Drupal如何实现类的自动加载？

Drupal通过spl_autoload_register()注册类加载器实现自动加载：

function _drupal_bootstrap_database() {
  // ... ....
  spl_autoload_register('drupal_autoload_class');
  spl_autoload_register('drupal_autoload_interface');
}

再来看看类加载器是如何实现的？

function drupal_autoload_interface($interface) {
  return _registry_check_code('interface', $interface);
}

function drupal_autoload_class($class) {
  return _registry_check_code('class', $class);
}

function _registry_check_code($type, $name = NULL) {
  static $lookup_cache, $cache_update_needed;

  if ($type == 'class' && class_exists($name) || $type == 'interface' && interface_exists($name)) {
    return TRUE;
  }

  if (!isset($lookup_cache)) {
    $lookup_cache = array();
    if ($cache = cache_get('lookup_cache', 'cache_bootstrap')) {
      $lookup_cache = $cache->data;
    }
  }

  // When we rebuild the registry, we need to reset this cache so
  // we don't keep lookups for resources that changed during the rebuild.
  if ($type == REGISTRY_RESET_LOOKUP_CACHE) {
    $cache_update_needed = TRUE;
    $lookup_cache = NULL;
    return;
  }

  // Called from drupal_page_footer, we write to permanent storage if there
  // changes to the lookup cache for this request.
  if ($type == REGISTRY_WRITE_LOOKUP_CACHE) {
    if ($cache_update_needed) {
      cache_set('lookup_cache', $lookup_cache, 'cache_bootstrap');
    }
    return;
  }

  // $type is either 'interface' or 'class', so we only need the first letter to
  // keep the cache key unique.
  $cache_key = $type[0] . $name;
  if (isset($lookup_cache[$cache_key])) {
    if ($lookup_cache[$cache_key]) {
      require_once DRUPAL_ROOT . '/' . $lookup_cache[$cache_key];
    }
    return (bool) $lookup_cache[$cache_key];
  }

  // This function may get called when the default database is not active, but
  // there is no reason we'd ever want to not use the default database for
  // this query.
  $file = Database::getConnection('default', 'default')->query("SELECT filename FROM {registry} WHERE name = :name AND type = :type", array(
      ':name' => $name,
      ':type' => $type,
    ))
    ->fetchField();

  // Flag that we've run a lookup query and need to update the cache.
  $cache_update_needed = TRUE;

  // Misses are valuable information worth caching, so cache even if
  // $file is FALSE.
  $lookup_cache[$cache_key] = $file;

  if ($file) {
    require_once DRUPAL_ROOT . '/' . $file;
    return TRUE;
  }
  else {
    return FALSE;
  }
}

留意下面这段代码：

$file= Database::getConnection('default','default')->query("SELECT filename FROM {registry} WHERE name = :name AND type = :type",array(
    ':name' => $name,
    ':type' => $type,
))->fetchField();

类加载器要自动加载DatabaseLog这个类，需要去哪里找对应的文件？

Drupal采用了注册表的方式解决这个问题，它将所有可用的类和接口以及它们对应的文件保存一个名为registry的数据表中。类加载器依据类名就可以简单地找到对应的文件。