composer 提供的 autoload 机制使得咱们组织代码和引入新类库十分不便，然而也使我的项目的性能降落了不少 。
composer autoload 慢的次要起因在于来自对 PSR-0 和 PSR-4 的反对，加载器失去一个类名时须要到文件系统里查找对应的类文件地位，能够看到 PSR-4 或者 PSR-0 的主动加载都是一件很累人的事儿。根本是个 O(n2) 的复杂度。另外有一大堆 is_file之类的 IO 操作所以性能堪忧。所以明天咱们就来聊聊composer主动加载的优化，聊之前咱们先来简略说一下composer主动加载的原理。以下是相干局部源码：
<phpclass ClassLoader{ private $vendorDir; // 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; private static $registeredLoaders = array(); public function __construct($vendorDir = null) { $this->vendorDir = $vendorDir; } /** * 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; }} ?>从源码中咱们看出，Compsoer\ClassLoader 会优先查看 autoload_classmap 中所有生成的注册类。如果在classmap 中没有发现再 fallback 到 psr-4 而后 psr-0。所以咱们就从classmap动手。
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