A global point of access also defeats the purpose of code design. So when should we use the singleton pattern? In the article Use Your Singletons Wisely , Rainsberger suggests three tests to see if you do need to use this pattern: Will every application use this class exactly the same way? Alternatives to singleton pattern We need to first understand that singleton Pattern breaks the Single Responsibility Principle, which says one class should only have one responsibility In the case of singleton, the class not only holds the related business logic but is also responsible for ensuring the class has only one instance "Why should the class itself be responsible for being a singleton?
Letting go of the result. How are you feeling? Time you enjoy wasting is not wasted time. An API is an example of a singleton at a protocol level. You access Twitter, Google etc through what are essentially singletons.
So why do singletons become bad within a program? It depends on how you think of a program. If you think of a program as society of services rather than randomly bound cached instances then singletons make perfect sense. Singletons are a service access point. The public interface to a tightly bound library of functionality that hides perhaps a very sophisticated internal architecture.
So I don't see a singleton as that different from a factory. The singleton can have constructor parameters passed in. It can be created by some context that knows how to resolve the default printer against all the possible selection mechanisms, for example. For testing you can insert your own mock. So it can be quite flexible. The key is internally in a program when I execute and need a bit of functionality I can access the singleton with full confidence that the service is up and ready for use.
This is key when there are different threads starting in a process that must go through a state machine to be considered ready.
Typically I would wrap a XxxService class that wraps a singleton around class Xxx. The singleton isn't in class Xxx at all, it's separated out into another class, XxxService. This is because Xxx can have multiple instances, although it's not likely, but we still want to have one Xxx instance globally accessible on each system. XxxService provides a nice separation of concerns. Xxx doesn't have to enforce a singleton policy, yet we can use Xxx as a singleton when we need to.
First question, do you find a lot of bugs in the application? Ok this isnt nessacary but i recommend it. Really you just initialize the class like a normal class and PASS the pointer in. Don't frigen say ClassIWant. Its more work but really, its less confusing. Some places where you shouldnt change things you now cannot since its no longer global. All my manager classes are regular classes, just treat it as that. IMO, your example sounds okay.
I'd suggest factoring out as follows: cache object for each and behind each data object; cache objects and the db accessor objects have the same interface. This gives the ability to swap caches in and out of the code; plus it gives an easy expansion route. DB accessor and cache can inherit from the same object or duck type into looking like the same object, whatever. This decouples things so you can add new caches without having to go in and modify some Uber-Cache object. Singleton is a tool in a toolbox, just as anything else.
Hopefully you have more in your toolbox than a just a single hammer. Its all about how you use it. This is called Inversion of Control, and allows you to swap out the controller when configuration changes and in tests. Additionally, you can run several instances of your application or parts of your application in parallell good for testing!
Lastly your manager will die with its owning object the startup class. So structure your app like a tree, where things above owns everything used below them.
Don't implement an app like a mesh, where everybody knows everybody and find each other through global methods.
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Create a free Team What is Teams? Learn more. So Singletons are bad, then what? Ask Question. Asked 10 years, 9 months ago.
Active 1 year, 11 months ago. Viewed k times. Here is a real example from a major recent project I was involved in: The application was a thick client with many separate screens and components which uses huge amounts of data from a server state which isn't updated too often.
Improve this question. Sazzad Hissain Khan 2 2 silver badges 15 15 bronze badges. Bobby Tables Bobby Tables What problem is the use of a Singleton supposed to solve? How is it better at solving that problem than the alternatives such as a static class?
Anon: How does using a static class make the situation better. There is still tight coupling? Martin: I'm not suggesting it makes it "better". I'm suggesting that in most cases, a singleton is a solution in search of a problem. Anon: Not true. Static classes give you almost no control over instantiation and make multi-threading even more difficult than Singletons do since you have to serialize access to every individual method instead of just the instance.
Singletons can also at least implement an interface, which static classes cannot. Static classes certainly have their advantages, but in this case the Singleton is definitely the lesser of two considerable evils. Aaronaught: If you're just synchronizing access to the singleton, then your concurrency is broken. Your thread could be interrupted just after fetching the singleton object, another thread comes on, and blam, race condition.
Using a Singleton instead of a static class, in most cases, is just taking the warning signs away and thinking that solves the problem. Show 19 more comments. Active Oldest Votes. The Singleton design pattern is a very specific type of single instance, specifically one that is: Accessible via a global, static instance field; Created either on program initialization or upon first access; No public constructor cannot instantiate directly ; Never explicitly freed implicitly freed on program termination.
None of these symptoms are actually endemic to single instances, just the Singleton pattern. What can you do instead? Simply don't use the Singleton pattern. Quoting from the question: The idea was to have this one place in the app which keeps the data stored and synced, and then any new screens that are opened can just query most of what they need from there, without making repetitive requests for various supporting data from the server.
Why should you use the interface-based design? Three reasons: It makes the code easier to read; you can clearly understand from the interfaces exactly what data the dependent classes depend on. Improve this answer. Aaronaught Aaronaught First post I have ever read which actually explains DI as an alternative to global state. Thanks for the time and effort put into this. We are all better off as a result of this post. Why can't the Cache be a singleton?
Is it not a singleton if you pass it around and use dependency injection? Singleton is just about limiting ourselves to one instance, not about how it is accessed right? See my take on this: assoc. AdamSmith: Did you actually read any of this answer? Your question is answered in the first two paragraphs.
Singleton Pattern! Cawas and Adam Smith - Reading your links I get the feeling you didn't really read this answer - everything is already in there. Cawas I feel that the meat of this answer is the distinction between single-instance and singleton. Singleton is bad, single-instance is not. Dependency Injection is a nice, general way to use single-instances without having to use singletons.
Show 31 more comments. Again, we are looking at large-scale architectural and design issues. There is a gigantic amount of data that certain screens might need, but don't necessarily need. And you don't know until user actions have been taken which define this - and there are many, many combinations. The Cocoa frameworks often refer to a shared object or a shared instance.
Take a look at these examples. With the definition of the singleton pattern in mind, it appears to be a useful design pattern. Unfortunately, many developers misuse the singleton pattern and use it to conveniently access an object from anywhere in the project. Having global access to the singleton object is no more than a side effect of the singleton pattern. It's not what the singleton pattern is about.
I suppose it is tempting, if the only tool you have is a hammer, to treat everything as if it were a nail. A surprising number of developers struggle with this problem.
The singleton pattern appears to be solving a common problem, accessing an object in various places of the project. A singleton, no more than a fancy global, looks like the perfect fit for the problem. And it is the perfect fit if all you want to do is solve that problem.
There's more to the story, though. Have you ever wondered why some experienced developers consider the singleton pattern an anti-pattern? Why is it that such a useful pattern is often avoided by more senior developers? By using singletons, you almost always sacrifice transparency for convenience. But how much are you willing to sacrifice for that little bit of convenience? Convenience should not be high on your priority list if you're working on a software project.
Let me show you with an example what the problem is. A common problem in software projects is user management. It means that a user object needs to be created and managed. There are many solutions to this problem. If you're a fan of the singleton pattern, then you might create a singleton or a manager class that manages the currently signed in user. Only a single user can be signed in at a time. Problem solved? Often, multiple classes depend on the concrete implementation of a singleton.
This increases coupling in your system: When you want to change the behavior of the singleton, you probably have to check and change all the callers. You always have common coupling between all classes that use the singleton : They share a global variable - the singleton! In a system with low cohesion, it is hard to even find out where to make a change: It can be any one of 10 different modules.
Or it could be all of them. If you really want only one instance of a certain class, implement inversion of control for your dependencies. If you are already using a dependency injection container like Guice or Spring , just make sure all your singleton objects are managed by that container.
All those containers have a way to declare objects as singleton. Just construct one, and inject it into your classes as desired. It will pay off. There are basically two types of singletons:.
The first category is not a problem. This overhead is probably negligible for most applications but not all! Thanks to Samir Talwar who convinced me to write this blog post.
He also helped me review the very first draft of this post. Want to read more like this? Never miss one of my articles: Readers of my newsletter get my articles before anyone else. Subscribe here! My name is David and I help my clients to write better software and to get better at writing software.
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