Django 源码小剖: 初探中间件(middleware)
2013-09-14 00:36 捣乱小子 阅读(4913) 评论(2) 编辑 收藏 举报因为考虑到文章的长度, 所以 BaseHandler 的展开被推迟了. 在 BaseHandler 中隐藏着中间件的信息, 较常见的 SessionMiddleware 就已经默认安装. BaseHandler 的展开主要是以代码为主, 但已经加入了注释; 文章的最后附一张美图 .
最后, 祝程序员们节日快乐, 别太宅了 ;)
BaseHandler 详解
BaseHandler 在 django.core.handlers.base.py 中定义, 有两个核心的成员方法不得不提, 里面就涉及了中间件的信息, 照抄如下(有点长, 但已经加入注释):
# 好经典的 handler
class BaseHandler(object):
# Changes that are always applied to a response (in this order).
response_fixes = [
http.fix_location_header,
http.conditional_content_removal,
http.fix_IE_for_attach,
http.fix_IE_for_vary,
]
初始化函数, 初始化请求中间件, 视图中间件, 模版中间件, 响应中间件和异常中间件.
def __init__(self):
self._request_middleware = self._view_middleware =
self._template_response_middleware =
self._response_middleware =
self._exception_middleware = None 视图, 模版相应, 相应, 异常中间件, 请求中间件
根据 mysite.settings.py 中的 `MIDDLEWARE_CLASSES` 添加所有的中间件.
def load_middleware(self):
"""
Populate middleware lists from settings.MIDDLEWARE_CLASSES.
从 settings 中加载各种中间件
Must be called after the environment is fixed (see __call__ in subclasses).
"""
# 初始化四种中间件
self._view_middleware = []
self._template_response_middleware = []
self._response_middleware = []
self._exception_middleware = []
# 临时的请求中间件, 因为在加入中间件的过程中, 可能会出现异常, 而出现异常都导致加载中间件的不成功, 因此将 self._request_middleware 的赋值放在最后, 表示已经成功.
request_middleware = []
# settings.MIDDLEWARE_CLASSES 设置项指定需要预装的中间件
for middleware_path in settings.MIDDLEWARE_CLASSES:
try:
mw_module, mw_classname = middleware_path.rsplit('.', 1)
except ValueError:
raise exceptions.ImproperlyConfigured('%s isn\'t a middleware module' % middleware_path)
try:
尝试导入中间件所在模块.
mod = import_module(mw_module)
except ImportError as e:
raise exceptions.ImproperlyConfigured('Error importing middleware %s: "%s"' % (mw_module, e))
try:
尝试得到某种中间件类
mw_class = getattr(mod, mw_classname)
except AttributeError:
raise exceptions.ImproperlyConfigured('Middleware module "%s" does not define a "%s" class' % (mw_module, mw_classname))
try:
尝试实例化
mw_instance = mw_class()
except exceptions.MiddlewareNotUsed:
continue
和 urllib 的处理方法类似: 请求预处理, 视图处理?, 模版处理, 相应处理, 错误处理(详见我的 urllib 源码剖析)
if hasattr(mw_instance, 'process_request'):
# 这里 request_middleware 用的是 append(), 这里是有讲究的:
# django 规定, 多个请求中间件调用的次序是其出现的次序, 下同
request_middleware.append(mw_instance.process_request)
if hasattr(mw_instance, 'process_view'):
self._view_middleware.append(mw_instance.process_view)
if hasattr(mw_instance, 'process_template_response'):
# 这里 _template_response_middleware 用的是 insert() 头插法, 这里是有讲究的:
# django 规定, 多个模版相应中间件调用的次序是其出现次序的逆序, 下同
self._template_response_middleware.insert(0, mw_instance.process_template_response)
if hasattr(mw_instance, 'process_response'):
self._response_middleware.insert(0, mw_instance.process_response)
if hasattr(mw_instance, 'process_exception'):
self._exception_middleware.insert(0, mw_instance.process_exception)
# We only assign to this when initialization is complete as it is used
# as a flag for initialization being complete.
# 结束的标识, 表明中间件加载成功
self._request_middleware = request_middleware
# 处理请求的函数, 并返回 response
def get_response(self, request):
"Returns an HttpResponse object for the given HttpRequest"
根据请求, 得到响应
try:
为该线程提供默认的 url 处理器
# Setup default url resolver for this thread, this code is outside
# the try/except so we don't get a spurious "unbound local
# variable" exception in the event an exception is raised before
# resolver is set
#ROOT_URLCONF = 'mysite.urls'
urlconf = settings.ROOT_URLCONF
# set_urlconf() 会设置 url 配置即 settings.ROOT_URLCONF
urlresolvers.set_urlconf(urlconf)
# 实例化 RegexURLResolver, 暂且将其理解为一个 url 的匹配处理器, 下节展开
resolver = urlresolvers.RegexURLResolver(r'^/', urlconf)
try:
response = None
# Apply request middleware 调用请求中间件
for middleware_method in self._request_middleware:
response = middleware_method(request)
# 如果此 response 有效, 即不走下面的逻辑
if response:
break
# 如果没有结果
if response is None:
# 尝试 request 中是否有 urlconf, 一般没有, 可以忽略此段代码!!!
if hasattr(request, 'urlconf'):
# Reset url resolver with a custom urlconf. 自定义的 urlconf
urlconf = request.urlconf
urlresolvers.set_urlconf(urlconf)
resolver = urlresolvers.RegexURLResolver(r'^/', urlconf)
# 调用 RegexURLResolver.resolve(), 可以理解为启动匹配的函数; 返回 ResolverMatch 实例
resolver_match = resolver.resolve(request.path_info)
# resolver_match 对象中存储了有用的信息, 譬如 callback 就是我们在 views.py 中定义的函数.
callback, callback_args, callback_kwargs = resolver_match
# 将返回的 resolver_match 挂钩到 request
request.resolver_match = resolver_match
# Apply view middleware 调用视图中间件
for middleware_method in self._view_middleware:
response = middleware_method(request, callback, callback_args, callback_kwargs)
# 如果此 response 有效, 即不走下面的逻辑
if response:
break
# response 还是为空
if response is None:
try:
# 这里调用的是真正的处理函数, 我们一般在 view.py 中定义这些函数
response = callback(request, *callback_args, **callback_kwargs)
except Exception as e:
# If the view raised an exception, run it through exception
# middleware, and if the exception middleware returns a
# response, use that. Otherwise, reraise the exception.
# 出现异常, 调用异常中间件
for middleware_method in self._exception_middleware:
response = middleware_method(request, e)
# 如果此 response 有效, 即不走下面的逻辑
if response:
break
if response is None:
raise
# response 还是为空, 可能就要异常了
# Complain if the view returned None (a common error).
if response is None:
if isinstance(callback, types.FunctionType): # FBV
view_name = callback.__name__
else: # CBV
view_name = callback.__class__.__name__ + '.__call__'
raise ValueError("The view %s.%s didn't return an HttpResponse object." % (callback.__module__, view_name))
# If the response supports deferred rendering, apply template
# response middleware and the render the response 如果 response 实现了 render, 那么渲染返回.
if hasattr(response, 'render') and callable(response.render):
for middleware_method in self._template_response_middleware:
response = middleware_method(request, response)
response = response.render()
except http.Http404 as e:
logger.warning('Not Found: %s', request.path,
extra={
'status_code': 404,
'request': request
})
# 如果是调试下, 直接要返回 404 页面
if settings.DEBUG:
response = debug.technical_404_response(request, e)
else:
try:
# 非调试模式下, 获取 url 处理器的默认 404 处理
callback, param_dict = resolver.resolve404()
response = callback(request, **param_dict)
except:
signals.got_request_exception.send(sender=self.__class__, request=request)
response = self.handle_uncaught_exception(request, resolver, sys.exc_info())
# 访问拒绝
except exceptions.PermissionDenied:
logger.warning(
'Forbidden (Permission denied): %s', request.path,
extra={
'status_code': 403,
'request': request
})
try:
callback, param_dict = resolver.resolve403()
response = callback(request, **param_dict)
except:
signals.got_request_exception.send(
sender=self.__class__, request=request)
response = self.handle_uncaught_exception(request,
resolver, sys.exc_info())
except SystemExit:
# Allow sys.exit() to actually exit. See tickets #1023 and #4701
raise
except: # Handle everything else, including SuspiciousOperation, etc.
# Get the exception info now, in case another exception is thrown later.
signals.got_request_exception.send(sender=self.__class__, request=request)
response = self.handle_uncaught_exception(request, resolver, sys.exc_info())
finally:
# Reset URLconf for this thread on the way out for complete
# isolation of request.urlconf 重置, 因为前面有两种 url resolver 的可能, 拒绝混淆
urlresolvers.set_urlconf(None)
try:
# Apply response middleware, regardless of the response 调用响应中间件
for middleware_method in self._response_middleware:
response = middleware_method(request, response)
response = self.apply_response_fixes(request, response)
except: # Any exception should be gathered and handled
signals.got_request_exception.send(sender=self.__class__, request=request)
response = self.handle_uncaught_exception(request, resolver, sys.exc_info())
return response
def handle_uncaught_exception(self, request, resolver, exc_info):
"""
处理未能捕捉的错误
Processing for any otherwise uncaught exceptions (those that will
generate HTTP 500 responses). Can be overridden by subclasses who want
customised 500 handling. 子类中可以重写 500 状态的处理
Be *very* careful when overriding this because the error could be
caused by anything, so assuming something like the database is always
available would be an error.
"""
if settings.DEBUG_PROPAGATE_EXCEPTIONS:
raise
logger.error('Internal Server Error: %s', request.path,
exc_info=exc_info,
extra={
'status_code': 500,
'request': request
}
)
调试模式特殊处理
if settings.DEBUG:
return debug.technical_500_response(request, *exc_info)
# If Http500 handler is not installed, re-raise last exception 如果http500 处理器都没有安装, 可能会崩溃
if resolver.urlconf_module is None:
six.reraise(*exc_info)
# Return an HttpResponse that displays a friendly error message.
#这是自定义的 500 处理器
callback, param_dict = resolver.resolve500()
return callback(request, **param_dict)
def apply_response_fixes(self, request, response):
"""
Applies each of the functions in self.response_fixes to the request and
response, modifying the response in the process. Returns the new
response.
"""
for func in self.response_fixes:
response = func(request, response)
return response
故此总结
load_middleware() 函数会根据 mysite.settings.py 中的 MIDDLEWARE_CLASSES 导入所有的中间件. 在 eclipse + pydev 创建 django 的默认设置当中就有默认的中间件:
MIDDLEWARE_CLASSES = (
'django.middleware.common.CommonMiddleware',
'django.contrib.sessions.middleware.SessionMiddleware',
'django.middleware.csrf.CsrfViewMiddleware',
'django.contrib.auth.middleware.AuthenticationMiddleware',
'django.contrib.messages.middleware.MessageMiddleware',
# Uncomment the next line for simple clickjacking protection:
# 'django.middleware.clickjacking.XFrameOptionsMiddleware',
)
每一个中间件都是一个类, 其内部会实现 process_request(),process_view(),process_template_response(),process_response() 或者 process_exception() 方法. 不一定都实现, 看需求. 而这些方法如果存在, 都会被保存响应的函数列表中, 待将来调用.
get_response() 方法, 中间件调用执行的顺序是请求中间件, 视图中间件, 模版中间件, 异常中间件(可选), 响应中间件. 习惯上, 我把这些简称为请求预处理和响应善后处理.get_response() 返回了 response, 但一长串的 url 是如何匹配的, 并且自己在 views.py 中的函数是在什么时候调用的?
我已经在 github 备份了 Django 源码的注释: Decode-Django, 有兴趣的童鞋 fork 吧.
图解中间件:
捣乱 2013-9-14
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