Creating the Singleton design pattern in PHP5
如何创建一个使用PHP5类的Singleton类？
/**
* Singleton class
*
*/
final class UserFactory
{
/**
* Call this method to get singleton
*
* @return UserFactory
*/
public static function Instance()
{
static $inst = null;
if ($inst === null) {
$inst = new UserFactory();
}
return $inst;
}
/**
* Private ctor so nobody else can instantiate it
*
*/
private function __construct()
{
}
}
To use:
$fact = UserFactory::Instance();
$fact2 = UserFactory::Instance();
$fact == $fact2;
But:
$fact = new UserFactory()
Throws an error.
See http://php.net/manual/en/language.variables.scope.php#language.variables.scope.static to understand static variable scopes and why setting static $inst = null; works.
PHP 5.3 allows the creation of an inheritable Singleton class via late static binding:
class Singleton
{
protected static $instance = null;
protected function __construct()
{
//Thou shalt not construct that which is unconstructable!
}
protected function __clone()
{
//Me not like clones! Me smash clones!
}
public static function getInstance()
{
if (!isset(static::$instance)) {
static::$instance = new static;
}
return static::$instance;
}
}
This solves the problem, that prior to PHP 5.3 any class that extended a Singleton would produce an instance of its parent class instead of its own.
Now you can do:
class Foobar extends Singleton {};
$foo = Foobar::getInstance();
And $foo will be an instance of Foobar instead of an instance of Singleton.
Unfortunately Inwdr's answer breaks when there are multiple subclasses.
Here is a correct inheritable Singleton base class.
class Singleton
{
private static $instances = array();
protected function __construct() {}
protected function __clone() {}
public function __wakeup()
{
throw new Exception("Cannot unserialize singleton");
}
public static function getInstance()
{
$cls = get_called_class(); // late-static-bound class name
if (!isset(self::$instances[$cls])) {
self::$instances[$cls] = new static;
}
return self::$instances[$cls];
}
}
Test code:
class Foo extends Singleton {}
class Bar extends Singleton {}
echo get_class(Foo::getInstance()) . "n";
echo get_class(Bar::getInstance()) . "n";
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