如何使用PHP5类创建单例类?
当前回答
PHP 5.3允许通过后期静态绑定创建可继承的单例类:
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;
}
}
这解决了一个问题,在PHP 5.3之前,任何扩展了Singleton的类都会生成父类的实例,而不是它自己的实例。
现在你可以做:
class Foobar extends Singleton {};
$foo = Foobar::getInstance();
$foo将是Foobar的一个实例而不是Singleton的一个实例。
其他回答
protected static $_instance;
public static function getInstance()
{
if(is_null(self::$_instance))
{
self::$_instance = new self();
}
return self::$_instance;
}
这段代码可以应用于任何类,而无需关心类名。
class Database{
//variable to hold db connection
private $db;
//note we used static variable,beacuse an instance cannot be used to refer this
public static $instance;
//note constructor is private so that classcannot be instantiated
private function __construct(){
//code connect to database
}
//to prevent loop hole in PHP so that the class cannot be cloned
private function __clone() {}
//used static function so that, this can be called from other classes
public static function getInstance(){
if( !(self::$instance instanceof self) ){
self::$instance = new self();
}
return self::$instance;
}
public function query($sql){
//code to run the query
}
}
Access the method getInstance using
$db = Singleton::getInstance();
$db->query();
这篇文章涵盖了相当广泛的主题: http://www.phptherightway.com/pages/Design-Patterns.html#singleton
Note the following: The constructor __construct() is declared as protected to prevent creating a new instance outside of the class via the new operator. The magic method __clone() is declared as private to prevent cloning of an instance of the class via the clone operator. The magic method __wakeup() is declared as private to prevent unserializing of an instance of the class via the global function unserialize(). A new instance is created via late static binding in the static creation method getInstance() with the keyword static. This allows the subclassing of the class Singleton in the example.
下面是我的例子,它提供了调用as $var = new Singleton()的能力,并创建了3个变量来测试它是否创建了新对象:
class Singleton{
private static $data;
function __construct(){
if ($this::$data == null){
$this->makeSingleton();
}
echo "<br/>".$this::$data;
}
private function makeSingleton(){
$this::$data = rand(0, 100);
}
public function change($new_val){
$this::$data = $new_val;
}
public function printme(){
echo "<br/>".$this::$data;
}
}
$a = new Singleton();
$b = new Singleton();
$c = new Singleton();
$a->change(-2);
$a->printme();
$b->printme();
$d = new Singleton();
$d->printme();
/**
* Singleton class
*
*/
final class UserFactory
{
private static $inst = null;
// Prevent cloning and de-serializing
private function __clone(){}
private function __wakeup(){}
/**
* Call this method to get singleton
*
* @return UserFactory
*/
public static function Instance()
{
if ($inst === null) {
$inst = new UserFactory();
}
return $inst;
}
/**
* Private ctor so nobody else can instantiate it
*
*/
private function __construct()
{
}
}
使用方法:
$fact = UserFactory::Instance();
$fact2 = UserFactory::Instance();
$fact == $fact2;
But:
$fact = new UserFactory()
抛出错误。
参见http://php.net/manual/en/language.variables.scope.php#language.variables.scope.static了解静态变量的作用域以及为什么设置static $inst = null;的工作原理。
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