Django中间件初始化过程

def load_middleware(self):
    """
    Populate middleware lists from settings.MIDDLEWARE.

    Must be called after the environment is fixed (see __call__ in subclasses).
    """
    self._view_middleware = []
    self._template_response_middleware = []
    self._exception_middleware = []

    handler = convert_exception_to_response(self._get_response) #为返回视图的函数，包一层出错函数处理异常。如果出错，返回出错的response，如果没出错，返回_get_response返回的东西
    for middleware_path in reversed(settings.MIDDLEWARE):   #从底向上遍历settings中的中间件列表
        middleware = import_string(middleware_path)
        try:
            mw_instance = middleware(handler)   #中间件实例：初始化每个中间件类，将他们的get_response初始化为，有异常处理的调用视图函数
        except MiddlewareNotUsed as exc:
            if settings.DEBUG:
                if str(exc):
                    logger.debug('MiddlewareNotUsed(%r): %s', middleware_path, exc)
                else:
                    logger.debug('MiddlewareNotUsed: %r', middleware_path)
            continue

        if mw_instance is None:
            raise ImproperlyConfigured(
                'Middleware factory %s returned None.' % middleware_path
            )

        if hasattr(mw_instance, 'process_view'):   #如果有view函数，放在类中，之后调用中间件时候使用
            self._view_middleware.insert(0, mw_instance.process_view)
        if hasattr(mw_instance, 'process_template_response'):
            self._template_response_middleware.append(mw_instance.process_template_response)
        if hasattr(mw_instance, 'process_exception'):
            self._exception_middleware.append(mw_instance.process_exception)

        handler = convert_exception_to_response(mw_instance)  #为初始化好的中间件实例包一层异常处理
        #每次循环，这里的handler都作为下一个中间件的get_response，也就是，每个中间件的get_response为上一个中间件实例（外面包一层异常）

    # We only assign to this when initialization is complete as it is used
    # as a flag for initialization being complete.
    self._middleware_chain = handler  #循环结束，chain为顶层中间件（settings中的第一个），顶层中间件实例中的get_response包了下一层中间件实例

def _get_response(self, request):
    """
    Resolve and call the view, then apply view, exception, and
    template_response middleware. This method is everything that happens
    inside the request/response middleware.
    """
    response = None

    if hasattr(request, 'urlconf'):
        urlconf = request.urlconf
        set_urlconf(urlconf)
        resolver = get_resolver(urlconf)
    else:
        resolver = get_resolver()  

    resolver_match = resolver.resolve(request.path_info)  #从请求url获取视图
    callback, callback_args, callback_kwargs = resolver_match  #获取视图函数，以及传参
    request.resolver_match = resolver_match

    # Apply view middleware
    for middleware_method in self._view_middleware:
        response = middleware_method(request, callback, callback_args, callback_kwargs) #在调用视图前调用view中间件，注意，如果有一个中间件返回response，将不调用视图函数
        if response:
            break

    if response is None:
        wrapped_callback = self.make_view_atomic(callback)  #为每个视图封装一个事务
        try:
            response = wrapped_callback(request, *callback_args, **callback_kwargs)  #调用视图函数
        except Exception as e:
            response = self.process_exception_by_middleware(e, request)

    # Complain if the view returned None (a common error).   #调用视图之后返回response为空的异常处理
    if response is None:
        if isinstance(callback, types.FunctionType):    # FBV
            view_name = callback.__name__
        else:                                           # CBV
            view_name = callback.__class__.__name__ + '.__call__'

        raise ValueError(
            "The view %s.%s didn't return an HttpResponse object. It "
            "returned None instead." % (callback.__module__, view_name)
        )

    # If the response supports deferred rendering, apply template
    # response middleware and then render the response
    elif hasattr(response, 'render') and callable(response.render):
        for middleware_method in self._template_response_middleware:
            response = middleware_method(request, response)
            # Complain if the template response middleware returned None (a common error).
            if response is None:
                raise ValueError(
                    "%s.process_template_response didn't return an "
                    "HttpResponse object. It returned None instead."
                    % (middleware_method.__self__.__class__.__name__)
                )

        try:
            response = response.render()
        except Exception as e:
            response = self.process_exception_by_middleware(e, request)

    return response