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The use keyword in PHP and file loading

May 30, 2018 pm 01:46 PM
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最近有个朋友问我,php中的use关键字是不是会进行文件的自动引入?其实use关键字与文件加载没有一点关系,use关键字的作用就是声明要使用的类所处那个命名空间之下。

前言

可能在大家经常使用框架,写一个Controller或者Model的时候,写了好多use,但是并没有写文件加载的代码,就以为use可以进行文件的自动加载了。

详细介绍

其实,现在流行的php框架,都是基于MVC模式的,大量的使用了命名空间,以提高程序的灵活性。那么框架是怎么实现将use关键字所声明的类库对应的脚本文件进行加载的那?

(1):在通过use关键字进行声明类库的声明的时候,并不会进行脚本的加载,而是在脚本文件真正使用到所对应的类库的时候才会进行加载(这就是所谓延迟加载)。

(2):框架一般在初始化阶段,就声明了__autoload()函数或者是spl_autoload_register()函数(一般会在入口文件中进行声明)。在调用通过use声明的类库的时候,会自动的调用上述的函数,将调用的类库名称传递到函数中(这里的类库名称是包括命名空间的全名称)

<?php
namespace Home\Controller;
function __autoload($class_name) { 
 //echo $class_name."<br/>"; 
 require_once ("./".$class_name . ".php");  
}
use Admin\Controller\Index;
new Index; //new Index会触发__autoload()函数,返回Admin\Controller\Index到函数中
?>
Copy after login

(3):框架中定义的__autoload()函数或者是spl_autoload_register()函数在接收到传递过来的类库名称以后,框架会对名称进行处理,分析出类库名称所对应的文件路径,然后进行文件的加载。这里需要注意的是,不同的框架在解析类库名称,分析文件路径的方式是各不一致的,毕竟各自的目录结构是不一样的。

比如说,thinkphp:

 /**
  * 类库自动加载
  * @param string $class 对象类名
  * @return void
  */
 public static function autoload($class) {
  // 检查是否存在映射
  if(isset(self::$_map[$class])) {
   include self::$_map[$class];
  }elseif(false !== strpos($class,&#39;\\&#39;)){
   $name   = strstr($class, &#39;\\&#39;, true);
   if(in_array($name,array(&#39;Think&#39;,&#39;Org&#39;,&#39;Behavior&#39;,&#39;Com&#39;,&#39;Vendor&#39;)) || is_dir(LIB_PATH.$name)){ 
    // Library目录下面的命名空间自动定位
    $path  = LIB_PATH;
   }else{
    // 检测自定义命名空间 否则就以模块为命名空间
    $namespace = C(&#39;AUTOLOAD_NAMESPACE&#39;);
    $path  = isset($namespace[$name])? dirname($namespace[$name]).&#39;/&#39; : APP_PATH;
   }
   $filename  = $path . str_replace(&#39;\\&#39;, &#39;/&#39;, $class) . EXT;
   if(is_file($filename)) {
    // Win环境下面严格区分大小写
    if (IS_WIN && false === strpos(str_replace(&#39;/&#39;, &#39;\\&#39;, realpath($filename)), $class . EXT)){
     return ;
    }
    include $filename;
   }
  }elseif (!C(&#39;APP_USE_NAMESPACE&#39;)) {
   // 自动加载的类库层
   foreach(explode(&#39;,&#39;,C(&#39;APP_AUTOLOAD_LAYER&#39;)) as $layer){
    if(substr($class,-strlen($layer))==$layer){
     if(require_cache(MODULE_PATH.$layer.&#39;/&#39;.$class.EXT)) {
      return ;
     }
    }   
   }
   // 根据自动加载路径设置进行尝试搜索
   foreach (explode(&#39;,&#39;,C(&#39;APP_AUTOLOAD_PATH&#39;)) as $path){
    if(import($path.&#39;.&#39;.$class))
     // 如果加载类成功则返回
     return ;
   }
  }
 }
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这里只是举例说明use与文件的自动加载没有关系,不强调各个框架是具体怎么完成自动加载,具体的可以自己看框架代码进行研究。

以上就是本文的全部内容,希望对大家的学习有所帮助。


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