Yii2框架类自动加载机制实例分析

<?php
defined('YII_DEBUG') or define('YII_DEBUG', true);//运行模式
defined('YII_ENV') or define('YII_ENV', 'dev');//运行环境
require(__DIR__ . '/../../vendor/autoload.php');//composer的类自动加载文件
require(__DIR__ . '/../../vendor/yiisoft/yii2/Yii.php');//yii的工具类文件（包含了yii类自动加载）
require(__DIR__ . '/../../common/config/bootstrap.php');//主要用于执行一些yii应用引导的代码
require(__DIR__ . '/../config/bootstrap.php');
$config = yii\helpers\ArrayHelper::merge(
  require(__DIR__ . '/../../common/config/main.php'),
  require(__DIR__ . '/../../common/config/main-local.php'),
  require(__DIR__ . '/../config/main.php'),
  require(__DIR__ . '/../config/main-local.php')
);
(new yii\web\Application($config))->run();

require(__DIR__ . '/BaseYii.php');
class Yii extends \yii\BaseYii
{
}
spl_autoload_register(['Yii', 'autoload'], true, true);//注册yii的类自动加载器
Yii::$classMap = require(__DIR__ . '/classes.php');//引入类名到类文件路径的映射
Yii::$container = new yii\di\Container();

return [
 'yii\base\Action' => YII2_PATH . '/base/Action.php',
 'yii\base\ActionEvent' => YII2_PATH . '/base/ActionEvent.php',
  ....//省略n多元素
 'yii\widgets\Pjax' => YII2_PATH . '/widgets/Pjax.php',
 'yii\widgets\PjaxAsset' => YII2_PATH . '/widgets/PjaxAsset.php',
 'yii\widgets\Spaceless' => YII2_PATH . '/widgets/Spaceless.php',
];

/**
 * 类自动加载器
 * @param type $className：要加载的类的名称
 * @return type
 * @throws UnknownClassException
 */
public static function autoload($className)
{
  if (isset(static::$classMap[$className])) {//要加载的类在 类名=>类文件路径 映射中找到
    $classFile = static::$classMap[$className];
    if ($classFile[0] === '@') {//若类文件路径使用了别名，进行别名解析获得完整路径
      $classFile = static::getAlias($classFile);
    }
  } elseif (strpos($className, '\\') !== false) {//类名需要包含'\'才符合规范
    $classFile = static::getAlias('@' . str_replace('\\', '/', $className) . '.php', false);//进行别名解析（说明类名必须以有效的根别名打头）
    if ($classFile === false || !is_file($classFile)) {
      return;
    }
  } else {
    return;
  }
  include($classFile);//引入需要加载的类文件
  if (YII_DEBUG && !class_exists($className, false) && !interface_exists($className, false) && !trait_exists($className, false)) {
    throw new UnknownClassException("Unable to find '$className' in file: $classFile. Namespace missing?");
  }
}