有没有可行的替代单例模式？

You shouldn't have to go out of your way to avoid any pattern. The use of a pattern is either a design decision or a natural fit (it just falls into place). When you are designing a system, you have a choice to either use a pattern or not use the pattern. However, you shouldn't go out of your way to avoid anything that is ultimately a design choice.

I don't avoid the Singleton Pattern. Either it's appropriate and I use it or it's not appropriate and I don't use it. I believe that it is as simple as that.

The appropriateness (or lack thereof) of the Singleton depends on the situation. It's a design decision that must be made and the consequences of that decision must be understood (and documented).

Use a plain object and a factory object. The factory is responsible for policing the instance and the plain object details only with the configuration information (it contains for example) and behaviour.

Spring or any other IoC-Container does a reasonably good job in that. Since the classes are created and managed outside the app itself, the container can make simple classes singletons and inject them where needed.

If you're using a Singleton to represent a single data object, you could instead pass a data object around as a method parameter.

(although, I would argue this is the wrong way to use a Singleton in the first place)

If your issue is that you want to keep state, you want a MumbleManager class. Before you start working with a system, your client creates a MumbleManager, where Mumble is the name of the system. State is retained through that. Chances are your MumbleManager will contain a property bag which holds your state.

This type of style feels very C-like and not very object like - you'll find that objects that define your system will all have a reference to the same MumbleManager.

Actually if you design right from scratch on avoiding Singeltons, you may not have to work around not using Singletons by using static variables. When using static variables, you are also creating a Singleton more or less, the only difference is you are creating different object instances, however internally they all behave as if they were using a Singleton.

Can you maybe give a detailed example where you use a Singleton or where a Singleton is currently used and you are trying to avoid using it? This could help people to find a more fancy solution how the situation could be handled without a Singleton at all.

BTW, I personally have no problems with Singletons and I can't understand the problems other people have regarding Singletons. I see nothing bad about them. That is, if you are not abusing them. Every useful technique can be abused and if being abused, it will lead to negative results. Another technique that is commonly misused is inheritance. Still nobody would say inheritance is something bad just because some people horribly abuse it.

Alex Miller in "Patterns I Hate" quotes the following:

"When a singleton seems like the answer, I find it is often wiser to:

1. Create an interface and a default implementation of your singleton
2. Construct a single instance of your default implementation at the “top” of your system. This might be in a Spring config, or in code, or defined in a variety of ways depending on your system.
3. Pass the single instance into each component that needs it (dependency injection)

Personally for me а much more sensible way to implement something that behaves like singleton is to use fully static class(static members , static methods , static properties). Most of the time I implement it in this way (I can not think of any behaviour differences from user point of view)

What do you mean, what are my techniques to avoid it?

To "avoid" it, that implies that there are many situations that I come across in which the singleton pattern is a naturally good fit, and hence that I have to take some measures to defuse these situations.

But there are not. I don't have to avoid the singleton pattern. It simply doesn't arise.

I think the best place to police the singleton is at the class design level. At this stage, you should be able to map out the interactions between classes and see if something absolutely, definitely requires that only 1 instance of this class is ever in existence at any time of the applications life.

If that is the case, then you have a singleton. If you are throwing singletons in as a convenience during coding then you should really be revisiting your design and also stop coding said singletons :)

And yes, 'police' is the word I meant here rather than 'avoid'. The singleton isn't something to be avoided (in the same way that goto and global variables aren't something to be avoided). Instead, you should be monitoring it's use and ensuring that it is the best method to get what you want done effectively.

I use singleton mostly as "methods container", with no state at all. If I need to share these methods with many classes and want to avoid the burden of instantiation and initialization I create a context/session and initialize all the classes there; everything which refers to the session has also access to the "singleton" thereby contained.

The singleton pattern exists because there are situations when a single object is needed to provide a set of services.

Even if this is the case I still consider the approach of creating singletons by using a global static field/property representing the instance, inappropriate. It's inappropriate because it create a dependency in the code between the static field and the object not, the services the object provides.

So instead of the classic, singleton pattern, I recommend to use the service 'like' pattern with serviced containers, where instead of using your singleton through a static field, you obtain a reference to it through a a method requesting the type of service required.

*pseudocode* currentContainer.GetServiceByObjectType(singletonType)
//Under the covers the object might be a singleton, but this is hidden to the consumer.

instead of single global

*pseudocode* singletonType.Instance

This way when you want to change type of an object from singleton to something else, you'll have and easy time doing it. Also as an and added benefit you don't have to pass around allot of object instances to every method.

Also see Inversion of Control, the idea is that by exposing singletons directly to the consumer, you create a dependency between the consumer and the object instance, not the object services provided by the object.

My opinion is to hide the use of the singleton pattern whenever possible, because it is not always possible to avoid it, or desirable.

The finest solution I have came across is using the factory pattern to construct instances of your classes. Using the pattern, you can assure that there is only one instance of a class that is shared among the objects that use it.

I though it would be complicated to manage, but after reading this blog post "Where Have All the Singletons Gone?", it seems so natural. And as an aside, it helps a lot with isolating your unit tests.

In summary, what you need to do? Whenever an object depends on another, it will receive an instance of it only through its constructor (no new keyword in your class).

class NeedyClass {


private ExSingletonClass exSingleton;


public NeedyClass(ExSingletonClass exSingleton){
this.exSingleton = exSingleton;
}


// Here goes some code that uses the exSingleton object
}

And then, the factory.

class FactoryOfNeedy {


private ExSingletonClass exSingleton;


public FactoryOfNeedy() {
this.exSingleton = new ExSingletonClass();
}


public NeedyClass buildNeedy() {
return new NeedyClass(this.exSingleton);
}
}

As you will instantiate your factory only once, there will be a single instantiation of exSingleton. Every time you call buildNeedy, the new instance of NeedyClass will be bundled with exSingleton.

I hope this helps. Please point out any mistakes.

To understand the proper way to workaround Singletons, you need to understand what is wrong with Singletons (and global state in general):

Singletons hide dependencies.

Why is that important?

Because If you hide the dependencies you tend to lose track of the amount of coupling.

You might argue that

void purchaseLaptop(String creditCardNumber, int price){
CreditCardProcessor.getInstance().debit(creditCardNumber, amount);
Cart.getInstance().addLaptop();
}

is simpler than

void purchaseLaptop(CreditCardProcessor creditCardProcessor, Cart cart,
String creditCardNumber, int price){
creditCardProcessor.debit(creditCardNumber, amount);
cart.addLaptop();
}

but at least the second API makes it clear exactly what the method's collaborators are.

So the way to workaround Singletons is not to use static variables or service-locators, but to change the Singleton-classes into instances, which are instantiated in the scope where they make sense and injected into the components and methods that need them. You might use a IoC-framework to handle this, or you might do it manually, but the important thing is to get rid of your global state and make the dependencies and collaborations explicit.

Having not programmed in an intensely object-oriented environment (e.g. Java), I'm not completely up on the intricacies of the discussion. But I have implemented a singleton in PHP 4. I did it as a way of creating a 'black-box' database handler that automatically initialized and didn't have to be passed up and down function calls in an incomplete and somewhat broken framework.

Having read some links of singleton patterns, I'm not completely sure I would implement it in quite the same way again. What was really needed was multiple objects with shared storage (e.g. the actual database handle) and this is pretty much what my call turned into.

Like most patterns and algorithms, using a singleton 'just because it's cool' is The Wrong Thing To Do. I needed a truly 'black-box' call that happened to look a lot like a singleton. And IMO that's the way to tackle the question: be aware of the pattern, but also look at it's wider scope and at what level it's instance needs to be unique.

Monostate (described in Robert C. Martin's Agile Software Development) is an alternative to singleton. In this pattern the class's data are all static but the getters/setters are non-static.

For example:

public class MonoStateExample
{
private static int x;


public int getX()
{
return x;
}


public void setX(int xVal)
{
x = xVal;
}
}


public class MonoDriver
{
public static void main(String args[])
{
MonoStateExample m1 = new MonoStateExample();
m1.setX(10);


MonoStateExample m2 = new MonoStateExample();
if(m1.getX() == m2.getX())
{
//singleton behavior
}
}
}

Monostate has similar behavior to singleton but does so in a way where the programmer is not necessarily aware of the fact that a singleton is being used.