PHP办理依靠(dependency)干系东西 Composer的主动加载（autoload）

举例来讲，假定咱们的名目想要操纵 monolog 这个日记东西，就须要在composer.json里告知composer咱们须要它：

{
 "require": {
  "monolog/monolog": "1.*"
 }
}

以后履行：

php composer.phar install

好，此刻装置完了，该怎样操纵呢？Composer主动天生了一个autoload文件，你只须要援用它

require "/path/to/vendor/autoload.php";

而后就能够很是便利的去操纵第三方的类库了，是不是是感受很棒啊！对咱们须要的monolog，就能够如许用了：

use MonologLogger;
use MonologHandlerStreamHandler;
// create a log channel
$log = new Logger("name");
$log->pushHandler(new StreamHandler("/path/to/log/log_name.log", Logger::WARNING));
// add records to the log
$log->addWarning("Foo");
$log->addError("Bar");

在这个进程中，Composer做了甚么呢？它天生了一个autoloader，再根据各个包本身的autoload设置装备摆设，从而帮咱们停止主动加载的使命。（若是对autoload这局部内容不太领会，能够看我之前的 一篇文章
）接上去让咱们看看Composer是怎样做的吧。

对第三方包的主动加载，Composer供给了四种体例的撑持，别离是 PSR-0和PSR-4的主动加载（我的一篇文章也有先容过它们)，天生class-map，和间接包罗files的体例。

PSR-4是composer保举操纵的一种体例，由于它更容易操纵并能带来更简练的目次布局。在composer.json里是如许停止设置装备摆设的：

{
  "autoload": {
    "psr-4": {
      "Foo": "src/",
    }
  }
}

key和value就界说出了namespace和到呼应path的映照。根据PSR-4的法则，当试图主动加载 "FooBarBaz" 这个class时，会去寻觅 "src/Bar/Baz.php" 这个文件，若是它存在则停止加载。注重， "Foo"
并不出此刻文件途径中，这是与PSR-0差别的一点，若是PSR-0有此设置装备摆设，那末会去寻觅

"src/Foo/Bar/Baz.php"

这个文件。

别的注重PSR-4和PSR-0的设置装备摆设里，"Foo"开头的定名空间分开符必须加上并且停止本义，以防呈现"Foo"婚配到了"FooBar"如许的不测产生。

在composer装置或更新完以后，psr-4的设置装备摆设换被转换成namespace为key，dir path为value的Map的情势，并写入天生的 vendor/composer/autoload_psr4.php 文件当中。

{
  "autoload": {
    "psr-0": {
      "Foo": "src/",
    }
  }
}

终究这个设置装备摆设也以Map的情势写入天生的

vendor/composer/autoload_namespaces.php

文件当中。

Class-map体例，则是经由过程设置装备摆设指定的目次或文件，而后在Composer装置或更新时，它会扫描指定目次下以.php或.inc开头的文件中的class，天生class到指定file path的映照，并插手新天生的 vendor/composer/autoload_classmap.php 文件中，。

{
  "autoload": {
    "classmap": ["src/", "lib/", "Something.php"]
  }
}

比方src/下有一个BaseController类，那末在autoload_classmap.php文件中，就会天生如许的设置装备摆设:

"BaseController" => $baseDir . "/src/BaseController.php"

Files体例，便是手动指定供间接加载的文件。比方说咱们有一系列全局的helper functions，能够放到一个helper文件里而后间接停止加载

{
  "autoload": {
    "files": ["src/MyLibrary/functions.php"]
  }
}

它会天生一个array，包罗这些设置装备摆设中指定的files，再写入新天生的

vendor/composer/autoload_files.php

文件中，以供autoloader间接停止加载。

下面来看看composer autoload的代码吧

<?php
// autoload_real.php @generated by Composer
class ComposerAutoloaderInit73612b48e6c3d0de8d56e03dece61d11
{
  private static $loader;
  public static function loadClassLoader($class)
  {
 if ("ComposerAutoloadClassLoader" === $class) {
   require __DIR__ . "/ClassLoader.php";
 }
  }
  public static function getLoader()
  {
 if (null !== self::$loader) {
   return self::$loader;
 }
 spl_autoload_register(array("ComposerAutoloaderInit73612b48e6c3d0de8d56e03dece61d11", "loadClassLoader"), true, true);
 self::$loader = $loader = new ComposerAutoloadClassLoader();
 spl_autoload_unregister(array("ComposerAutoloaderInit73612b48e6c3d0de8d56e03dece61d11", "loadClassLoader"));
 $vendorDir = dirname(__DIR__); //verdor第三方类库供给者目次
 $baseDir = dirname($vendorDir); //全部操纵的目次
 $includePaths = require __DIR__ . "/include_paths.php";
 array_push($includePaths, get_include_path());
 set_include_path(join(PATH_SEPARATOR, $includePaths));
 $map = require __DIR__ . "/autoload_namespaces.php";
 foreach ($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);
 $includeFiles = require __DIR__ . "/autoload_files.php";
 foreach ($includeFiles as $file) {
   composerRequire73612b48e6c3d0de8d56e03dece61d11($file);
 }
 return $loader;
  }
}
function composerRequire73612b48e6c3d0de8d56e03dece61d11($file)
{
  require $file;
}

起首初始化ClassLoader类，而后顺次用下面提到的4种加载体例来注册/间接加载，ClassLoader的一些焦点代码以下：

/**
  * @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,
  * 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;
  }
 }
 /**
  * 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);
 }
 /**
  * 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)
 {
  //这是PHP5.3.0 - 5.3.2的一个bug 详见http://bugs.php.net/50731
  if ("" == $class[0]) {
   $class = substr($class, 1);
  }
  // class map 体例的查找
  if (isset($this->classMap[$class])) {
   return $this->classMap[$class];
  }
  //psr-0/4体例的查找
  $file = $this->findFileWithExtension($class, ".php");
  // Search for Hack files if we are running on HHVM
  if ($file === null && defined("HHVM_VERSION")) {
   $file = $this->findFileWithExtension($class, ".hh");
  }
  if ($file === null) {
   // Remember that this class does not exist.
   return $this->classMap[$class] = false;
  }
  return $file;
 }
 private function findFileWithExtension($class, $ext)
 {
  // PSR-4 lookup
  $logicalPathPsr4 = strtr($class, "", DIRECTORY_SEPARATOR) . $ext;
  $first = $class[0];
  if (isset($this->prefixLengthsPsr4[$first])) {
   foreach ($this->prefixLengthsPsr4[$first] as $prefix => $length) {
    if (0 === strpos($class, $prefix)) {
     foreach ($this->prefixDirsPsr4[$prefix] as $dir) {
      if (file_exists($file = $dir . DIRECTORY_SEPARATOR . substr($logicalPathPsr4, $length))) {
       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;
  }
 }

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