loose coupling.
I would recommend DI as a general programming principle all of the time so that you have the choice on how to 'layer' your application later.
Give it a look.
R
So, I'm looking at using Smalltalk/Squeak for a couple of hobby/academic interest projects, and while trying to read up on the language I came across this nice article. However, this paragraph had me a bit dumbfounded:
"Unfortunately, there is a complete lack of standardization for providing or dealing with modules/packages in Smalltalk. Some dialects provide very strong, comprehensive support for modules/packages (including versioning and distributed access by programming teams,) and other dialects provide little or nothing in this regard. Some dialects provide a robust implementation of multiple, shareable namespaces, others don't. The only commonality is that, when either modules/packages or namespaces are provided, they are implemented as reified objects, in the same way that classes and methods are implemented as reified objects."
So, I have tried googling for it, and this shows up on the Squeak wiki: http://wiki.squeak.org/squeak/734. Does anyone know if this (or something similar) is now part of the standard distribution?
As Mue says, it is not perceived as a big problem in the Squeak community. Prefixing is "good enough". A while back I tried hard to do something better and still maintain the unique feeling of Smalltalk:
http://swiki.krampe.se/gohu/32
...but even though lots of people thought it was nice it didn't catch on. Code more or less works though, but there are several other approaches too - unfortunately most of them just copy some stupid approach from a lesser language thus destroying the feeling of Smalltalk.
Namespaces are not part of Squeak today. But it's a common agreement to prefix all classes of the own project with two or three letters. That's not as save as real namespaces, but it's leightweighted, simple, and works. +smile+
The Google Summer of Code supported a namespace project called Environments. Chris Cunnington is currently investigating it, but he says it looks promising.
Not necessariy related except by name, Squeak 4.5 has a taken another run at the problem, with Colin Putney's Environments package.
Sounds like you should check out Newspeak.
I've been hearing and reading about cases when people had come across cases of overused design patterns. Ok, missused design patterns are understandable phenomenon. What does it actually mean overused design patterns?
Do you have any examples and why do you think there are too many patterns?
The singleton is probably the most overused design pattern. I often see it used in many cases when it's out of scope and much more appropriate to directly instantiate objects.
After that, I believe the factory pattern is way overused as a shortcut of instantiating objects, many times without a real need.
Object Orientation, which is no longer a design pattern but a way of life. I have seen a lot of procedural code munged up in objects and a lot of objects for the sake of objects because the zeitgeist says "presumably you are object oriented", when a few lines of C and a struct would do just as well.
I cite it as the most over-ued design pattern because it is (probably) the most widely used design pattern and its merits are rarely questioned.
I vote for ActiveRecord.
Many popular data access frameworks use ActiveRecord as the only data access pattern, a sort of one-size-fits-all solution, even though Martin Fowler's book "Patterns of Enterprise Application Architecture" describes several other data access patterns, and details strengths of each pattern and how to decide when to use each pattern.
The (sometimes) so-called JavaBeans-Pattern: getters and setters for every field. Highly questionable and extremely widespread.
I guess Singleton gets easily overused (though it certainly has its legitime uses).
Addiction to the Singleton pattern is called Singletonitis. :) Symptoms include, at least, unnecessarily high coupling, and testing becoming more difficult.
Edit: As a prescribed cure for Singletonitis, you could try Inline Singleton, described in Refactoring to Patterns by Joshua Kerievsky.
Edit 2: For a good discussion on Singletons, see this older question: What is so bad about Singletons
PREAMBLE: Generally, Singleton is considered the most abused pattern, if for nothing more than the fact that many will use it to write in-line programming in fact, if not in actuality, while others use it as a substitute for global variables.
BODY:There is a book out there called, "A Pattern Language" which predates the illustrious GoF by several years. It calls for a similar language among different aspects of a project — it was apparently a major influence on "Design Patterns" and those who know both texts consider it superior.
My personal experience is that the GoF is only useful in certain circumstances, and a far cry from encompassing all of OOP. I actually find it quite amusing that several of the patterns are made obsolete in other languages, and others are merely redundantly describing the same scenario (Is there really that much difference between something which adapts and translates?)
Patterns, in general, are a good thing. It is good that Singletons generally use a static getInstance method. It is good that many MVC structures use similar naming conventions. On the other hand, Patterns are not everything and that needs to be remembered.
Recommended reading:
http://perl.plover.com/yak/design/
The singleton pattern, which is only suitable in a very few cases and makes testing harder. It's not only over-used, but it's often badly implemented in Java and C# - people often rush into double-checked locking when it's not only inappropriate but also relatively hard to get right.
EDIT: I really should have realised that everyone would post the same thing.
Next example, the factory pattern and in particular its use in the Java DOM API. Blech.
I would say the Singleton is well overused. There are often much better solutions than using what's essentially a global variable.
I'm going to weigh in on the much-overused Singleton. Quite often, developers learn only this one pattern and use it when a static class would be just as effective.
I think a worse problem than overused design patterns is patterns misapplied by enthusiastic developers who've recently learned a new pattern tool and decide they need to try it out. Recently I've been reading some of Misko Hevery's blog (http://misko.hevery.com/2008/08/17/singletons-are-pathological-liars/) entries on dependency injection. One of his major assertions is that the singleton pattern implemented as a global instance severely limits testability and should be avoided.
A few days ago I read an interesting opinion on patterns from Christian Gruber's blog. He suggests they are useful as a tool for discussing architectures, but shouldn't be used during design conception lest software architecture deteriorate into what he calls "paint by numbers." See the paragraph on Design Patterns: http://www.geekinasuit.com/2008/12/testability-re-discovering-what-we.html.
So possibly the issue with design patterns is misapplication and tunnel vision induced by the perception that all well designed software must fit into one of the patterns described in Gang of Four.
This was actually discussed by our one and only Jeff Atwood on Coding Horror:
http://www.codinghorror.com/blog/archives/000380.html
I keep seeing the Provider pattern used where there is only one provider. This seems like an awful lot of extra work with no benefit.
I too vote for Singleton: a global in abstracted clothing.
And Factory, since that makes it easier not to think about how objects are connected together in a given program.
What's the penetration of design patterns in the real world? Do you use them in your day to day job - discussing how and where to apply them with your coworkers - or do they remain more of an academic concept?
Do they actually provide actual value to your job? Or are they just something that people talk about to sound smart?
Note: For the purpose of this question ignore 'simple' design patterns like Singleton. I'm talking about designing your code so you can take advantage of Model View Controller, etc.
Any large program that is well written will use design patterns, even if they aren't named or recognized as such. That's what design patterns are, designs that repeatedly and naturally occur. If you're interfacing with an ugly API, you'll likely find yourself implementing a Facade to clean it up. If you've got messaging between components that you need to decouple, you may find yourself using Observer. If you've got several interchangeable algorithms, you might end up using Strategy.
It's worth knowing the design patterns because you're more likely to recognize them and then converge on a clean solution more quickly. However, even if you don't know them at all, you'll end up creating them eventually (if you are a decent programmer).
And of course, if you are using a modern language, you'll probably be forced to use them for some things, because they're baked into the standard libraries.
In my opinion, the question: "Do you use design pattern?", alone is a little flawed because the answer is universally YES.
Let me explain, we, programmers and designers, all use design patterns... we just don't always realise it. I know this sounds cliché, but you don't go to patterns, patterns come to you. You design stuff, it might look like an existing pattern, you name it that way so everyone understand what you are talking about and the rationale behind your design decision is stronger, knowing it has been discussed ad nauseum before.
I personally use patterns as a communication tool. That's it. They are not design solutions, they are not best practices, they are not tools in a toolbox.
Don't get me wrong, if you are a beginner, books on patterns will show you how a solution is best solved "using" their patterns rather than another flawed design. You will probably learn from the exercise. However, you have to realise that this doesn't mean that every situation needs a corresponding pattern to solve it. Every situation has a quirk here and there that will require you to think about alternatives and take a difficult decision with no perfect answer. That's design.
Anti-pattern however are on a totally different class. You actually want to actively avoid anti-patterns. That's why the name anti-pattern is so controversial.
To get back to your original question:
"Do I use design patterns?", Yes!
"Do I actively lean toward design patterns?", No.
Yes. Design patterns can be wonderful when used appropriately. As you mentioned, I am now using Model-View-Controller (MVC) for all of my web projects. It is a very common pattern in the web space which makes server-side code much cleaner and well-organized.
Beyond that, here are some other patterns that may be useful:
MVVM (Model-View-ViewModel): a similar pattern to MVC; used for WPF and Silverlight applications.
Composition: Great for when you need to use a hierarchy of objects.
Singleton: More elegant than using globals for storing items that truly need a single instance. As you mentioned, a simple pattern but it does have its uses.
It is worth noting a design pattern can also highlight a lack of language features and/or deficiencies in a language. For example, iterators are now built in as part of newer languages.
In general design patterns are quite useful but you should not use them everywhere; just where they are a good fit for your needs.
I try to, yes. They do indeed help maintainability and readability of your code. However, there are people who do abuse them, usually (from what I've seen) by forcing a system into a pattern that doesn't exist.
I try to use patterns if they are applicable. I think it's kind of sad seeing developers implement design patterns in code just for the sake of it. For the right task though, design patterns can be very useful and powerful.
There are many design patterns beyond the simple that are used in "real world". Good example Stackoverflow uses the Model View Controller Pattern. I have used Class Factories multiple times in projects for my employer, and I have seen many already written projects using them as well.
I am not saying every design pattern is being used but many are.
Yes we do, it usually happens when we start designing something and then someone notices that it resembles an existing pattern. We then take a look at it and see how it would help us achieve our goal.
We also use patterns that are not documented but that emerge from designing a lot.
Mind you, we don't use them a lot.
Yes, Factory, Chain of Responsibility, Command, Proxy, Visitor, and Observer, among others, are in use in a codebase I work with daily. As far as MVC goes, this site seems to use it quite well, and the devs couldn't say enough good things in the latest podcast.
Yes, I use a lot of well known design patterns, but I also end up building some software that I later find out uses a 'named' design pattern. Most elegant, reusable designs could be called a 'pattern'. It's a lot like dance moves. We all know the waltz, and the 2-step, but not everyone has a name for the 'bump and scoot' although most of us do it.
MVC is very well known so yes we use design patterns quite a lot. Now if your asking about the Gang of Four patterns, there are several that I use because other maintainers will know the design and what we are working towards in the code. There are several though that remain fairly obscure for what we do, so if I use one I don't get the full benefits of using a pattern.
Are they important, yes because it gives you a method of talking about software design in a quick efficient and generally accepted way. Can you do better custom solutions, well yes (sorta)?
The original GoF patterns were pulled from production code, so they catalogued what was already being used in the wild. They aren't purely or even mostly an academic thing.
I find the MVC pattern really useful to isolate your model logic, which can than be reused or worked on without too much trouble. It also helps de-coupling your classes and makes unit testing easier. I wrote about it recently (yes, shameless plug here...)
Also, I've recently used a factory pattern from a base class to generate and return the proper DataContext class that I needed on the fly, using LINQ.
Bridges are used when trying when trying to glue together two different technologies (like Cocoa and Ruby on the Mac, for example)
I find, however, that whenever I implement a pattern, it's because I knew about it before hand. Some extra thought generally goes into it as I find I must modify the original pattern slightly to accommodate my needs.
You just need to be careful not to become and architecture astronaut!
Yes, design patterns are largely used in the real world - and daily by many of the people I work with.
In my opinion the biggest value provided by design patterns is that they provide a universal, high level language for you to convey software design to other programmers.
For instance instead of describing your new class as a "utility that creates one of several other classes based on some combination of input criteria", you can simply say it's an "abstract factory" and everyone instantly understands what you're talking about.
Yes, design patterns or abstractly patterns are part of my life, where I look, I begin to see them. Therefore, I am surrounded by them. But, as you know, little knowledge is a dangerous thing. Therefore, I strongly recommend you to read GoF book.
One of the main problems about design patterns, most developers just do not get the idea, or do not believe in them. And most of the time they argue about the variables, loops, or switches. But, I strongly believe that if you do not speak the pattern language, your software will not go far and you will find yourselves in a maintenance nightmare.
As you know, anti-pattern is also dangerous thing and it happens when you have little expertise on design patterns. And refactoring anti-patterns is much more harder. As a recommended book about this problem please read "AntiPatterns: Refactoring Software, Architectures, and Projects in Crisis".
Yes.
We are even using them in my current job: Mainframe coding with COBOL and PL/I.
So far I have seen Adaptor, Visitor, Facade, Module, Observer and something very close to Composite and Iterator. Due to the nature of the languages it's mostly strutural patterns that are used. Also, I'm not always sure that the people who use them do so conciously :D
I absolutely use design patterns. At this point I take MVC for granted as a design pattern. My primary reason for using them is that I am humble enough to know that I am likely not the first person to encounter a particular problem. I rarely start a piece of code knowing which pattern I am going to use; I constantly watch the code to see if it naturally develops into an existing pattern.
I am also very fond of Martin Fowler's Patterns of Enterprise Application Architecture. When a problem or task presents itself, I flip to related section (it's mostly a reference book) and read a few overviews of the patterns. Once I have a better idea of the general problem and the existing solutions, I begin to see the long term path my code will likely take via the experience of others. I end up making much better decisions.
Design patterns definitely play a big role in all of my "for the future" ideas.