DZone Snippets: singleton code

Java Singleton Template for Eclipse

marcelotmelo (Marcelo Melo) — Fri, 02 Mar 2007 19:42:35 GMT

This is a template for easily creating an implementation of the Singleton Pattern on Eclipse. Open Window->Preferences->Java->Editor->Templates click on New and insert the code below on the pattern text area; add a name (I suggest "singleton") - whenever you type this name and press Ctrl+Space the code will be inserted in your class - and you're good to go.



private static ${enclosing_type} instance;



private ${enclosing_type}(){}



public static ${enclosing_type} getInstance(){

	if(null == instance){

		instance = new ${enclosing_type}();

	}

	return instance;

}

Singleton

wjchay (Chay Weei Jye) — Wed, 14 Feb 2007 02:42:59 GMT

The proper way of implementing a Singleton.



public class MySingleton {

    private static final MySingleton INSTANCE = new MySingleton();



    private MySingleton() {}



    public static final MySingleton getInstance() {

      return INSTANCE;

    }

	 

	/**

	* Normal deserialization returns a new instance of an object. This ensures that only one instance is in existence. 

         * Deserialization can either create a new instance and leave the deserialized object to be garbage collected or

         * reuse the deserialized instance.

	*/

	private Object readResolve() throws ObjectStreamException {

	  return INSTANCE;

	}

}

singleton in ruby

ovhaag (Oliver Haag) — Sat, 07 Oct 2006 14:47:26 GMT

// simple singleton



class Singleton 

  private_class_method :new

  @@singleton = nil

  

  def Singleton.create

    @@singleton = new unless @@singleton

    @@singleton

  end

end

Singleton pattern in python

korakot (Korakot Chaovavanich) — Sun, 18 Sep 2005 13:01:10 GMT



class Borg:

    __shared_state = {}

    def __init__(self):

        self.__dict__ = self.__shared_state

What a pythonic way to use Singleton. It uses shared-state
approach where you can actually have many instances as you want
but they all share the same state. See Alex's recipe.



>>> b = Borg()

>>> b.x = 1

>>> c = Borg()

>>> c.x

1

python singleton and singleton borg

manatlan (manatlan) — Thu, 08 Sep 2005 00:12:20 GMT

Singleton with same states



# http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/66531

class Borg:

    __shared_state = {}

    def __init__(self):

        self.__dict__ = self.__shared_state



a=Borg()

a.toto = 12



b=Borg()

print b.toto

print id(a),id(b) # different ! but states are sames

real Singleton instance



class Singleton(object):

    def __new__(type):

        if not '_the_instance' in type.__dict__:

            type._the_instance = object.__new__(type)

        return type._the_instance



a=Singleton()

a.toto = 12



b=Singleton()

print b.toto

print id(a),id(b) # the same !!

Flexible Singleton

drwitt (Carsten Witt) — Mon, 11 Apr 2005 05:52:59 GMT

PHP function Singleton allows you holding exactly one object of a class in memory. If an object of a given class doesn't exist, Singleton will create and store a new one in static array singleton. It then (and otherwise) will return a reference to it.

Adding an optional ID allows holding more than one object a class. Assigned ID will distinguish array keys.



/**

 * Singleton Repository

 * @param string $class PHP Class Name

 * @param string $id Optional Object ID

 * @return reference Reference to existing Object

 */

function &Singleton($class, $id='') {

  static $singleton = array();

  if (!array_key_exists($class.$id, $singleton))

    $singleton[$class.$id] = &new $class();

  $reference = &$singleton[$class.$id];

  return $reference;

}

Use like this:
[edit] I'm sorry there was a mistake in the first exmaple for three days or so. Fixed.



# first call: create object

$site_user=&Singleton('Student');

$site_user->Drink_Beer(5);



# second call: get a reference

$current_user=&Singleton('Student');

echo $current_user->Show_Beers_Counter();

#will be 5



#Two different objects

$one=&Singleton('Some_Class','one');

$two=&Singleton('Some_Class','two');

Works fine with PHP4, not tested on PHP5