Composer 原理

历史
自动加载原理
参考资料

历史

Composer是一个非常流行的PHP包依赖管理工具，已经取代PEAR包管理器。

PHP最早加载类的方法

如果不使用framework，直接开发，便常出现类似这样的code：

// whatever.php
// 这档案需要用到几个类别
require  'xxx_class.php';
require  'yyy_class.php';
require  'zzz_class.php';
// ...

然后在其他档案会出现：

// another.php
// 这档案需要用到几个类别
require  'yyy_class.php';
require  'zzz_class.php';
// ...

这样的结果，会产生至少两个问题：

许多档案用到同样几个class，于是在不同地方都需要载入一次。当类别多了起来，会显得很乱、忘记载入时还会出现error。那么，不如试试一种懒惰的作法？

写一个php，负责载入所有类别：

// load_everything.php
require  'xxx_class.php';
require  'yyy_class.php';
require  'zzz_class.php';
require  'aaa_class.php';
require  'bbb_class.php';
require  'ccc_class.php';

然后在其他档案都载入这支档案即可：

require  'load_everything.php'

结果新问题又来了：当类别很多的时候，随便一个web page都会载入一堆code，占用大量内存，怎么办呢？

__autoload

为解决这个问题，PHP 5开始提供__autoload这种俗称“magic method”的函式。

当你要使用的类别PHP找不到时，它会将类别名称当成字串丢进这个函式，在PHP喷error投降之前，做最后的尝试：

<?php
$m = new TestClass();
$m->show();

function __autoload ($className)
{
   require $className . '.php';
}

如果复杂一点：

<?php
// autoload.php
function  __autoload($classname)  {
    if  ($classname  ===  'xxx'){
        $filename  =  "./".  $classname  .".php";
        include_once($filename);
    }  else  if  ($classname  ===  'yyy'){
        $filename  =  "./other_library/".  $classname  .".php";
        include_once($filename);
    }  else  if  ($classname  ===  'zzz'){
        $filename  =  "./my_library/".  $classname  .".php";
        include_once($filename);
    }
    // blah
}

如此一来，问题似乎解决了？

可惜还是有小缺点，就是这个__autoload函式内容会变得很巨大。以上面的例子来说，一下会去根目录找、一下会去other_library资料夹、一下会去my_library资料夹寻找。在整理档案的时候，显得有些混乱。

spl_autoload_register

于是PHP从5.1.2开始，多提供了一个函式。可以多写几个autoload函式，然后注册起来，效果跟直接使用__autoload相同。现在可以针对不同用途的类别，分批autoload了。

<?php
spl_autoload_register('my_library_loader');
spl_autoload_register('other_library_loader');
spl_autoload_register('basic_loader');

function  my_library_loader($classname)  {
    $filename  =  "./my_library/".  $classname  .".php";
    include_once($filename);
}

function  other_library_loader($classname)  {
    $filename  =  "./other_library/".  $classname  .".php";
    include_once($filename);
}

function  basic_loader($classname)  {
    $filename  =  "./".  $classname  .".php";
    include_once($filename);
}

每个loader内容可以做很多变化。可以多写判断式让它更智慧、可以进行字串处理。自动载入类别的问题终于解决了，但是一大串一大串的autoload，手动去写这些，很麻烦，这个时候就会想到能不能用一种工具直接去生成呢？这个时候就有了Composer了。

Composer

建立一个composer.json档，里面输入这些：

{
    "autoload":  {
        "classmap":  [
            "my_library",
            "other_library"
        ]
    }
}

再来，在terminal输入

composer install

执行成功之后，你会看到一个vendor资料夹，内含一个autoload.php。没错，跟你梦想的一样。你只要载入这个档案：

require  'vendor/autoload.php';

你需要的所有类别，都会在适当的时候、以适当的方式自动载入。php再也不会error说你“类别尚未定义”了！

最后提供一个扩展包下载，直接用composer就好了。

查询一下那几个套件在“https://packagist.org/”的名称、还有你需要的版本号。

把刚刚的composer.json改成这样：

{
    "require":  {
        "google/apiclient":  "1.0.*@beta",
        "guzzlehttp/guzzle":  "~4.0",
        "doctrine/dbal":  "~2.4"
    },

    "autoload":  {
        "classmap":  [
            "my_library"
        ]
    }
}

然后composer install指令除了自动载入你的类别之外、还会自动下载你需要的类别、然后自动载入它们。

自动加载原理

使用Composer构建的php项目，在项目根目录下生成vendor目录，vendor具体目录如下：

vendor/
    composer/
        autoload_classmap.php
        autoload_namespace.php
        autoload_psr4.php
        autoload_real.php
        autoload_static.php
        ClassLoader.php
        installed.json
        LICENSE
    autoload.php

一般在项目的入口文件中就会引入vendor/autoload.php ，如：

<?php
require(__DIR__ . '/vendor/autoload.php');

vendor/autoload.php 文件是composer自动加载的入口文件(引导文件)，此文件做了两件事情：

加载文件 vendor/composer/autoload_real.php文件
实例化类Composer\Autoload\ClassLoader

vendor/autoload.php 自动加载入口文件源码：

<?php
require_once __DIR__ . '/composer/autoload_real.php';

return ComposerAutoloaderInit90a2538306db6d5fdf7185fa09792d9a::getLoader();

vendor/composer/autoload_real.php 自动加载核心文件，此文件的核心任务是分别加载composer的配置文件：

<?php
class ComposerAutoloaderInit90a2538306db6d5fdf7185fa09792d9a
{
    private static $loader;

    public static function loadClassLoader($class)
    {
        if ('Composer\Autoload\ClassLoader' === $class) {
            require __DIR__ . '/ClassLoader.php';
        }
    }

    /**
     * @return \Composer\Autoload\ClassLoader
     */
    public static function getLoader()
    {
        if (null !== self::$loader) {
            return self::$loader;
        }

        spl_autoload_register(array('ComposerAutoloaderInit90a2538306db6d5fdf7185fa09792d9a', 'loadClassLoader'), true, true);
        self::$loader = $loader = new \Composer\Autoload\ClassLoader();
        spl_autoload_unregister(array('ComposerAutoloaderInit90a2538306db6d5fdf7185fa09792d9a', '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\ComposerStaticInit90a2538306db6d5fdf7185fa09792d9a::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);

        return $loader;
    }
}

vendor/composer/autoload_classmap.php 自动加载配置源码：

<?php
$vendorDir = dirname(dirname(__FILE__));
$baseDir = dirname($vendorDir);

return array(
);

vendor/composer/autoload_namespace.php PSR0自动加载配置源码：

<?php
$vendorDir = dirname(dirname(__FILE__));
$baseDir = dirname($vendorDir);

return array(
    'Highlight\\' => array($vendorDir . '/scrivo/highlight.php')
);

vendor/composer/autoload_psr4.php PSR4自动加载配置 (推荐使用) ：

<?php
$vendorDir = dirname(dirname(__FILE__));
$baseDir = dirname($vendorDir);

return array(
    'GuzzleHttp\\Psr7\\' => array($vendorDir . '/guzzlehttp/psr7/src'),
    'App\\' => array($baseDir . '/app')
);

vendor/composer/ClassLoader.php (是核心文件，上面重点调用的下面的register()方法)

public function register($prepend = false)
{
    spl_autoload_register(array($this, 'loadClass'), true, $prepend);
}

当实例化一个类，在这个类未加载进来的时候，会执行loadClass()方法，loadClass方法使用前文已经加载到内存中的类与路径之间的关系，找到需要加载的文件，然后将其加载进来：

public function loadClass($class)
{
    if ($file = $this->findFile($class)) {
        includeFile($file);

        return true;
    }
}

...

function includeFile($file)
{
    include $file;
}

vendor/composer/ClassLoader.php 源码：

<?php

/*
 * This file is part of Composer.
 *
 * (c) Nils Adermann <naderman@naderman.de>
 *     Jordi Boggiano <j.boggiano@seld.be>
 *
 * For the full copyright and license information, please view the LICENSE
 * file that was distributed with this source code.
 */

namespace Composer\Autoload;

/**
 * ClassLoader implements a PSR-0, PSR-4 and classmap class loader.
 *
 *     $loader = new \Composer\Autoload\ClassLoader();
 *
 *     // register classes with namespaces
 *     $loader->add('Symfony\Component', __DIR__.'/component');
 *     $loader->add('Symfony',           __DIR__.'/framework');
 *
 *     // activate the autoloader
 *     $loader->register();
 *
 *     // to enable searching the include path (eg. for PEAR packages)
 *     $loader->setUseIncludePath(true);
 *
 * In this example, if you try to use a class in the Symfony\Component
 * namespace or one of its children (Symfony\Component\Console for instance),
 * the autoloader will first look for the class under the component/
 * directory, and it will then fallback to the framework/ directory if not
 * found before giving up.
 *
 * This class is loosely based on the Symfony UniversalClassLoader.
 *
 * @author Fabien Potencier <fabien@symfony.com>
 * @author Jordi Boggiano <j.boggiano@seld.be>
 * @see    http://www.php-fig.org/psr/psr-0/
 * @see    http://www.php-fig.org/psr/psr-4/
 */
class ClassLoader
{
    // PSR-4
    private $prefixLengthsPsr4 = array();
    private $prefixDirsPsr4 = array();
    private $fallbackDirsPsr4 = array();

    // PSR-0
    private $prefixesPsr0 = array();
    private $fallbackDirsPsr0 = array();

    private $useIncludePath = false;
    private $classMap = array();
    private $classMapAuthoritative = false;
    private $missingClasses = array();
    private $apcuPrefix;

    public function getPrefixes()
    {
        if (!empty($this->prefixesPsr0)) {
            return call_user_func_array('array_merge', array_values($this->prefixesPsr0));
        }

        return array();
    }

    public function getPrefixesPsr4()
    {
        return $this->prefixDirsPsr4;
    }

    public function getFallbackDirs()
    {
        return $this->fallbackDirsPsr0;
    }

    public function getFallbackDirsPsr4()
    {
        return $this->fallbackDirsPsr4;
    }

    public function getClassMap()
    {
        return $this->classMap;
    }

    /**
     * @param array $classMap Class to filename map
     */
    public function addClassMap(array $classMap)
    {
        if ($this->classMap) {
            $this->classMap = array_merge($this->classMap, $classMap);
        } else {
            $this->classMap = $classMap;
        }
    }

    /**
     * Registers a set of PSR-0 directories for a given prefix, either
     * appending or prepending to the ones previously set for this prefix.
     *
     * @param string       $prefix  The prefix
     * @param array|string $paths   The PSR-0 root directories
     * @param bool         $prepend Whether to prepend the directories
     */
    public function add($prefix, $paths, $prepend = false)
    {
        if (!$prefix) {
            if ($prepend) {
                $this->fallbackDirsPsr0 = array_merge(
                    (array) $paths,
                    $this->fallbackDirsPsr0
                );
            } else {
                $this->fallbackDirsPsr0 = array_merge(
                    $this->fallbackDirsPsr0,
                    (array) $paths
                );
            }

            return;
        }

        $first = $prefix[0];
        if (!isset($this->prefixesPsr0[$first][$prefix])) {
            $this->prefixesPsr0[$first][$prefix] = (array) $paths;

            return;
        }
        if ($prepend) {
            $this->prefixesPsr0[$first][$prefix] = array_merge(
                (array) $paths,
                $this->prefixesPsr0[$first][$prefix]
            );
        } else {
            $this->prefixesPsr0[$first][$prefix] = array_merge(
                $this->prefixesPsr0[$first][$prefix],
                (array) $paths
            );
        }
    }

    /**
     * Registers a set of PSR-4 directories for a given namespace, either
     * appending or prepending to the ones previously set for this namespace.
     *
     * @param string       $prefix  The prefix/namespace, with trailing '\\'
     * @param array|string $paths   The PSR-4 base directories
     * @param bool         $prepend Whether to prepend the directories
     *
     * @throws \InvalidArgumentException
     */
    public function addPsr4($prefix, $paths, $prepend = false)
    {
        if (!$prefix) {
            // Register directories for the root namespace.
            if ($prepend) {
                $this->fallbackDirsPsr4 = array_merge(
                    (array) $paths,
                    $this->fallbackDirsPsr4
                );
            } else {
                $this->fallbackDirsPsr4 = array_merge(
                    $this->fallbackDirsPsr4,
                    (array) $paths
                );
            }
        } elseif (!isset($this->prefixDirsPsr4[$prefix])) {
            // Register directories for a new namespace.
            $length = strlen($prefix);
            if ('\\' !== $prefix[$length - 1]) {
                throw new \InvalidArgumentException("A non-empty PSR-4 prefix must end with a namespace separator.");
            }
            $this->prefixLengthsPsr4[$prefix[0]][$prefix] = $length;
            $this->prefixDirsPsr4[$prefix] = (array) $paths;
        } elseif ($prepend) {
            // Prepend directories for an already registered namespace.
            $this->prefixDirsPsr4[$prefix] = array_merge(
                (array) $paths,
                $this->prefixDirsPsr4[$prefix]
            );
        } else {
            // Append directories for an already registered namespace.
            $this->prefixDirsPsr4[$prefix] = array_merge(
                $this->prefixDirsPsr4[$prefix],
                (array) $paths
            );
        }
    }

    /**
     * Registers a set of PSR-0 directories for a given prefix,
     * replacing any others previously set for this prefix.
     *
     * @param string       $prefix The prefix
     * @param array|string $paths  The PSR-0 base directories
     */
    public function set($prefix, $paths)
    {
        if (!$prefix) {
            $this->fallbackDirsPsr0 = (array) $paths;
        } else {
            $this->prefixesPsr0[$prefix[0]][$prefix] = (array) $paths;
        }
    }

    /**
     * Registers a set of PSR-4 directories for a given namespace,
     * replacing any others previously set for this namespace.
     *
     * @param string       $prefix The prefix/namespace, with trailing '\\'
     * @param array|string $paths  The PSR-4 base directories
     *
     * @throws \InvalidArgumentException
     */
    public function setPsr4($prefix, $paths)
    {
        if (!$prefix) {
            $this->fallbackDirsPsr4 = (array) $paths;
        } else {
            $length = strlen($prefix);
            if ('\\' !== $prefix[$length - 1]) {
                throw new \InvalidArgumentException("A non-empty PSR-4 prefix must end with a namespace separator.");
            }
            $this->prefixLengthsPsr4[$prefix[0]][$prefix] = $length;
            $this->prefixDirsPsr4[$prefix] = (array) $paths;
        }
    }

    /**
     * Turns on searching the include path for class files.
     *
     * @param bool $useIncludePath
     */
    public function setUseIncludePath($useIncludePath)
    {
        $this->useIncludePath = $useIncludePath;
    }

    /**
     * Can be used to check if the autoloader uses the include path to check
     * for classes.
     *
     * @return bool
     */
    public function getUseIncludePath()
    {
        return $this->useIncludePath;
    }

    /**
     * Turns off searching the prefix and fallback directories for classes
     * that have not been registered with the class map.
     *
     * @param bool $classMapAuthoritative
     */
    public function setClassMapAuthoritative($classMapAuthoritative)
    {
        $this->classMapAuthoritative = $classMapAuthoritative;
    }

    /**
     * Should class lookup fail if not found in the current class map?
     *
     * @return bool
     */
    public function isClassMapAuthoritative()
    {
        return $this->classMapAuthoritative;
    }

    /**
     * APCu prefix to use to cache found/not-found classes, if the extension is enabled.
     *
     * @param string|null $apcuPrefix
     */
    public function setApcuPrefix($apcuPrefix)
    {
        $this->apcuPrefix = function_exists('apcu_fetch') && filter_var(ini_get('apc.enabled'), FILTER_VALIDATE_BOOLEAN) ? $apcuPrefix : null;
    }

    /**
     * The APCu prefix in use, or null if APCu caching is not enabled.
     *
     * @return string|null
     */
    public function getApcuPrefix()
    {
        return $this->apcuPrefix;
    }

    /**
     * Registers this instance as an autoloader.
     *
     * @param bool $prepend Whether to prepend the autoloader or not
     */
    public function register($prepend = false)
    {
        spl_autoload_register(array($this, 'loadClass'), true, $prepend);
    }

    /**
     * Unregisters this instance as an autoloader.
     */
    public function unregister()
    {
        spl_autoload_unregister(array($this, 'loadClass'));
    }

    /**
     * Loads the given class or interface.
     *
     * @param  string    $class The name of the class
     * @return bool|null True if loaded, null otherwise
     */
    public function loadClass($class)
    {
        if ($file = $this->findFile($class)) {
            includeFile($file);

            return true;
        }
    }

    /**
     * Finds the path to the file where the class is defined.
     *
     * @param string $class The name of the class
     *
     * @return string|false The path if found, false otherwise
     */
    public function findFile($class)
    {
        // class map lookup
        if (isset($this->classMap[$class])) {
            return $this->classMap[$class];
        }
        if ($this->classMapAuthoritative || isset($this->missingClasses[$class])) {
            return false;
        }
        if (null !== $this->apcuPrefix) {
            $file = apcu_fetch($this->apcuPrefix.$class, $hit);
            if ($hit) {
                return $file;
            }
        }

        $file = $this->findFileWithExtension($class, '.php');

        // Search for Hack files if we are running on HHVM
        if (false === $file && defined('HHVM_VERSION')) {
            $file = $this->findFileWithExtension($class, '.hh');
        }

        if (null !== $this->apcuPrefix) {
            apcu_add($this->apcuPrefix.$class, $file);
        }

        if (false === $file) {
            // Remember that this class does not exist.
            $this->missingClasses[$class] = true;
        }

        return $file;
    }

    private function findFileWithExtension($class, $ext)
    {
        // PSR-4 lookup
        $logicalPathPsr4 = strtr($class, '\\', DIRECTORY_SEPARATOR) . $ext;

        $first = $class[0];
        if (isset($this->prefixLengthsPsr4[$first])) {
            $subPath = $class;
            while (false !== $lastPos = strrpos($subPath, '\\')) {
                $subPath = substr($subPath, 0, $lastPos);
                $search = $subPath . '\\';
                if (isset($this->prefixDirsPsr4[$search])) {
                    $pathEnd = DIRECTORY_SEPARATOR . substr($logicalPathPsr4, $lastPos + 1);
                    foreach ($this->prefixDirsPsr4[$search] as $dir) {
                        if (file_exists($file = $dir . $pathEnd)) {
                            return $file;
                        }
                    }
                }
            }
        }

        // PSR-4 fallback dirs
        foreach ($this->fallbackDirsPsr4 as $dir) {
            if (file_exists($file = $dir . DIRECTORY_SEPARATOR . $logicalPathPsr4)) {
                return $file;
            }
        }

        // PSR-0 lookup
        if (false !== $pos = strrpos($class, '\\')) {
            // namespaced class name
            $logicalPathPsr0 = substr($logicalPathPsr4, 0, $pos + 1)
                . strtr(substr($logicalPathPsr4, $pos + 1), '_', DIRECTORY_SEPARATOR);
        } else {
            // PEAR-like class name
            $logicalPathPsr0 = strtr($class, '_', DIRECTORY_SEPARATOR) . $ext;
        }

        if (isset($this->prefixesPsr0[$first])) {
            foreach ($this->prefixesPsr0[$first] as $prefix => $dirs) {
                if (0 === strpos($class, $prefix)) {
                    foreach ($dirs as $dir) {
                        if (file_exists($file = $dir . DIRECTORY_SEPARATOR . $logicalPathPsr0)) {
                            return $file;
                        }
                    }
                }
            }
        }

        // PSR-0 fallback dirs
        foreach ($this->fallbackDirsPsr0 as $dir) {
            if (file_exists($file = $dir . DIRECTORY_SEPARATOR . $logicalPathPsr0)) {
                return $file;
            }
        }

        // PSR-0 include paths.
        if ($this->useIncludePath && $file = stream_resolve_include_path($logicalPathPsr0)) {
            return $file;
        }

        return false;
    }
}

/**
 * Scope isolated include.
 *
 * Prevents access to $this/self from included files.
 */
function includeFile($file)
{
    include $file;
}

vendor/composer/autoload_static.php 源码：

<?php
namespace Composer\Autoload;

class ComposerStaticInit90a2538306db6d5fdf7185fa09792d9a
{
    public static function getInitializer(ClassLoader $loader)
    {
        return \Closure::bind(function () use ($loader) {

        }, null, ClassLoader::class);
    }
}

历史

PHP最早加载类的方法

__autoload

spl_autoload_register

Composer

自动加载原理

参考资料