nova/api/openstack/__init__.py
APIRouter类：
def __init__(self, ext_mgr=None, init_only=None):
if ext_mgr is None:
if self.ExtensionManager:
ext_mgr = self.ExtensionManager()
else:
raise Exception(_("Must specify an ExtensionManager class"))
...
其中的ExtensionManager由子类nova/api/openstack/computer/__init__.py中的APIRouter指定为nova/api/openstack/computer/extensions.py中的ExtensionManager类，下面看其构造函数：
class ExtensionManager(base_extensions.ExtensionManager):
def __init__(self):
LOG.audit(_(‘Initializing extension manager.‘))
self.cls_list = CONF.osapi_compute_extension
self.extensions = {}
self.sorted_ext_list = []
self._load_extensions()
其中self.cls_list是扩展包的引导类列表，此处为[nova.api.openstack.compute.contrib.standard_extensions]。接着看self._load_extensions()的内容，由于ExtensionManager的类继承关系，该函数为nova.api.openstack. extensions ExtensionManager的函数，内容如下：
def _load_extensions(self):
extensions = list(self.cls_list)
for ext_factory in extensions:
try:
self.load_extension(ext_factory)
except Exception as exc:
LOG.warn(_(‘Failed to load extension %(ext_factory)s: ‘
‘%(exc)s‘),
{‘ext_factory‘: ext_factory, ‘exc‘: exc})
其中self.load_extension函数内容如下：
def load_extension(self, ext_factory):
LOG.debug(_("Loading extension %s"), ext_factory)
if isinstance(ext_factory, basestring):
factory = importutils.import_class(ext_factory)
else:
factory = ext_factory
LOG.debug(_("Calling extension factory %s"), ext_factory)
factory(self)
整个过程就是调用nova.api.openstack.compute.contrib.standard_extensions。接下来看standard_extensions函数，内容如下：
def standard_extensions(ext_mgr):
extensions.load_standard_extensions(ext_mgr, LOG, __path__, __package__)
其中extensions为nova.api.openstack.extensions，看load_standard_extensions内容如下：
def load_standard_extensions(ext_mgr, logger, path, package, ext_list=None):
our_dir = path[0]
for dirpath, dirnames, filenames in os.walk(our_dir):
relpath = os.path.relpath(dirpath, our_dir)
if relpath == ‘.‘:
relpkg = ‘‘
else:
relpkg = ‘.%s‘ % ‘.‘.join(relpath.split(os.sep))
for fname in filenames:
root, ext = os.path.splitext(fname)
if ext != ‘.py‘ or root == ‘__init__‘:
continue
classname = "%s%s" % (root[0].upper(), root[1:])
classpath = ("%s%s.%s.%s" %
(package, relpkg, root, classname))
if ext_list is not None and classname not in ext_list:
logger.debug("Skipping extension: %s" % classpath)
continue
try:
ext_mgr.load_extension(classpath)
except Exception as exc:
logger.warn(_(‘Failed to load extension %(classpath)s: ‘
‘%(exc)s‘),
{‘classpath‘: classpath, ‘exc‘: exc})
...
实际上也就是调用之前ExtensionManager的 load_extension函数加载模块，并构造模块中与模块名对应的类，由于这些类都是nova.api.openstack.extensions.ExtensionDescriptor的子类，且看其__init__函数：
def __init__(self, ext_mgr):
ext_mgr.register(self)
self.ext_mgr = ext_mgr
可知构造这些扩展类会在ExtensionManager中进行注册，注册过程就是在ExtensionManag中将扩展类的实例存入ExtensionManager的extensions列表中，对应的key为扩展类的alias。
综上所述，ExtensionManager的构造就是对其中的self.cls_list进行加载调用，self.cls_list包含扩展包的引导类或引导函数，此处为nova.api.openstack.compute.contrib.standard_extensions，该引导函数负责加载扩展包中模块，并构造相应的类实例，这些类实例在构造时会向ExtensionManager进行注册。