Singleton in Gosu

[Jeffrey Backes]

Is this a (the) correct way to implement a thread safe singleton in gosu?

uses java.util.concurrent.locks.ReentrantLock

class Factors {

  private static var _lock = new ReentrantLock()
  private static var _instance: Factors;

  private construct() {
  }

  static property get Instance(): Factors {
    if (_instance == null) {
      using (_lock) {
        if (_instance == null) {
          _instance = new Factors()
          ...
        }
      }
    }
  }
}

  return _instance
}

[Justin Sampson]

Hi Jeff,

 

That's only going to be correct if _instance is volatile, which isn't (yet) supported in Gosu. Better to avoid hand-coding the double-checked locking logic altogether.

 

The first option should always be to initialize the singleton eagerly. Only bother making it lazy if it's really expensive to construct.

 

If you do really want to make it lazy, my favorite tool in Java is Suppliers.memoize() in Google's Guava libarary. You should be able to use it in Gosu like _instance = Suppliers.memoize(\ -> new Factors()) if you have Guava in your classpath.

 

If you don't want to depend on Guava, consider LockingLazyVar in Gosu's own standard library, but be sure to always provide an explicit lock in the constructor because otherwise it uses a global lock, which could cause scalability issues.

 

Cheers,

Justin

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