I have a singleton class which needs to be intialized by castle? I'm a little newbie in castle. I have looked for singleton in castle and I saw setting the lifestyle attribute of component to "Singleton" seems enough. But in that case it seems I don't need to implement the class in "Singleton Pattern". Just plain class definition for my class seems ok.
Is castle provide my class as singleton to me even I define it as a normal class? Or I am missing something?
No, you are absolutely right - the container will instantiate your class the first time it is needed, and from then on the instance will live inside your container.
Stay away from the singleton pattern - it is evil! :)
(no, seriously - there are very few cases where implementing a singleton is acually necessary... and if you are using an IoC container, you will never have to implement singletons by using static instances, which can seriously hurt your design by coupling your classes to those static instances)
Related
I read so many bad things about singleton pattern (What is so bad about singletons?), however, #Singleton is used everywhere in this Micronaut doc to illustrate inversion of control https://docs.micronaut.io/1.3.0.M2/guide/index.html#ioc
When is it appropriate to use #Singleton? For example, if I have a UserInfoService that has getUserInfo, createUserInfo, updateUserInfo method, is it a good idea to use #Singleton?
Another separate question is when do I use #Prototype, because if I don't use any annotation for a function/class, isn't it by default a prototype (as in I initiate a new instance of it in another class/function)?
When is it appropriate to use #Singleton? For example, if I have a
UserInfoService that has getUserInfo, createUserInfo, updateUserInfo
method, is it a good idea to use #Singleton?
UserInfoService should almost certainly be a stateless singleton.
Another separate question is when do I use #Prototype, because if I
don't use any annotation for a function/class, isn't it by default a
prototype (as in I initiate a new instance of it in another
class/function)?
If you initiate a new instance, then the annotation doesn't matter. The annotation only affects instances that the Micronaut container creates for you. For #Singleton the container creates a single instance and injects the same instance at all injection points that require it. For #Prototype the container creates a new instance for each injection point.
All of this is about application design more than it is about Micronaut. Micronaut provides a simple mechanism for you to declaratively express (by way of #Singleton or #Prototype) whether or not you want the same instance to be shared or not but the issue is really about application design. In general you should prefer stateless singletons. If for some reason you have a bean that must be stateful and you have good reasons to not want to share instances from different contexts, then #Prototype might be appropriate.
I hope that helps.
Let's be 100% clear that the GoF Singleton design pattern and the #Singleton annotation are two different things.
Consider that the Singleton design pattern requires you to change your code. You must write the code in such a way as to prevent multiple instantiations.
On the other hand, you can add the #Singleton annotation to any class you like, without modifying its code to prevent instantiation; and as #Jeff-Scott-Brown points out, you can instantiate this Micronaut "Singleton" repeatedly by simply calling its constructor.
The #Singleton annotation does not exhibit those negative consequences you've read about in the Singleton design pattern, because it doesn't implement the Singleton design pattern. The term is overloaded to mean two different things here.
On a related note the #Prototype annotation doesn't follow the GoF Prototype design pattern either. Spring prototype following prototype design pattern
As Jeff Scott Brown covers most parts of your question, your have to be careful to the nuance between Singleton scope in Spring and singleton pattern.
Some of the main differences between these 2 are
Singleton pattern ensures one instance of a particular class of per
class loader.
Spring Singleton is “per container per bean”.
Hope that helps.
Apparently, singletons are bad, and a factory method is recommended. I'm wondering if a singleton toolbox is any better than a singleton.
In my opinion, It's really weak to think that singletons are bad,factory methods are good.
Each of them has preferences. As consequence, I'm sure that there is misunderstanding here.
I know that wikipedia is not the best source. But check out the definition of them. The range of situations are not the same for these patterns.
In software engineering, the singleton pattern is a design pattern that restricts the instantiation of a class to one object. This is useful when exactly one object is needed to coordinate actions across the system. The concept is sometimes generalized to systems that operate more efficiently when only one object exists, or that restrict the instantiation to a certain number of objects. The term comes from the mathematical concept of a singleton.
In class-based programming, the factory method pattern is a creational pattern which uses factory methods to deal with the problem of creating objects without specifying the exact class of object that will be created. This is done by creating objects via calling a factory method—either specified in an interface and implemented by child classes, or implemented in a base class and optionally overridden by derived classes—rather than by calling a constructor.
I have a direct question: Is a good pratice use Singleton pattern to control a class responsable for Login tasks, or another pattern is more appropriated? Or do not exist a patter to do this kind os issue?
Thanks.
Few people use singletons these days as they are almost becoming anti-patterns. I would recommend to learn Dependency Injection (DI).
With DI you can register the object you wish to use as singleton with a container and that container will serve or give that object to all other objects that need it. Of course you can register the object as a singleton - but not the usual singleton - and the container will guarantee that all objects which need it will receive the same instance.
Nevertheless, if you are building a small application then It would be better to use the Singleton pattern and avoid DI.
Jon Skeet has a very nice article about Singleton pattern or if you are using Java then you could use Enumerations to implement it, look implementations techniques on Google.
The singleton pattern is used when you have to prevent the creation of more than one instance of the same class. I do not really see the situation in which a login class should only have one instance so i would say using this pattern for a login class is overkill.
Then again, introducing a DI framework when you only need a simple singleton... now thats overkill :)
Yes and no. There's no 'best' or 'worse' practise.
Just do it, if using a singleton makes it easier to test and if the approach will get you to the pub earlier than learning and implementing DI just for the purposes of login.
Once the singleton pattern is understood, writing subsequent singleton classes in C# is a brainless exercise. I would hope that the framework would help you by providing an interface or a base class to do that. Here is how I envision it:
public sealed class Schablone : ISingleton<Schablone>
{
// Stuff forced by the interface goes here
// Extra logic goes here
}
Does what I am looking for exist?
Is there some syntactic sugar for constructing a singleton class - whether with an interface, a class attribute, etc.?
Can one write a useful and bullet-proof ISingleton themselves? Care to try?
Thanks!
How often do you find yourself writing singleton classes?
I'd suggest that if it's a sufficiently frequent task for you that you feel you're losing productivity, you may well be overusing the pattern. I find that it's very rarely appropriate - but can easily lead to hard-to-test code.
Writing a singleton class in C# isn't just brainless - it's almost entirely painless. The boiler plate required is tiny, assuming you don't require absolute laziness to the point where the singleton shouldn't be instantiated just by calling other static methods.
public sealed class Singleton
{
private static readonly Singleton instance = new Singleton();
public static Singleton Instance { get { return instance; } }
private Singleton() {}
// Omit if you don't care about type initializer laziness
static Singleton() {}
}
Obviously you'd need a class declaration anyway, so you've saved either three or four lines, depending on whether you need the static constructor. Personally I don't think that's so time consuming for the rare occasions on which a singleton is valid that it's worth being part of the framework.
Indeed, it's hard to see how it could be a framework feature rather than a language feature... at the very least the compiler would have to be aware of it, to understand that there will be some magic "Instance" property (or whatever) that it should make available to callers.
As Eric Lippert is fond of pointing out (entirely reasonably, I might add) language features have great costs associated with them in terms of design, implementation and testing. Furthermore, there's the burden of each developer learning about the new language feature. In short, they need to really earn their value - and for me, the singleton pattern isn't sufficiently useful and important that it makes the bar.
What is there to force? A singleton can be implemented with as little as a single property and a field. It doesn't seem like there is a very good value/cost ratio here for any framework developers to bother with.
Not to mention, the singleton pattern is often a poor one to use, and IMO it'd be bad to have the framework promoting them. To me, a singleton is almost always a code smell.
As others and me (in comments) have pointed out, the singleton pattern is rarely appropriate.
Still, since some believe that this could not work, here's the helper class to implement such a singleton (inspired by the linked article about how to implement singletons):
public static class Singleton<T> where T: class, new() {
public static T Instance {
get {
return Nested.instance;
}
}
private static class Nested {
public static readonly T instance = new T();
static Nested() {}
}
}
The class to be used as singleton needs to have a public default constructor.
You could then use it as follows:
Singleton<MyClass>.Instance.DoSomething();
No, there is nothing in the .Net framework like this. You have to write singletons yourself. It would be a bit hard to have a definative API for singletons as there are so many ways of implementing them, and depending on your goals depends on what approach you will take.
I would suggest you read this article on singletons. (Written by Jon - He's a wise guy, listen to him) for guidance and tips on the various techniques. It covers some stuff that most people might not have thought about.
[A lot of people consider the singleton pattern to be a dubious design pattern. I won't go into the reasons here, but if your interested google has loads of info. Just be careful you don't overuse it unnecessarily.]
I don't think that is possible.
How would an interface prohibit you from adding a public constructor to your class?
After all, writing a singleton is not that much work once you understood the basic pattern (and the caveats of making it thread-safe).
Cut. Paste. Next.
Use an Inversion of Control container to manage it for you.
With StructureMap, you can declare any type as a singleton:
ObjectFactory.Initilialize(x => x.ForSingletonOf<Schablone>());
To retrieve it:
ObjectFactory.GetInstance<Schablone>();
.Net 4.0 comes with MEF. And you can use MEF to instantiate a singleton. Thru the shared creation policy. I'll dig out a code sample....
EDIT ----
better still, here's some doco....
http://mef.codeplex.com/wikipage?title=Parts%20Lifetime&referringTitle=Guide
It was recommended to me that, when using an IOC container, I should change this:
class Foobar: IFoobar, IDisposable {};
Into this:
interface IFoobar: IDisposable{};
class Foobar : IFoobar{};
I'm wondering if this is ok, or if it solves one problem and creates another. It certainly solves the problem where I badly want to do this:
using( IFoobar = myContainer.Resolve<IFoobar>() )
{ ... }
And now I know that any substitute won't cause a run-time error.
On the other hand, now all my mock objects must handle IDisposable too. Am I right that most any mocking framework handles this easily? If yes, then perhaps this is a non-issue.
Or is it? Is there another hidden gotcha I should watch for? It certainly occurs to me that if I were using an IOC container not for unit tests / mocking, but for true service independence, then this might be a problem because perhaps only one of my swappable services actually deals with unmanaged resources (and now I'm having to implement empty "IDispose" operations in these other services).
Even this latter issue I suppose I could live with, for the sake of gaining the ability to employ the "using" statement as I demoed above. But am I following a popular convention, or am I missing an entirely different and better solution?
Deriving an interface from IDisposable is in my opinion a design smell that indicates a Leaky Abstraction. As Nicholas Blumhardt put it:
an interface [...] generally shouldn't be disposable. There's no way for the one defining an interface to foresee all possible implementations of it - you can always come up with a disposable implementation of practically any interface.
Consider why you want to add IDisposable to your interface. It's probably because you have a particular implementation in mind. Hence, the implementation leaks into the abstraction.
An DI Container worth its salt should know when it creates an instance of a disposable type. When you subsequently ask the container to release an object graph, it should automatically dispose the disposable components (if their time is up according to their lifestyles).
I know that at least Castle Windsor and Autofac does this.
So in your case, you should keep your type like this:
class Foobar: IFoobar, IDisposable {};
You may find Nicholas Blumhardt's post The Relationship Zoo interesting as well - particularly the discussion about Owned<T>.