如何使用PHP5类创建单例类?
当前回答
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.
其他回答
我同意第一个答案,但我也会将类声明为final,这样它就不能被扩展,因为扩展单例违背了单例模式。此外,实例变量应该是私有的,这样就不能直接访问它。还要将__clone方法设为私有,这样就不能克隆单例对象。
下面是一些示例代码。
/**
* Singleton class
*
*/
final class UserFactory
{
private static $_instance = null;
/**
* Private constructor
*
*/
private function __construct() {}
/**
* Private clone method
*
*/
private function __clone() {}
/**
* Call this method to get singleton
*
* @return UserFactory
*/
public static function getInstance()
{
if (self::$_instance === null) {
self::$_instance = new UserFactory();
}
return self::$_instance;
}
}
示例使用
$user_factory = UserFactory::getInstance();
这阻止你做什么(这将违反单例模式..
你不能这么做!
$user_factory = UserFactory::$_instance;
class SecondUserFactory extends UserFactory { }
<?php
/**
* Singleton patter in php
**/
trait SingletonTrait {
protected static $inst = null;
/**
* call this method to get instance
**/
public static function getInstance(){
if (static::$inst === null){
static::$inst = new static();
}
return static::$inst;
}
/**
* protected to prevent clonning
**/
protected function __clone(){
}
/**
* protected so no one else can instance it
**/
protected function __construct(){
}
}
使用方法:
/**
* example of class definitions using SingletonTrait
*/
class DBFactory {
/**
* we are adding the trait here
**/
use SingletonTrait;
/**
* This class will have a single db connection as an example
**/
protected $db;
/**
* as an example we will create a PDO connection
**/
protected function __construct(){
$this->db =
new PDO('mysql:dbname=foodb;port=3305;host=127.0.0.1','foouser','foopass');
}
}
class DBFactoryChild extends DBFactory {
/**
* we repeating the inst so that it will differentiate it
* from UserFactory singleton
**/
protected static $inst = null;
}
/**
* example of instanciating the classes
*/
$uf0 = DBFactoryChild::getInstance();
var_dump($uf0);
$uf1 = DBFactory::getInstance();
var_dump($uf1);
echo $uf0 === $uf1;
respose:
object(DBFactoryChild)#1 (0) {
}
object(DBFactory)#2 (0) {
}
如果你使用的是PHP 5.4: trait,它是一个选项,所以你不必为了拥有单例模式而浪费继承层次结构
还要注意你是使用trait还是扩展Singleton类 如果你不添加以下代码行,就会创建子类的单例:
protected static $inst = null;
在儿童班
意想不到的结果将是:
object(DBFactoryChild)#1 (0) {
}
object(DBFactoryChild)#1 (0) {
}
我很久以前就写过,想在这里分享
class SingletonDesignPattern {
//just for demo there will be only one instance
private static $instanceCount =0;
//create the private instance variable
private static $myInstance=null;
//make constructor private so no one create object using new Keyword
private function __construct(){}
//no one clone the object
private function __clone(){}
//avoid serialazation
public function __wakeup(){}
//ony one way to create object
public static function getInstance(){
if(self::$myInstance==null){
self::$myInstance=new SingletonDesignPattern();
self::$instanceCount++;
}
return self::$myInstance;
}
public static function getInstanceCount(){
return self::$instanceCount;
}
}
//now lets play with singleton design pattern
$instance = SingletonDesignPattern::getInstance();
$instance = SingletonDesignPattern::getInstance();
$instance = SingletonDesignPattern::getInstance();
$instance = SingletonDesignPattern::getInstance();
echo "number of instances: ".SingletonDesignPattern::getInstanceCount();
/**
* 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 $inst = null;的工作原理。
所有这些复杂性(“后期静态绑定”……对我来说,harumph)只是PHP对象/类模型破碎的标志。如果类对象是一级对象(参见Python),那么"$_instance"将是一个类实例变量——类对象的成员,而不是其实例的成员/属性,也与它的后代共享相反。在Smalltalk世界中,这就是“类变量”和“类实例变量”之间的区别。
在PHP中,在我看来,我们似乎需要牢记模式是编写代码的指南——我们可能会考虑Singleton模板,但试图编写从实际的“Singleton”类继承的代码对PHP来说是错误的(尽管我认为一些有进取心的人可以创建一个合适的SVN关键字)。
我将继续使用共享模板分别编写每个单例。
请注意,我绝对不会参与单身是邪恶的讨论,生命太短暂了。