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 = WSGIRequest

    def __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 = None

    def 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_response
        handler = convert_exception_to_response(get_response)
        handler_is_async = is_async
        for 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 = False
            else:
                middleware_is_async = middleware_can_async
            try:
                # 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)
                continue
            else:
                handler = adapted_handler

            if 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：

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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 response

        return inner
    else:
        @wraps(get_response)
        def inner(request):
            try:
                response = get_response(request)
            except Exception as exc:
                response = response_for_exception(request, exc)
            return response

        return 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