Django中间件的源码解析流程（上）—

Django中间件的源码解析流程（上）——中间件载入的前置

1. 前言

2. 请求的入口

3. 中间件加载的入口

4. 源码中的闭包实现

5. 最后

1. 前言

哈喽，大家好，我是小K,今天咋们分享的内容是：在学会Django中间件之后，我们继续深入底层源码。

在执行中间件时请求到来总是从前往后逐一匹配，但当响应返回时，执行的中间件顺序往往是反着执行的。

在Django中间件中，采用了多层闭包的形式，以来达到逐一执行中间件，本篇文章主要是从请求的入口开始，简单介绍一下前置。

如果还不会使用中间件的小伙伴可以跳转到这里：

Django之五种中间件定义类型

2. 请求的入口

在同步请求中，请求的入口其实就是 wsgi.py 文件，不清楚的可以跳转这里，看第2点：

Django源码之路由匹配（下）——图解逐步分析底层源码

3. 中间件加载的入口

在请求到来时，最后返回的是一个WSGIHandler的一个对象

application = get_wsgi_application()def get_wsgi_application():django.setup(set_prefix=False)return WSGIHandler()class WSGIHandler(base.BaseHandler):request_class = WSGIRequestdef __init__(self, *args, **kwargs):super().__init__(*args, **kwargs)self.load_middleware()

在加载对象时，肯定是少不了执行__init__的初始化方法的

self.load_middleware()   # 这其实就是加载中间件的方法

在执行load_middleware()函数时，首先是在自己的类中寻找，如果没有就去父类中寻找

显然，这里的代码是需要我们去父类BaseHandler类中寻找的

这里BaseHandler类和load_middleware的源码：

class BaseHandler:_view_middleware = None_template_response_middleware = None_exception_middleware = None_middleware_chain = Nonedef load_middleware(self, is_async=False):"""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 = []get_response = self._get_response_async if is_async else self._get_responsehandler = convert_exception_to_response(get_response)handler_is_async = is_asyncfor middleware_path in reversed(settings.MIDDLEWARE):middleware = import_string(middleware_path)middleware_can_sync = getattr(middleware, "sync_capable", True)middleware_can_async = getattr(middleware, "async_capable", False)if not middleware_can_sync and not middleware_can_async:raise RuntimeError("Middleware %s must have at least one of ""sync_capable/async_capable set to True." % middleware_path)elif not handler_is_async and middleware_can_sync:middleware_is_async = Falseelse:middleware_is_async = middleware_can_asynctry:# Adapt handler, if needed.adapted_handler = self.adapt_method_mode(middleware_is_async,handler,handler_is_async,debug=settings.DEBUG,name="middleware %s" % middleware_path,)mw_instance = middleware(adapted_handler)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)continueelse:handler = adapted_handlerif mw_instance is None:raise ImproperlyConfigured("Middleware factory %s returned None." % middleware_path)if hasattr(mw_instance, "process_view"):self._view_middleware.insert(0,self.adapt_method_mode(is_async, mw_instance.process_view),)if hasattr(mw_instance, "process_template_response"):self._template_response_middleware.append(self.adapt_method_mode(is_async, mw_instance.process_template_response),)if hasattr(mw_instance, "process_exception"):# The exception-handling stack is still always synchronous for# now, so adapt that way.self._exception_middleware.append(self.adapt_method_mode(False, mw_instance.process_exception),)handler = convert_exception_to_response(mw_instance)handler_is_async = middleware_is_async# Adapt the top of the stack, if needed.handler = self.adapt_method_mode(is_async, handler, handler_is_async)# We only assign to this when initialization is complete as it is used# as a flag for initialization being complete.self._middleware_chain = handler

ok，我们现在着重来分析load_middleware的源码

4. 源码中的闭包实现

先来看看什么是闭包：

Python奇幻之旅(从入门到入狱基础篇)——附相关资料【中】-CSDN博客

这篇文章里面有一部分介绍了闭包和装饰器

现在来看具体的闭包实现函数：

def convert_exception_to_response(get_response):if iscoroutinefunction(get_response):@wraps(get_response)async def inner(request):try:response = await get_response(request)except Exception as exc:response = await sync_to_async(response_for_exception, thread_sensitive=False)(request, exc)return responsereturn innerelse:@wraps(get_response)def inner(request):try:response = get_response(request)except Exception as exc:response = response_for_exception(request, exc)return responsereturn inner

最后返回了一个inner函数，而这个inner调用的是get_response()

最后都被封装进了handler里面