Composer 实现自动加载原理-新手也能看懂

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