python-django中间件session源码

settings.py

MIDDLEWARE = [
    'django.contrib.sessions.middleware.SessionMiddleware',
]

1. 看看SessionMiddleware类   from django.contrib.sessions.middleware import SessionMiddleware

class SessionMiddleware(MiddlewareMixin):
　　 #1.1
    def __init__(self, get_response=None):
        self.get_response = get_response
        engine = import_module(settings.SESSION_ENGINE)
        self.SessionStore = engine.SessionStore
　　 #2
    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

1.1步：每个中间件的类在调用的时候都会先执行init 构造方法

class SessionMiddleware(MiddlewareMixin):
    def __init__(self, get_response=None):
        self.get_response = get_response
        #1.1.1 到配置文件中获取一个类
        engine = import_module(settings.SESSION_ENGINE)
        self.SessionStore = engine.SessionStore

1.1.1步：django自带的settings中没有SESSION_ENGINE这个属性，那就需要去全局的global_settings文件中找

        SESSION_ENGINE = 'django.contrib.sessions.backends.db'     #这个时候self.SessionStore =db 文件中的 SessionStore类

class SessionStore(SessionBase):
    """
    Implements database session store.
    """
    def __init__(self, session_key=None):
        super(SessionStore, self).__init__(session_key)

    @classmethod
    def get_model_class(cls):
        # Avoids a circular import and allows importing SessionStore when
        # django.contrib.sessions is not in INSTALLED_APPS.
        from django.contrib.sessions.models import Session
        return Session

    @cached_property
    def model(self):
        return self.get_model_class()

    def load(self):
        try:
            s = self.model.objects.get(
                session_key=self.session_key,
                expire_date__gt=timezone.now()
            )
            return self.decode(s.session_data)
        except (self.model.DoesNotExist, SuspiciousOperation) as e:
            if isinstance(e, SuspiciousOperation):
                logger = logging.getLogger('django.security.%s' % e.__class__.__name__)
                logger.warning(force_text(e))
            self._session_key = None
            return {}

    def exists(self, session_key):
        return self.model.objects.filter(session_key=session_key).exists()

    def create(self):
        while True:
            self._session_key = self._get_new_session_key()
            try:
                # Save immediately to ensure we have a unique entry in the
                # database.
                self.save(must_create=True)
            except CreateError:
                # Key wasn't unique. Try again.
                continue
            self.modified = True
            return

    def create_model_instance(self, data):
        """
        Return a new instance of the session model object, which represents the
        current session state. Intended to be used for saving the session data
        to the database.
        """
        return self.model(
            session_key=self._get_or_create_session_key(),
            session_data=self.encode(data),
            expire_date=self.get_expiry_date(),
        )

    def save(self, must_create=False):
        """
        Saves the current session data to the database. If 'must_create' is
        True, a database error will be raised if the saving operation doesn't
        create a *new* entry (as opposed to possibly updating an existing
        entry).
        """
        if self.session_key is None:
            return self.create()
        data = self._get_session(no_load=must_create)
        obj = self.create_model_instance(data)
        using = router.db_for_write(self.model, instance=obj)
        try:
            with transaction.atomic(using=using):
                obj.save(force_insert=must_create, force_update=not must_create, using=using)
        except IntegrityError:
            if must_create:
                raise CreateError
            raise
        except DatabaseError:
            if not must_create:
                raise UpdateError
            raise

    def delete(self, session_key=None):
        if session_key is None:
            if self.session_key is None:
                return
            session_key = self.session_key
        try:
            self.model.objects.get(session_key=session_key).delete()
        except self.model.DoesNotExist:
            pass

    @classmethod
    def clear_expired(cls):
        cls.get_model_class().objects.filter(expire_date__lt=timezone.now()).delete()

2.这时候就该走中间件的process_request方法了

    def process_request(self, request):
        session_key = request.COOKIES.get(settings.SESSION_COOKIE_NAME)    #从cookies中获取 'sessionid' 对应的值
        request.session = self.SessionStore(session_key)                  #把获取的值给db中的SessionStore类进行实例化并赋值给request.session

class SessionStore(SessionBase):
    def __init__(self, session_key=None):
        super(SessionStore, self).__init__(session_key)   #调用父类的 init 构造方

class SessionBase(object):
　　 session_key = property(_get_session_key)
　　 _session_key = property(_get_session_key, _set_session_key)   #在代码加载的时候要类的字段在方法前面加载，这时绑定了property方法，如果是获取操作就走_get_session_key方法，设置操作就走_set_session_key方法
    
　　 def __init__(self, session_key=None):
　　　　　#2.1
        self._session_key = session_key   #给_session_key赋值，走_set_session_key方法，把session_key传进去
        self.accessed = False
        self.modified = False
        self.serializer = import_string(settings.SESSION_SERIALIZER)   #SESSION_SERIALIZER = 'django.contrib.sessions.serializers.JSONSerializer'
　　　　 #表示 django 的session中间件是用Json进行序列化

2.1步：

class SessionBase(object):
    def _set_session_key(self, value):

        if self._validate_session_key(value):   #value = None
            self.__session_key = value
        else:
            self.__session_key = None
            
            
    def _validate_session_key(self, key):
        #如果sessionid有值并且长度大于等于8
        return key and len(key) >= 8

3. 然后该走视图中 对request.session进行操作，如果对session就行设置 值就执行 SessionStore 类中的__setitem__方法，如果是取session中的值就执行 __getitem__方法，删除执行__delitem__方法

，但是在SessionStore类中没有发现这个方法，那就去它的父类中找

class SessionBase(object):
　　 #self 是 request.session 这个SessionBase类对象
    def __getitem__(self, key):
        return self._session[key]   #到字典中获取值

    def __setitem__(self, key, value):
        self._session[key] = value  #给字典设置值，并且改变modified 状态为 True
        self.modified = True        

    def __delitem__(self, key):
        del self._session[key]      #删除字典中的键值对，并且改变modified 状态为 True
        self.modified = True　　　　 

在上面的代码中，我们看到self._session，点进去看看

class SessionBase(object):
    _session = property(_get_session) 

class SessionBase(object):
　　 #self 是 request.session 这个SessionBase类对象
    　
    def _get_session(self, no_load=False):

        self.accessed = True          #在这步给session设置值的时候 把这个accessed 状态改成了 True
        try:
            return self._session_cache   # 获取self 的_session_cache 属性，如果没有就走下面的except方法
        except AttributeError:
            if self.session_key is None or no_load:
                self._session_cache = {}   #赋值操作 
            else:
                self._session_cache = self.load()
        return self._session_cache

4. 走中间件的process_response 方法

class SessionMiddleware(MiddlewareMixin):
    def process_response(self, request, response):
        
        try:
            accessed = request.session.accessed     # TRUE
            modified = request.session.modified     #当设置或删除操作时 为TRUE
　　
　　　　　　　#4.1
            empty = request.session.is_empty()      # Flase
        except AttributeError:
            pass
        else:
            # 4.2 如果sessionid在cookies中 并且值为空，从cookie中把sessionid键值删除
            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',))
　　　　　　　　　　4.3  有效期和 过期时间相关
                if (modified or settings.SESSION_SAVE_EVERY_REQUEST) and not empty:      #True and True
　　　　　　　　　　　　#在浏览器关闭的时候 session 的有效期为 None ， 过期时间 为 None
                    if request.session.get_expire_at_browser_close():
                        max_age = None
                        expires = None
                    else:
　　　　　　　　　　　　　　 # 设置 有效期 和 过期时间
                        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:
　　　　　　　　　　　　　　　　　#4.4 访问正常 
                            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."
                            )
　　　　　　　　　　　　　　 #4.5  对 response 设置 cookie
                        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

4.1步：

class SessionBase(object): 
   def is_empty(self):
        "Returns True when there is no session_key and the session is empty"
        try:
            return not bool(self._session_key) and not self._session_cache   # 获取_session_key 执行property(_get_session_key)方法
　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　#  此时 self._session_cache 中已经有值了
　　　　　　　　#  True and   Flase

        except AttributeError:
            return True

class SessionBase(object):
    def _get_session_key(self):
        return self.__session_key    #从2.1步可以得出 为None

4.4步：

class SessionStore(SessionBase):
    def save(self, must_create=False):
　　　　　#4.4.1
        if self.session_key is None:   return self.create()
        
        data = self._get_session(no_load=must_create)
        obj = self.create_model_instance(data)
        using = router.db_for_write(self.model, instance=obj)
        try:
            with transaction.atomic(using=using):
                obj.save(force_insert=must_create, force_update=not must_create, using=using)
        except IntegrityError:
            if must_create:
                raise CreateError
            raise
        except DatabaseError:
            if not must_create:
                raise UpdateError
            raise

4.4.1步：

#session_key 获取，执行session_key = property(_get_session_key)  _get 方法

class SessionBase(object):
    def _get_session_key(self):
        return self.__session_key    # None

执行 self.create()  方法

class SessionStore(SessionBase):
    def create(self):
        while True:
　　　　　　　#4.4.1.1
            self._session_key = self._get_new_session_key()   #对_session_key 赋值操作，执行_set 方法

            try:
　　　　　　　　　 #4.4.1.2   传参 must_create=True 
                self.save(must_create=True)
            except CreateError:
                # Key wasn't unique. Try again.
                continue
            self.modified = True
            return

4.4.1.1步：

class SessionBase(object):
    def _get_new_session_key(self):
        while True:
            session_key = get_random_string(32, VALID_KEY_CHARS)
            if not self.exists(session_key):
                break
        return session_key   #随机字符串

class SessionBase(object):
    def _set_session_key(self, value):

        if self._validate_session_key(value):   #此时 value 是一个随机字符串
            self.__session_key = value          #self.__session_key 是 一个随机字符串
        else:
            self.__session_key = None

4.4.1.2步：

class SessionStore(SessionBase):
    def save(self, must_create=False):
        
        if self.session_key is None:    #获取session_key 操作，执行_get方法    见下面
            return self.create()
        data = self._get_session(no_load=must_create)   #must_create =True  见下面
        obj = self.create_model_instance(data)          #django创建session表对象
        using = router.db_for_write(self.model, instance=obj)
        try:
            with transaction.atomic(using=using):
　　　　　　　　　 # session对象 执行save方法  参数must_create = True 表示是创建 一条数据
                obj.save(force_insert=must_create, force_update=not must_create, using=using)
        except IntegrityError:
            if must_create:
                raise CreateError
            raise
        except DatabaseError:
            if not must_create:
                raise UpdateError
            raise
            

            
            
class SessionBase(object):        
    
    session_key = property(_get_session_key)
    
    def _get_session_key(self):
        return self.__session_key    #此时是一个随机字符串


class SessionBase(object):		
    def _get_session(self, no_load=False):
        
        self.accessed = True
        try:
            return self._session_cache    #此时 字典中已经有值
        except AttributeError:
            if self.session_key is None or no_load:
                self._session_cache = {}
            else:
                self._session_cache = self.load()
        return self._session_cache