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Detailed explanation of composer automatic loading mechanism

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2020-07-06 13:34:353482browse

The following tutorial column of composer will introduce the composer automatic loading mechanism from shallow to deep. I hope it will be helpful to friends in need!

Detailed explanation of composer automatic loading mechanism

Preface

Because for composer automatically The only memory left in the loading mechanism is "spl_auto???" and "deriving file paths based on namespace". . . Still incomplete. .

I wanted to collect a detailed explanation online, but I couldn’t find an article that fits my opinion of "from the simplest to the deeper".

So I have this note.

The following knowledge points are coming soon:

1. Learn about spl_autoload_register
2. The story of composer update
3. Track the automatic loading of composer

Text

1. Learn about spl_autoload_register

First check the official php manual:


Detailed explanation of composer automatic loading mechanism(You can just look at the red part if you are lazy)

Is that right? Seemingly ignorant?

Let’s translate it in vernacular:

If we create a new class, we must first require or include the class file. If it is not loaded, an error will be reported. This creates a problem: in this case, the header of the file is full of requirements and includes, which is obviously not in line with the programmer's need to be "lazy".

In order to create a new class normally without requiring or including a class file, an automatic loading mechanism has emerged. The spl_autoload_register function is specifically designed to do this.

From the screenshot, we know that this function has three parameters:

ParametersDetailed explanationautoload_functionFill in here is the name of a ***"function"***, a string or an array. The function of this function is to load the file that needs new Require or include as much as possible to avoid reporting errors when using new. To put it simply, you need to encapsulate a ***function that automatically loads files***throwWhen the automatically loaded function cannot be registered, whether to throw ExceptionprependWhether to add the function to the head of the function queue, if true, it is the head, otherwise the tail

来一波代码,印象深刻一些:

//文件 testClass.php ,即将new的类
class TestClass{
    public function __construct() {
        echo '你已经成功new了我了';
    }
}

//文件autoloadDemo.php文件
spl_autoload_register('autoLoad_function', true, true);
function autoLoad_function($class_name){
    echo "所有的require或者include文件工作都交给我吧!\r\n";
    $class_filename = "./{$class_name}.php";
    echo "我来加载{$class_filename}文件\r\n";
    require_once("./{$class_name}.php");
}
$obj_demo = new TestClass();

输出:

所有的require或者include文件工作都交给我吧!
我来加载testClass.php文件
你已经成功new了我了

明白了这个加载的原理,看下文就顺利多了。

2.composer update发生的故事

将自动加载之前,必须要先说一下composer update,这里头承载了自动加载的前提。

composer项目都包含一个composer.json的配置文件。
Detailed explanation of composer automatic loading mechanism
这里头有一个关键的字段"autoload",包含psr-4和files两个字段。

psr-4:说明是基于psr-4规范的类库,都支持自动加载,只要在后面的对象中以**“命名空间:路径”**的方式写入自己的类库信息即可。
files:这就就更直接了,写入路径就自动加载。

按照以上配置每回composer update之后呢,都会更新一个很重要的文件:./vender/composer/autoload_psr4.php。
Detailed explanation of composer automatic loading mechanism

这个文件只做了一件事情:把命名空间和文件路径对应起来,这样后续自动加载就有映射根据了。

3.追踪一下composer的自动加载

composer的故事从唯一的一个require说起:

require '../vendor/autoload.php'

这个脚本执行了一个函数:

ComposerAutoloaderInitd9b31141b114fcbee3cf55d0e97b7f87::getLoader()

继续跟getloader函数做了什么?

public static function getLoader() {
   if (null !== self::$loader) {
        return self::$loader;
    }

    spl_autoload_register(array('ComposerAutoloaderInitd9b31141b114fcbee3cf55d0e97b7f87', 'loadClassLoader'), true, true);
    self::$loader = $loader = new \Composer\Autoload\ClassLoader();
    spl_autoload_unregister(array('ComposerAutoloaderInitd9b31141b114fcbee3cf55d0e97b7f87', '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\ComposerStaticInitd9b31141b114fcbee3cf55d0e97b7f87::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) {
        $includeFiles = Composer\Autoload\ComposerStaticInitd9b31141b114fcbee3cf55d0e97b7f87::$files;
    } else {
        $includeFiles = require __DIR__ . '/autoload_files.php';
    }
    foreach ($includeFiles as $fileIdentifier => $file) {
        composerRequired9b31141b114fcbee3cf55d0e97b7f87($fileIdentifier, $file);
    }

    return $loader;
}

这个函数主要做了两件事情:
1.将各种存有命名空间和文件映射关系的文件autoload_xxx.php加载了进来,并作了一些处理(比如:setPsr4将相关映射加载了进去,这个留意下,下文会有呼应。)。
2.注册了函数register

继续跟踪register做了什么:

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

原来调用了spl_autoload_register函数,当类没加载的时候使用loadClass来加载类。(这个前文讲的很清楚了,应该很熟了)

继续跟踪loadClass实现:

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

大概可以看出,是做了文件的include。
继续跟踪下是怎么查找文件的,看findFile函数:

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;
}

这个函数做了一件事:就是寻找类从上文的autoload_xxx.php初始化的数据中来寻找映射的文件路径。
其中这个函数findFileWithExtension,适用于寻找psr-4规范的文件的映射信息的。

继续跟踪findFileWithExtension:

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;
}

这个函数做了件事:将命名空间\类这样的类名,转换成目录名/类名.php这样的路径,再从前文setPsr4设置的映射信息中寻找映射信息,然后完成返回路径。

至此composer的自动加载机制结束。

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