中间件
一、url请求经过中间件的执行顺序
中间件描述起来可能会很麻烦,一张图片更能简洁明了的django中间件的大概作用。
django 中的中间件(middleware),在django中,中间件其实就是一个类,在请求到来和结束后,django会根据自己的规则在合适的时机执行中间件中相应的方法。
在django项目的settings模块中,有一个 MIDDLEWARE_CLASSES 变量,其中每一个元素就是一个中间件,如下图。
每一个中间件中都可以定义四个方法,分别是:
- process_request(self,request)
- process_view(self, request, callback, callback_args, callback_kwargs)
- process_exception(self, request, exception)
- process_response(self, request, response)
以上方法的返回值可以是None和HttpResonse对象,如果是None,则继续按照django定义的规则向下执行,如果是HttpResonse对象,则直接将该对象返回给用户。
二、自定义中间件
1、创建放置中间件的目录和文件
2、自定义中间件
class mmm(object):
def process_request(self, request):
print 'mmm.process_request'
def process_view(self, request, callback, callback_args, callback_kwargs):
print 'mmm.process_view'
def process_response(self, request, response):
print 'mmm.process_response'
return response
class xxx(object):
def process_request(self, request):
print 'xxx.process_request'
def process_view(self, request, callback, callback_args, callback_kwargs):
print 'xxx.process_view'
def process_response(self, request, response):
print 'xxx.process_response'
return response
3、在settings.py里注册自定义的中间件类
MIDDLEWARE_CLASSES = [
'django.middleware.security.SecurityMiddleware',
'django.contrib.sessions.middleware.SessionMiddleware',
'django.middleware.common.CommonMiddleware',
'django.middleware.csrf.CsrfViewMiddleware',
'django.contrib.auth.middleware.AuthenticationMiddleware',
'django.contrib.auth.middleware.SessionAuthenticationMiddleware',
'django.contrib.messages.middleware.MessageMiddleware',
'django.middleware.clickjacking.XFrameOptionsMiddleware',
'middleware.middle1.mmm',
'middleware.middle1.xxx',
]
在执行时,可以看到请求在经过这中间件时的执行顺序
只有当程序执行错误时,才会执行process_exception(self, request, exception)
可以在views里的函数里触发一个错误,然后执行程序。
三、django中间件源码剖析
中间件的源码可以再wsgi.py里找到
点击进入到WSGIHandler类:
在WSGIHandler类的里面的 _call_方法里面会执行self.load_middleware()
在load_middleware方法里,定义了5个空列表。此方法会去循环settings.py下的MIDDLEWARE_CLASSES列表中所有中间件的类名。然后,在通过hasattr反射的方法去查找所有的中间件类里面是否有对应的process_request,process_view,process_response,process_exception,如果有这些方法就会把他们分别一一添加到定义的五个空列表里,然后再去循环四个列表,并分别去执行列表下的函数
1 def load_middleware(self):
2 """
3 Populate middleware lists from settings.MIDDLEWARE_CLASSES.
4
5 Must be called after the environment is fixed (see __call__ in subclasses).
6 """
7 self._view_middleware = []
8 self._template_response_middleware = []
9 self._response_middleware = []
10 self._exception_middleware = []
11
12 request_middleware = []
13 for middleware_path in settings.MIDDLEWARE_CLASSES:
14 mw_class = import_string(middleware_path)
15 try:
16 mw_instance = mw_class()
17 except MiddlewareNotUsed as exc:
18 if settings.DEBUG:
19 if six.text_type(exc):
20 logger.debug('MiddlewareNotUsed(%r): %s', middleware_path, exc)
21 else:
22 logger.debug('MiddlewareNotUsed: %r', middleware_path)
23 continue
24
25 if hasattr(mw_instance, 'process_request'):
26 request_middleware.append(mw_instance.process_request)
27 if hasattr(mw_instance, 'process_view'):
28 self._view_middleware.append(mw_instance.process_view)
29 if hasattr(mw_instance, 'process_template_response'):
30 self._template_response_middleware.insert(0, mw_instance.process_template_response)
31 if hasattr(mw_instance, 'process_response'):
32 self._response_middleware.insert(0, mw_instance.process_response)
33 if hasattr(mw_instance, 'process_exception'):
34 self._exception_middleware.insert(0, mw_instance.process_exception)
35
36 # We only assign to this when initialization is complete as it is used
37 # as a flag for initialization being complete.
38 self._request_middleware = request_middleware