I'm working in a web app framework, and part of it consists of a number of services, all implemented as singletons. They all extend a Service class, where the singleton behaviour is implemented, looking something like this:
class Service {
protected static $instance;
public function Service() {
if (isset(self::$instance)) {
throw new Exception('Please use Service::getInstance.');
}
}
public static function &getInstance() {
if (empty(self::$instance)) {
self::$instance = new self();
}
return self::$instance;
}
}
Now, if I have a class called FileService implemented like this:
class FileService extends Service {
// Lots of neat stuff in here
}
... calling FileService::getInstance() will not yield a FileService instance, like I want it to, but a Service instance. I assume the problem here is the "self" keyword used in the Service constructor.
Is there some other way to achieve what I want here? The singleton code is only a few lines, but I'd still like to avoid any code redundance whenever I can.
Code:
abstract class Singleton
{
protected function __construct()
{
}
final public static function getInstance()
{
static $instances = array();
$calledClass = get_called_class();
if (!isset($instances[$calledClass]))
{
$instances[$calledClass] = new $calledClass();
}
return $instances[$calledClass];
}
final private function __clone()
{
}
}
class FileService extends Singleton
{
// Lots of neat stuff in here
}
$fs = FileService::getInstance();
If you use PHP < 5.3, add this too:
// get_called_class() is only in PHP >= 5.3.
if (!function_exists('get_called_class'))
{
function get_called_class()
{
$bt = debug_backtrace();
$l = 0;
do
{
$l++;
$lines = file($bt[$l]['file']);
$callerLine = $lines[$bt[$l]['line']-1];
preg_match('/([a-zA-Z0-9\_]+)::'.$bt[$l]['function'].'/', $callerLine, $matches);
} while ($matches[1] === 'parent' && $matches[1]);
return $matches[1];
}
}
Had I paid more attention in 5.3 class, I would have known how to solve this myself. Using the new late static binding feature of PHP 5.3, I believe Coronatus' proposition can be simplified into this:
class Singleton {
protected static $instance;
protected function __construct() { }
final public static function getInstance() {
if (!isset(static::$instance)) {
static::$instance = new static();
}
return static::$instance;
}
final private function __clone() { }
}
I tried it out, and it works like a charm. Pre 5.3 is still a whole different story, though.
This is fixed Johan's answer. PHP 5.3+
abstract class Singleton
{
protected function __construct() {}
final protected function __clone() {}
final public static function getInstance()
{
static $instance = null;
if (null === $instance)
{
$instance = new static();
}
return $instance;
}
}
I have found a good solution.
the following is my code
abstract class Singleton
{
protected static $instance; // must be protected static property ,since we must use static::$instance, private property will be error
private function __construct(){} //must be private !!! [very important],otherwise we can create new father instance in it's Child class
final protected function __clone(){} #restrict clone
public static function getInstance()
{
#must use static::$instance ,can not use self::$instance,self::$instance will always be Father's static property
if (! static::$instance instanceof static) {
static::$instance = new static();
}
return static::$instance;
}
}
class A extends Singleton
{
protected static $instance; #must redefined property
}
class B extends A
{
protected static $instance;
}
$a = A::getInstance();
$b = B::getInstance();
$c = B::getInstance();
$d = A::getInstance();
$e = A::getInstance();
echo "-------";
var_dump($a,$b,$c,$d,$e);
#object(A)#1 (0) { }
#object(B)#2 (0) { }
#object(B)#2 (0) { }
#object(A)#1 (0) { }
#object(A)#1 (0) { }
You can refer http://php.net/manual/en/language.oop5.late-static-bindings.php
for more info
Use trait instead of a abstract class allows to extend a singleton class.
Use the trait SingletonBase for a parent singleton class.
Use the trait SingletonChild for its singleton childs.
interface Singleton
{
public static function getInstance(): Singleton;
}
trait SingletonBase
{
private static $instance=null;
abstract protected function __construct();
public static function getInstance(): Singleton {
if (is_null(self::$instance)) {
self::$instance=new static();
}
return self::$instance;
}
protected function clearInstance(): void {
self::$instance=null;
}
public function __clone()/*: void*/ {
trigger_error('Class singleton '.get_class($this).' cant be cloned.');
}
public function __wakeup(): void {
trigger_error('Classe singleton '.get_class($this).' cant be serialized.');
}
}
trait SingletonChild
{
use SingletonBase;
}
class Bar
{
protected function __construct(){
}
}
class Foo extends Bar implements Singleton
{
use SingletonBase;
}
class FooChild extends Foo implements Singleton
{
use SingletonChild; // necessary! If not, the unique instance of FooChild will be the same as the unique instance of its parent Foo
}
