django源码分析：HTTP请求过程（wsgi）

本文环境python3.5.2，django1.10.x系列
python manage.py runserver 命令启动本地调试服务器后，接着要分析一下django对请求request的整个处理过程，

在此感谢在django源码学习中 小屋子大侠 的帮助，
小屋子大侠 博文地址：https://blog.youkuaiyun.com/qq_33339479/article/category/6800733

一、创建WSGIServer

上一节讲到在使用runserver运行Django项目，在启动时会调用django.core.servers.basehttp中的run()方法，创建一个django.core.servers.basehttp.WSGIServer类的实例，之后调用其serve_forever()方法启动HTTP服务。run方法的源码如下：

def run(addr, port, wsgi_handler, ipv6=False, threading=False): 
  server_address = (addr, port)                                                         # 服务端监听的地址和端口
  if threading:                                                                         # 如果多线程运行
    httpd_cls = type(str('WSGIServer'), (socketserver.ThreadingMixIn, WSGIServer), {})  # 创建WSGIServer服务器，生成一个
  else:
    httpd_cls = WSGIServer
  httpd = httpd_cls(server_address, WSGIRequestHandler, ipv6=ipv6)                      # 实例化该类
  # Sets the callable application as the WSGI application that will receive requests
  httpd.set_app(wsgi_handler)                                                           # 设置客服务端的处理handler， 即application
  httpd.serve_forever()

如上，我们可以看到：在创建WSGIServer实例的时候会指定HTTP请求的Handler，上述代码使用WSGIRequestHandler。当用户的HTTP请求到达服务器时，WSGIServer会创建WSGIRequestHandler实例，使用其handler方法来处理HTTP请求(其实最终是调用wsgiref.handlers.BaseHandler中的run方法处理)。WSGIServer通过set_app方法设置一个可调用(callable)的对象作为application，上面提到的handler方法最终会调用设置的application处理request，并返回response。

其中，WSGIServer继承自wsgiref.simple_server.WSGIServer，而WSGIRequestHandler继承自wsgiref.simple_server.WSGIRequestHandler，wsgiref是Python标准库给出的WSGI的参考实现。

class WSGIServer(HTTPServer):                                                           # WSGIServer继承自HTTPServer，HTTPServer继承自TCPServer

    """BaseHTTPServer that implements the Python WSGI protocol"""

    application = None

    def server_bind(self):
        """Override server_bind to store the server name."""
        HTTPServer.server_bind(self)
        self.setup_environ()

    def setup_environ(self):                                                            # 设置env环境参数
        # Set up base environment
        env = self.base_environ = {}
        env['SERVER_NAME'] = self.server_name
        env['GATEWAY_INTERFACE'] = 'CGI/1.1'
        env['SERVER_PORT'] = str(self.server_port)
        env['REMOTE_HOST']=''
        env['CONTENT_LENGTH']=''
        env['SCRIPT_NAME'] = ''

    def get_app(self):                                                                  # 获得处理器handler
        return self.application

    def set_app(self,application):                                                      # 设置处理器handler
        self.application = application

二、处理Request

第一步中说到的application，即为django.core.management.commands.runserver中的get_handler()方法

def get_internal_wsgi_application():
    """
    Loads and returns the WSGI application as configured by the user in
    ``settings.WSGI_APPLICATION``. With the default ``startproject`` layout,
    this will be the ``application`` object in ``projectname/wsgi.py``.

    This function, and the ``WSGI_APPLICATION`` setting itself, are only useful
    for Django's internal server (runserver); external WSGI servers should just
    be configured to point to the correct application object directly.

    If settings.WSGI_APPLICATION is not set (is ``None``), we just return
    whatever ``django.core.wsgi.get_wsgi_application`` returns.
    """
    from django.conf import settings                                                   # 加载生成project时生成的setting文件
    app_path = getattr(settings, 'WSGI_APPLICATION')                                   # 检查settings配置文件中是否设置“WSGI_APPLICATION”
    if app_path is None:                                                               # 如果没有设置，就使用默认的application
        return get_wsgi_application()

    try:
        return import_string(app_path)                                                 # 如果有设置，就返回设置的application
    except ImportError as e:
        msg = (
            "WSGI application '%(app_path)s' could not be loaded; "
            "Error importing module: '%(exception)s'" % ({
                'app_path': app_path,
                'exception': e,
            })
        )
        six.reraise(ImproperlyConfigured, ImproperlyConfigured(msg),
                    sys.exc_info()[2])

当settings文件中未设置WSGI_APPLICATION，就会使用django默认的内部wsgi方法，其中，get_wsgi_application()方法就是调用此方法，该函数在django.core.wsgi.py文件中

def get_wsgi_application():
    """
    The public interface to Django's WSGI support. Should return a WSGI
    callable.

    Allows us to avoid making django.core.handlers.WSGIHandler public API, in
    case the internal WSGI implementation changes or moves in the future.
    """
    django.setup(set_prefix=False)                                                   # 加载django初始环境
    return WSGIHandler()                                                             # 返回WSGI处理器

此方法在加载django处理化环境后，返回一个WSGIHandler类，下面我们看看这个处理器中的代码

class WSGIHandler(base.BaseHandler):
    request_class = WSGIRequest                                                 # 调用WSGIResquest对象

    def __init__(self, *args, **kwargs):
        super(WSGIHandler, self).__init__(*args, **kwargs)                      
        self.load_middleware()                                                  # 加载所有的中间件

    def __call__(self, environ, start_response):                                # 类实例化调用此方法
        set_script_prefix(get_script_name(environ))
        signals.request_started.send(sender=self.__class__, environ=environ)