Singleton Usage

Question

Enviroment:

I need to share some kind information (see HashMap) through different part of my code. I think that Singleton could be a perfect design pattern - except for one thing:

Information retrival (read init HashMap and populate it) is demand to another class (MVC). So, my question:

is possible to assign to my Singleton class this information from an external class, if yes, how?

And is a good usage of Singleton design pattern? Otherwise, what could you advise me?

I have a manager class that populates this Hashmap. HashMap contains information about "Users". Now, my manager starts n Threads. Every thread open a connection to db. In Db class handler, I have to check if this query contains users in my HashMap.

So at the moment I have: Manager -> Threads -> DBHandler

I want to avoid to pass my HashMap thourgh every constructor. That's why I thought about a Singleton.

Also, my HashMap, once populated, is just a READ-ONLY Map.

Answer 1

And is a good usage of Singleton design pattern?

No

The singleton pattern is used to enforce that there can only exist a single, unique instance of a class. You don't explicitly need that. It has the drawback of achieving this by holding a global state. This is what you want.

Do not pick a pattern when you don't need it's prime purpose and instead "need" what is the bad part of the pattern.

If you need global state in your program, there is no need to hide behind the singleton pattern. If you need a global state, go and create a global state.

There are multiple ways to do this without having a simple global variable. You could for example put this task into a specialized module. Most dependency injection containers will feature something like a "SingletonLifetime". This is not a singleton. It just means you get the same instance every time you ask your container for an instance.

Whether you use a dependency injection container or not, make sure you inject this state anywhere you need it, to make your program modular and unit-testable.

Answer 2

I would strongly recommend shying away from a singleton if at all possible. If you need your different components to reference the same object, then I would normally look to inject it into each component (preferably during construction).

At the very least it will make unit testing your components so much easier. I would refer you to this SO question/answer, which covers such issues.

You address the issue of possibly injecting this config into every component. I would take a higher level view, and configure your components using this config, and then inject those components e.g. (in Scala, but language largely immaterial here)

val entityDao = new EntityDao(config)
val entityMgr = new EntityManager(entityDao)

thus ensuring your components talk at a 'higher' level of abstraction where possible.

Answer 3

You could work it so that the external class defines the singleton, though it wouldn't quite be what people expect from a singleton since it's not always available.

That said, you shouldn't use a singleton for this. Singletons exist to restrict a type to only having a single instance ever. That is an exceptionally rare and limiting design, and nothing you've said makes me think it is warranted here. It seems you just want a global. If that's what you want, just use a global.

Answer 4

Singleton pattern is good for keeping config information that various parts of the application might need. That said, it is difficult to write unit tests for it. Therefore, if you absolutely must use singleton in your particular case and dependency injection is out of the question, write a regular class, write unit tests for it, and then just write a singleton wrapper around it, so that you have the best of both worlds.