Django-session中间件源码简单分析

settings里有关中间件的配置

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',
    'rbac.service.middleware.M1'
]

from django.contrib.sessions.middleware import SessionMiddleware

可以看到settings中都是字符串形式的，我们通过from django.contrib.sessions.middleware import SessionMiddleware导入，并查看SessionMiddleware类的内容

SessionMiddleware

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 cookie_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 = cookie_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,
                        )
        return response

初始化方法

我们先看这个类的init初始化方法

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

首先定义self.get_response=get_response这里默认为None

然后通过import_module导入模块，该方法可以通过字符串导入模块

我们到settings中找到SESSION_ENGINE参数的值

这里engine就等于上面导入的模块

然后self.SessionStore = engine.SessionStore，我们可以看看engine模块中都有什么

可以看到SessionStore就是一个类

self.SessionStore就是这个类

process_request

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

首先定义一个session_key

这里的session_key其实就是COOKIE里带的以sessionid为key，随机字符串为值的值

当我们第一次访问时其实session_key的值应该是None

然后定义request.session，我们可以看到我们一直使用的request.session其实是self.SessionStore这个类实例化出的对象

实例化的过程执行了什么呢

SessionStore类初始化时调用了其父类的__init__方法

父类的__init__方法首先定义了self._session_key=session_key，第一次访问和默认情况都是None

然后定义了两个变量self.accessed和self.modified都是False

最后的self.serializer是序列化时的方法，我们暂时不看

到此process_request就执行完了

视图函数

在执行视图函数时我们可能会对request.session进行操作，例如登录成功后我们要做以下设置

request.session["user_id"] = user.pk

我们在学习面向对象的知识时曾经学过这种类似于操作字典一样的方法，在对象的父类中需要有__getitem__,__setitem__,__delitem__方法，才能这么操作，而这么操作了也就相当于执行了前面的几个方法

在SessionStore的父类SessionBase中我们找到了这几个方法

我们这里的操作其实就是执行的下面的代码

def __setitem__(self, key, value):
    self._session[key] = value
    self.modified = True

首先self._session[key] = value，这个self._session是个什么呢

继续在SessionBase类中查找

从上面的代码可以看出self._session其实就是self._session_cache，如果第一次访问，self._session_cache不存在，所以会抛出异常，走except的内容，给self._session_cache赋一个空字典，这里给这个字典添加了我们设置的键值对

然后将self.accessed和self.modified的值变为True

当视图函数执行完成后，我们又要走中间件的process_response方法

process_response

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 = cookie_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,
                    )
    return response