简介
一般在框架中都会用到 composer 工具 ,用它来管理依赖。其中 composer 有类的自动加载机制，可以加载 composer 下载的库中的所有的类文件。那么 composer 的自动加载机制是怎么实现的呢？
composer 自动加载原理
以在 Laravel 框架中为例：
1. 首先在入口文件（/public/index.PHP）中引入了 autoload.PHP
``` require __DIR__.'/../vendor/autoload.PHP'; ```
2. 我们看看 autoload.PHP 的内容
``` require_once __DIR__ . '/composer/autoload_real.PHP'; return ComposerAutoloaderInit1215780529014c2b50a6fca7ce889273::getLoader(); ```
3. 我们再看看 autoload_real.PHP 的内容
``` <?PHP // autoload_real.PHP @generated by Composer class ComposerAutoloaderInit1215780529014c2b50a6fca7ce889273 { private static $loader; public static function loadClassLoader($class) { if ('Composer\Autoload\ClassLoader' === $class) { require __DIR__ . '/ClassLoader.PHP'; } } public static function getLoader() { if (null !== self::$loader) { return self::$loader; } spl_autoload_register(array('ComposerAutoloaderInit1215780529014c2b50a6fca7ce889273','loadClassLoader'),true,true); self::$loader = $loader = new \Composer\Autoload\ClassLoader(); spl_autoload_unregister(array('ComposerAutoloaderInit1215780529014c2b50a6fca7ce889273','loadClassLoader')); $useStaticLoader = PHP_VERSION_ID >= 50600 && !defined('HHVM_VERSION') && (!function_exists('zend_loader_file_encoded') || !zend_loader_file_encoded()); if ($useStaticLoader) { require_once __DIR__ . '/autoload_static.PHP'; call_user_func(\Composer\Autoload\ComposerStaticInit1215780529014c2b50a6fca7ce889273::getInitializer($loader)); } else { $map = require __DIR__ . '/autoload_namespaces.PHP'; foreach ($map as $namespace => $path) { $loader->set($namespace,$path); } $map = require __DIR__ . '/autoload_psr4.PHP'; foreach ($map as $namespace => $path) { $loader->setPsr4($namespace,$path); } $classMap = require __DIR__ . '/autoload_classmap.PHP'; if ($classMap) { $loader->addClassMap($classMap); } } $loader->register(true); if ($useStaticLoader) { $includeFiles = Composer\Autoload\ComposerStaticInit1215780529014c2b50a6fca7ce889273::$files; } else { $includeFiles = require __DIR__ . '/autoload_files.PHP'; } foreach ($includeFiles as $fileIdentifier => $file) { composerRequire1215780529014c2b50a6fca7ce889273($fileIdentifier,$file); } return $loader; } } function composerRequire1215780529014c2b50a6fca7ce889273($fileIdentifier,$file) { if (empty($GLOBALS['__composer_autoload_files'][$fileIdentifier])) { require $file; $GLOBALS['__composer_autoload_files'][$fileIdentifier] = true; } } ```
可以看出这一段是 composer 自动加载的重点，首先在 autoload.PHP 中调用
ComposerAutoloaderInit1215780529014c2b50a6fca7ce889273::getLoader () 方法，getLoader () 首先判断当前
