The use keyword in PHP and file loading
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 ?>
(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,'\\')){ $name = strstr($class, '\\', true); if(in_array($name,array('Think','Org','Behavior','Com','Vendor')) || is_dir(LIB_PATH.$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('AUTOLOAD_NAMESPACE'); $path = isset($namespace[$name])? dirname($namespace[$name]).'/' : APP_PATH; } $filename = $path . str_replace('\\', '/', $class) . EXT; if(is_file($filename)) { // Win环境下面严格区分大小写 if (IS_WIN && false === strpos(str_replace('/', '\\', realpath($filename)), $class . EXT)){ return ; } include $filename; } }elseif (!C('APP_USE_NAMESPACE')) { // 自动加载的类库层 foreach(explode(',',C('APP_AUTOLOAD_LAYER')) as $layer){ if(substr($class,-strlen($layer))==$layer){ if(require_cache(MODULE_PATH.$layer.'/'.$class.EXT)) { return ; } } } // 根据自动加载路径设置进行尝试搜索 foreach (explode(',',C('APP_AUTOLOAD_PATH')) as $path){ if(import($path.'.'.$class)) // 如果加载类成功则返回 return ; } } }
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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