Django2.0手册:中间件

编写自己的中间件¶

中间件工厂是一个可调用的程序，它接受 get_response 可调用并返回中间件。中间件是可调用的，它接受请求并返回响应，就像视图一样。

中间件可以被写成这样的函数：

def simple_middleware(get_response):
    # One-time configuration and initialization.

    def middleware(request):
        # Code to be executed for each request before
        # the view (and later middleware) are called.

        response = get_response(request)

        # Code to be executed for each request/response after
        # the view is called.

        return response

    return middleware

或者它可以写成一个类，它的实例是可调用的，如下：

class SimpleMiddleware:
    def __init__(self, get_response):
        self.get_response = get_response
        # One-time configuration and initialization.

    def __call__(self, request):
        # Code to be executed for each request before
        # the view (and later middleware) are called.

        response = self.get_response(request)

        # Code to be executed for each request/response after
        # the view is called.

        return response

Django 提供的 get_response 响应可能是实际视图（如果这是最后列出的中间件），或者它可能是链中的下一个中间件。不需要知道或关心当前的中间件到底是什么，它只是代表了下一步的内容。

以上是一个轻微的简化——链中最后一个中间件调用的 get_response 可不是实际视图，而是处理程序的包装方法，它负责应用 view middleware，调用具有适当URL参数的视图，并应用 template-response 和 exception 中间件。

中间件可以放在 Python 路径上的任何地方。

激活中间件¶

若要激活中间件组件，请将其添加到 Django 设置中的 MIDDLEWARE 列表中。

在 MIDDLEWARE 中，每个中间件组件由字符串表示：指向中间件工厂的类或函数名的完整 Python 路径。例如，这里创建的默认值是 django-admin startproject：

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',
]

Django 安装不需要任何中间件——如果您愿意的话，MIDDLEWARE 可以为空——但是强烈建议您至少使用 CommonMiddleware。

MIDDLEWARE 的顺序很重要，因为中间件会依赖其他中间件。例如：类 AuthenticationMiddleware 在会话中存储经过身份验证的用户；因此，它必须在 SessionMiddleware 后面运行 。中间件。Session中间件。请参阅 Middleware ordering ，用于一些关于 Django 中间件类排序的常见提示。

中间件顺序与分层¶

在请求阶段，在调用视图之前，Django 按照定义的顺序应用中间件 MIDDLEWARE，自顶向下。

你可以把它想象成一个洋葱：每个中间件类都是一个“层”，它覆盖了洋葱的核心。如果请求通过洋葱的所有层（每一个调用 get_response ）以将请求传递到下一层，一直到内核的视图，那么响应将在返回的过程中通过每个层（以相反的顺序）。

If one of the layers decides to short-circuit and return a response without
ever calling its get_response, none of the layers of the onion inside that
layer (including the view) will see the request or the response. The response
will only return through the same layers that the request passed in through.

Other middleware hooks¶

Besides the basic request/response middleware pattern described earlier, you
can add three other special methods to class-based middleware:

Dealing with streaming responses¶

Unlike HttpResponse,
StreamingHttpResponse does not have a content
attribute. As a result, middleware can no longer assume that all responses
will have a content attribute. If they need access to the content, they
must test for streaming responses and adjust their behavior accordingly:

if response.streaming:
    response.streaming_content = wrap_streaming_content(response.streaming_content)
else:
    response.content = alter_content(response.content)

def wrap_streaming_content(content):
    for chunk in content:
        yield alter_content(chunk)

Exception handling¶

Django automatically converts exceptions raised by the view or by middleware
into an appropriate HTTP response with an error status code. Certain
exceptions are converted to 4xx status codes, while an unknown
exception is converted to a 500 status code.

This conversion takes place before and after each middleware (you can think of
it as the thin film in between each layer of the onion), so that every
middleware can always rely on getting some kind of HTTP response back from
calling its get_response callable. Middleware don’t need to worry about
wrapping their call to get_response in a try/except and handling an
exception that might have been raised by a later middleware or the view. Even
if the very next middleware in the chain raises an
Http404 exception, for example, your middleware won’t see
that exception; instead it will get an HttpResponse
object with a status_code of 404.

Upgrading pre-Django 1.10-style middleware¶

class django.utils.deprecation.MiddlewareMixin¶

Django provides django.utils.deprecation.MiddlewareMixin to ease creating
middleware classes that are compatible with both MIDDLEWARE and the
old MIDDLEWARE_CLASSES. All middleware classes included with Django
are compatible with both settings.

The mixin provides an __init__() method that accepts an optional
get_response argument and stores it in self.get_response.

The __call__() method:

1. Calls self.process_request(request) (if defined).
2. Calls self.get_response(request) to get the response from later
   middleware and the view.
3. Calls self.process_response(request, response) (if defined).
4. Returns the response.

If used with MIDDLEWARE_CLASSES, the __call__() method will
never be used; Django calls process_request() and process_response()
directly.

In most cases, inheriting from this mixin will be sufficient to make an
old-style middleware compatible with the new system with sufficient
backwards-compatibility. The new short-circuiting semantics will be harmless or
even beneficial to the existing middleware. In a few cases, a middleware class
may need some changes to adjust to the new semantics.

These are the behavioral differences between using MIDDLEWARE and
MIDDLEWARE_CLASSES:

1. Under MIDDLEWARE_CLASSES, every middleware will always have its
   process_response method called, even if an earlier middleware
   short-circuited by returning a response from its process_request
   method. Under MIDDLEWARE, middleware behaves more like an onion:
   the layers that a response goes through on the way out are the same layers
   that saw the request on the way in. If a middleware short-circuits, only
   that middleware and the ones before it in MIDDLEWARE will see the
   response.
2. Under MIDDLEWARE_CLASSES, process_exception is applied to
   exceptions raised from a middleware process_request method. Under
   MIDDLEWARE, process_exception applies only to exceptions
   raised from the view (or from the render method of a
   TemplateResponse). Exceptions raised from
   a middleware are converted to the appropriate HTTP response and then passed
   to the next middleware.
3. Under MIDDLEWARE_CLASSES, if a process_response method raises
   an exception, the process_response methods of all earlier middleware are
   skipped and a 500 Internal Server Error HTTP response is always
   returned (even if the exception raised was e.g. an
   Http404). Under MIDDLEWARE, an exception
   raised from a middleware will immediately be converted to the appropriate
   HTTP response, and then the next middleware in line will see that
   response. Middleware are never skipped due to a middleware raising an
   exception.