php单例数据库连接，这个代码坏的做法吗？ [英] php singleton database connection, is this code bad practice?

问题描述

我想创建一个简单的使用singleton类连接到mysql数据库和查询，代码工作正常，我没有任何问题，但因为我是新的OOP我

I'm trying to create a simple to use singleton class to connect to mysql database and do queries, the code works fine and i haven't had any problems with it, but since I'm new to OOP I'm wondering whether this is bad practice or not.

以下是类

class Database {
private $databaseName = 'dbname';
private $host = 'localhost';
private $user = 'user';
private $password = 'pass'; 
private static $instance; //store the single instance of the database

private function __construct(){
    //This will load only once regardless of how many times the class is called
    $connection = mysql_connect($this->host, $this->user, $this->password) or die (mysql_error());
    $db = mysql_select_db($this->databaseName, $connection) or die(mysql_error()); 
    echo 'DB initiated<br>';
}

//this function makes sure there's only 1 instance of the Database class
public static function getInstance(){
    if(!self::$instance){
        self::$instance = new Database();
    }
    return self::$instance;     
}

public function connect() { 
    //db connection
} 
public function query($query) {
    //queries   
    $sql = mysql_query($query) or die(mysql_error()); 
    return $sql;
}

public function numrows($query) {
    //count number of rows  
    $sql = $this->query($query);
    return mysql_num_rows($sql);
}


}

//Intantiate the class
$database = Database::getInstance();

当我想使用类时，我会这样做：

and when i want to use the class I would do:

$query = "SELECT * FROM registrations";
echo $database->numrows($query);
$sql = $database->query($query);

推荐答案

单身人士是坏消息。

• 他们将全局状态引入程序。大多数程序员应该熟悉为什么全局状态是坏的。


• 他们在单例和使用它的任何类之间引入紧耦合。这意味着你不能重用这个类，而不重用单例。


• 他们对依赖于单例问题的类进行单元测试，因为你不能用模拟代替单例。


• 他们鼓励一种编码风格，其中类试图解决自己的依赖。这是坏的，因为它可以减少类的依赖关系的清晰度。


• PHP有一个Share Nothing架构，这意味着PHP单例实际上不是单例，可以有多个实例在任何一个时间活动（每个打开的请求一个）。


• 如果您稍后突然发现您实际上需要多个由单身人士提供的资源，会发生什么？这是比你想象的更常见的情况。

• They introduce global state into a program. Most programmers should be familiar with why global state is bad.
• They introduce tight coupling between the singleton and any class that uses it. This means you can't reuse the classes in question without reusing the singleton too.
• They make unit testing of classes that depend on the singleton problematic because you can't easily replace the singleton with a mock.
• They encourage a coding style where classes attempt to resolve their own dependencies. This is bad because it can reduce clarity regarding what dependencies the class has.
• PHP has a Share Nothing Architecture, meaning that PHP singletons aren't really singletons at all, there can be multiple instances alive at any one time (one per open request).
• What happens if you suddenly discover at some later date that you actually need more than one of the resource that's being provided by the singleton? It's a more common scenario than you might think

你最好看看依赖注入，因为它解决了上述问题。

You're better off looking at dependency-injection instead, as it resolves the above issues.

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