一，什么是中间件

中间件是 Django 请求/响应处理的钩子框架。它是一个轻量级的、低级的“插件”系统，用于全局改变 Django 的输入或输出。

二，中间件的作用

每个中间件组件负责做一些特定的功能。例如，Django 包含一个中间件组件 AuthenticationMiddleware，它使用会话将用户与请求关联起来。

既然中间件是一种钩子框架，那就好理解了：会在系统处于某种状态时自动调用。而这种系统状态就是django接收到请求request时的状态。中间件作用就是用于处理用户请求、产生响应内容。

其实，中间件就是Request到views、views到Response的一段处理过程。这个处理过程依赖于settings中激活的顺序：

MIDDLEWARE = [
    'django.middleware.security.SecurityMiddleware',
    'django.contrib.sessions.middleware.SessionMiddleware',
    'django.middleware.common.CommonMiddleware',
    'django.middleware.csrf.CsrfViewMiddleware',
    'django.contrib.auth.middleware.AuthenticationMiddleware',
    'django.contrib.messages.middleware.MessageMiddleware',
    'django.middleware.clickjacking.XFrameOptionsMiddleware',
    'MyDjango.myMiddleware.MyMiddleware'
]

中间件们从上至下对Request来接，从下至上对Response去送：

三，django的常用中间件

django默认提供了许多中间件，能满足绝大多数需要，参见Middleware。

中间件源码分析

以SessionMiddleware为例。

django\contrib\sessions\middleware.py:

import time
from importlib import import_module
from django.conf import settings
from django.contrib.sessions.backends.base import UpdateError
from django.core.exceptions import SuspiciousOperation
from django.utils.cache import patch_vary_headers
from django.utils.deprecation import MiddlewareMixin
from django.utils.http import http_date


class SessionMiddleware(MiddlewareMixin):
    def __init__(self, get_response=None):
        self.get_response = get_response
        engine = import_module(settings.SESSION_ENGINE)
        self.SessionStore = engine.SessionStore

    def process_request(self, request):
        session_key = request.COOKIES.get(settings.SESSION_COOKIE_NAME)
        request.session = self.SessionStore(session_key)

    def process_response(self, request, response):
        """
        If request.session was modified, or if the configuration is to save the
        session every time, save the changes and set a session cookie or delete
        the session cookie if the session has been emptied.
        """
        try:
            accessed = request.session.accessed
            modified = request.session.modified
            empty = request.session.is_empty()
        except AttributeError:
            pass
        else:
            # First check if we need to delete this cookie.
            # The session should be deleted only if the session is entirely empty
            if settings.SESSION_COOKIE_NAME in request.COOKIES and empty:
                response.delete_cookie(
                    settings.SESSION_COOKIE_NAME,
                    path=settings.SESSION_COOKIE_PATH,
                    domain=settings.SESSION_COOKIE_DOMAIN,
                )
            else:
                if accessed:
                    patch_vary_headers(response, ('Cookie',))
                if (modified or settings.SESSION_SAVE_EVERY_REQUEST) and not empty:
                    if request.session.get_expire_at_browser_close():
                        max_age = None
                        expires = None
                    else:
                        max_age = request.session.get_expiry_age()
                        expires_time = time.time() + max_age
                        expires = http_date(expires_time)
                    # Save the session data and refresh the client cookie.
                    # Skip session save for 500 responses, refs #3881.
                    if response.status_code != 500:
                        try:
                            request.session.save()
                        except UpdateError:
                            raise SuspiciousOperation(
                                "The request's session was deleted before the "
                                "request completed. The user may have logged "
                                "out in a concurrent request, for example."
                            )
                        response.set_cookie(
                            settings.SESSION_COOKIE_NAME,
                            request.session.session_key, max_age=max_age,
                            expires=expires, domain=settings.SESSION_COOKIE_DOMAIN,
                            path=settings.SESSION_COOKIE_PATH,
                            secure=settings.SESSION_COOKIE_SECURE or None,
                            httponly=settings.SESSION_COOKIE_HTTPONLY or None,
                            samesite=settings.SESSION_COOKIE_SAMESITE,
                        )
        return response

SessionMiddleware继承自MiddlewareMixin，而MiddlewareMixin只定义了__init__()与__call__()。SessionMiddleware重写了__init__()，还定义了两个钩子方法process_request和process_response来对请求与响应两个阶段的session进行处理。

实际上，各个中间件都用到了五个钩子函数process_request、process_response、process_view、process_exception和process_template_response中的一些来实现处理功能。

四，自定义中间件

没什么是比自定义一个中间件更能了解中间件运行机制的了。

首先，几个钩子函数的说明如下：

1，__init__(get_response)：初始化。

必须配置get_response

2，process_request(request)：在到达urls之前创建访问对象。

所有钩子函数与视图共用一个request

3，process_view(request, view_func, view_args, view_kwargs)：完成url地址匹配但还未执行视图。

view_func为视图功能名
view_args与view_kwargs为路由传入的参数

4，process_exception(request, exception)：在执行视图时发生的异常。

5，process_template_response(request, response)：在视图被完全执行后返回一个必须含有render方法类的对象。

6，process_response(request, response)：完成视图的执行但还未把它返回给浏览器。

response为返回的响应对象

1，先完成基本功能

MyDjango/urls.py：
from django.urls import path, include

urlpatterns = [
    path('', include(('index.urls', 'index'), namespace='index')),
]


index/urls.py：
from django.urls import path
from .views import *

urlpatterns = [
    path('', index, name='index'),
]


index/views.py：
from django.shortcuts import render
from django.shortcuts import Http404

def index(request):
    if request.GET.get('id', ''):
        raise Http404
    return render(request, 'index.html', locals())


templates/index.html：
<!DOCTYPE html>
<html lang="zh-hans">
<head>
    <title>首页</title>
</head>
<body>
    <div>Hello Django</div>
</body>
</html>

2，定义中间件

MyDjango/myMiddleware.py：
from django.utils.deprecation import MiddlewareMixin


class MyMiddleware(MiddlewareMixin):
    def __init__(self, get_response=None):
        """运行Django将自动执行"""
        super().__init__(get_response)
        self.get_response = get_response
        print('This is __init__')

    def process_request(self, request):
        """生成请求对象后，路由匹配之前"""
        print('This is process_request')

    def process_view(self, request, func, args, kwargs):
        """路由匹配后，视图函数调用之前"""
        print('This is process_view')

    def process_exception(self, request, exception):
        """视图函数发生异常时"""
        print('This is process_exception')

    def process_response(self, request, response):
        """视图函数执行后，响应内容返回浏览器之前"""
        print('This is process_response')
        return response

3，激活中间件

MIDDLEWARE = [
    'django.middleware.security.SecurityMiddleware',
    'django.contrib.sessions.middleware.SessionMiddleware',
    'django.middleware.common.CommonMiddleware',
    'django.middleware.csrf.CsrfViewMiddleware',
    'django.contrib.auth.middleware.AuthenticationMiddleware',
    'django.contrib.messages.middleware.MessageMiddleware',
    'django.middleware.clickjacking.XFrameOptionsMiddleware',
    'MyDjango.myMiddleware.MyMiddleware'
]