Django HTTP处理流程（自我总结）

Django中由wsgi模块接管http请求，核心处理方法为get_wsgi_application，其定义如下：

def get_wsgi_application():
    """
    The public interface to Django's WSGI support. Should return a WSGI
    callable.
    Allows us to avoid making django.core.handlers.WSGIHandler public API, in
    case the internal WSGI implementation changes or moves in the future.
    """
    django.setup()
    return WSGIHandler()

WSGIHandler(代码有节减)：

class WSGIHandler(base.BaseHandler):
    initLock = Lock()
    request_class = WSGIRequest

    def __call__(self, environ, start_response):
        # Set up middleware if needed. We couldn't do this earlier, because
        # settings weren't available.
        if self._request_middleware is None:
            with self.initLock:
                try:
                    # Check that middleware is still uninitialized.
                    if self._request_middleware is None:
                        self.load_middleware()
                except:
                    # Unload whatever middleware we got
                    self._request_middleware = None
                    raise

        set_script_prefix(get_script_name(environ))
        signals.request_started.send(sender=self.__class__, environ=environ)
        try:
            request = self.request_class(environ)
        except UnicodeDecodeError:
            logger.warning('Bad Request (UnicodeDecodeError)',
                exc_info=sys.exc_info(),
                extra={
                    'status_code': 400,
                }
            )
            response = http.HttpResponseBadRequest()
        else:
            response = self.get_response(request)

        response._handler_class = self.__class__

        status = '%s %s' % (response.status_code, response.reason_phrase)
        response_headers = [(str(k), str(v)) for k, v in response.items()]
        for c in response.cookies.values():
            response_headers.append((str('Set-Cookie'), str(c.output(header=''))))
        start_response(force_str(status), response_headers)
        if getattr(response, 'file_to_stream', None) is not None and environ.get('wsgi.file_wrapper'):
            response = environ['wsgi.file_wrapper'](response.file_to_stream)
        return response

从代码中可以看到，当WSGIHandler对象被调用的时候，首先会判断请求中间件是否为None，如果为None，则通过self.load_middleware()加载中间件。之后调用self.get_response(request)获得http响应对象。BaseHandler的load_middleware、get_response方法定义如下：

BaseHandler(代码有节减)：

class BaseHandler(object):
    # Changes that are always applied to a response (in this order).
    response_fixes = [
        http.conditional_content_removal,
    ]

    def __init__(self):
        self._request_middleware = None
        self._view_middleware = None
        self._template_response_middleware = None
        self._response_middleware = None
        self._exception_middleware = None

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

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

        request_middleware = []
        for middleware_path in settings.MIDDLEWARE_CLASSES:
            mw_class = import_string(middleware_path)
            try:
                mw_instance = mw_class()
            except MiddlewareNotUsed as exc:
                if settings.DEBUG:
                    if six.text_type(exc):
                        logger.debug('MiddlewareNotUsed(%r): %s', middleware_path, exc)
                    else:
                        logger.debug('MiddlewareNotUsed: %r', middleware_path)
                continue

            if hasattr(mw_instance, 'process_request'):
                request_middleware.append(mw_instance.process_request)
            if hasattr(mw_instance, 'process_view'):
                self._view_middleware.append(mw_instance.process_view)
            if hasattr(mw_instance, 'process_template_response'):
                self._template_response_middleware.insert(0, mw_instance.process_template_response)
            if hasattr(mw_instance, 'process_response'):
                self._response_middleware.insert(0, mw_instance.process_response)
            if hasattr(mw_instance, 'process_exception'):
                self._exception_middleware.insert(0, mw_instance.process_exception)

        # We only assign to this when initialization is complete as it is used
        # as a flag for initialization being complete.
        self._request_middleware = request_middleware

    def get_response(self, request):
        "Returns an HttpResponse object for the given HttpRequest"

        # Setup default url resolver for this thread, this code is outside
        # the try/except so we don't get a spurious "unbound local
        # variable" exception in the event an exception is raised before
        # resolver is set
        urlconf = settings.ROOT_URLCONF
        urlresolvers.set_urlconf(urlconf)
        resolver = urlresolvers.get_resolver(urlconf)
        # Use a flag to check if the response was rendered to prevent
        # multiple renderings or to force rendering if necessary.
        response_is_rendered = False
        try:
            response = None
            # Apply request middleware
            for middleware_method in self._request_middleware:
                response = middleware_method(request)
                if response:
                    break

            if response is None:
                if hasattr(request, 'urlconf'):
                    # Reset url resolver with a custom URLconf.
                    urlconf = request.urlconf
                    urlresolvers.set_urlconf(urlconf)
                    resolver = urlresolvers.get_resolver(urlconf)

                resolver_match = resolver.resolve(request.path_info)
                callback, callback_args, callback_kwargs = resolver_match
                request.resolver_match = resolver_match

                # Apply view middleware
                for middleware_method in self._view_middleware:
                    response = middleware_method(request, callback, callback_args, callback_kwargs)
                    if response:
                        break

            if response is None:
                wrapped_callback = self.make_view_atomic(callback)
                try:
                    response = wrapped_callback(request, *callback_args, **callback_kwargs)
                except Exception as e:
                    response = self.process_exception_by_middleware(e, request)

            # Complain if the view returned None (a common error).
            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
            if 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)

                response_is_rendered = True

        except http.Http404 as exc:
            logger.warning('Not Found: %s', request.path,
                        extra={
                            'status_code': 404,
                            'request': request
                        })
            if settings.DEBUG:
                response = debug.technical_404_response(request, exc)
            else:
                response = self.get_exception_response(request, resolver, 404, exc)

        try:
            # Apply response middleware, regardless of the response
            for middleware_method in self._response_middleware:
                response = middleware_method(request, response)
                # Complain if the response middleware returned None (a common error).
                if response is None:
                    raise ValueError(
                        "%s.process_response didn't return an "
                        "HttpResponse object. It returned None instead."
                        % (middleware_method.__self__.__class__.__name__))
            response = self.apply_response_fixes(request, response)
        except:  # Any exception should be gathered and handled
            signals.got_request_exception.send(sender=self.__class__, request=request)
            response = self.handle_uncaught_exception(request, resolver, sys.exc_info())

        response._closable_objects.append(request)

        # If the exception handler returns a TemplateResponse that has not
        # been rendered, force it to be rendered.
        if not response_is_rendered and callable(getattr(response, 'render', None)):
            response = response.render()

        return response

get_response方法中，循环遍历执行已经加载的request middleware，调用middleware将返回None 或者HttpResponse对象，如果返回前者，继续处理其它中间件，如果返回一个 HttpResponse，就处理中止。接着，应用view middleware，通过 url resolve匹配路由获取view functions(如果使用的通用视图，通过调用as_view方法，此方法为闭包，返回一个view方法)。如果middleware支持渲染(render)，则应用template response中间件，然后通过调用HttpResponse对象的render方法渲染模板响应输出。