第二篇：白话tornado源码之待请求阶段

上篇《白话tornado源码之一个脚本引发的血案》用上帝视角多整个框架做了一个概述，同时也看清了web框架的的本质，下面我们从tornado程序的起始来分析其源码。

概述

上图是tornado程序启动以及接收到客户端请求后的整个过程，对于整个过程可以分为两大部分：

• 启动程序阶段，又称为待请求阶段（上图1、2所有系列和3.0）
• 接收并处理客户端请求阶段（上图3系列）

简而言之：

1、在启动程序阶段，第一步，获取配置文件然后生成url映射（即：一个url对应一个XXRequestHandler，从而让XXRequestHandler来处理指定url发送的请求）；第二步，创建服务器socket对象并添加到epoll中；第三步，创建无线循环去监听epoll。

2、在接收并处理请求阶段，第一步，接收客户端socket发送的请求（socket.accept）；第二步，从请求中获取请求头信息，再然后根据请求头中的请求url去匹配某个XXRequestHandler；第三步，匹配成功的XXRequestHandler处理请求；第四步，将处理后的请求发送给客户端；第五步，关闭客户端socket。

本篇的内容主要剖析【启动程序阶段】，下面我们就来一步一步的剖析整个过程，在此阶段主要是有下面重点标注的三个方法来实现。

import tornado.ioloop
import tornado.web
 
class MainHandler(tornado.web.RequestHandler):
    def get(self):
        self.write("Hello, world")
 
application = tornado.web.Application([
    (r"/index", MainHandler),
])
if __name__ == "__main__":
    application.listen()
    tornado.ioloop.IOLoop.instance().start()

一、application = tornado.web.Application([(xxx,xxx)])

　　执行Application类的构造函数，并传入一个列表类型的参数，这个列表里保存的是url规则和对应的处理类，即：当客户端的请求url可以配置这个规则时，那么该请求就交由对应的Handler去执行。

注意：Handler泛指继承自RequestHandler的所有类
        Handlers泛指继承自RequestHandler的所有类的集合

class Application(object):
    def __init__(self, handlers=None, default_host="", transforms=None,wsgi=False, **settings):
        #设置响应的编码和返回方式，对应的http相应头：Content-Encoding和Transfer-Encoding
        #Content-Encoding:gzip 表示对数据进行压缩，然后再返回给用户，从而减少流量的传输。
        #Transfer-Encoding:chunck 表示数据的传送方式通过一块一块的传输。
        if transforms is None:
            self.transforms = []
            if settings.get("gzip"):
                self.transforms.append(GZipContentEncoding)
            self.transforms.append(ChunkedTransferEncoding)
        else:
            self.transforms = transforms
        #将参数赋值为类的变量
        self.handlers = []
        self.named_handlers = {}
        self.default_host = default_host
        self.settings = settings
        #ui_modules和ui_methods用于在模版语言中扩展自定义输出
        #这里将tornado内置的ui_modules和ui_methods添加到类的成员变量self.ui_modules和self.ui_methods中
        self.ui_modules = {'linkify': _linkify,
                           'xsrf_form_html': _xsrf_form_html,
                           'Template': TemplateModule,
                           }
        self.ui_methods = {}
        self._wsgi = wsgi
        #获取获取用户自定义的ui_modules和ui_methods，并将他们添加到之前创建的成员变量self.ui_modules和self.ui_methods中
        self._load_ui_modules(settings.get("ui_modules", {}))
        self._load_ui_methods(settings.get("ui_methods", {}))
 
        #设置静态文件路径，设置方式则是通过正则表达式匹配url，让StaticFileHandler来处理匹配的url
        if self.settings.get("static_path"):
            #从settings中读取key为static_path的值，用于设置静态文件路径
            path = self.settings["static_path"]
            #获取参数中传入的handlers，如果空则设置为空列表
            handlers = list(handlers or [])
            #静态文件前缀，默认是/static/
            static_url_prefix = settings.get("static_url_prefix","/static/")
            #在参数中传入的handlers前再添加三个映射：
            #【/static/.*】            -->  StaticFileHandler
            #【/(favicon\.ico)】    -->  StaticFileHandler
            #【/(robots\.txt)】        -->  StaticFileHandler
            handlers = [
                (re.escape(static_url_prefix) + r"(.*)", StaticFileHandler,dict(path=path)),
                (r"/(favicon\.ico)", StaticFileHandler, dict(path=path)),
                (r"/(robots\.txt)", StaticFileHandler, dict(path=path)),
            ] + handlers
        #执行本类的Application的add_handlers方法
        #此时，handlers是一个列表，其中的每个元素都是一个对应关系，即：url正则表达式和处理匹配该正则的url的Handler
        if handlers: self.add_handlers(".*$", handlers)
 
        # Automatically reload modified modules
        #如果settings中设置了 debug 模式，那么就使用自动加载重启
        if self.settings.get("debug") and not wsgi:
            import autoreload
            autoreload.start()

Application.__init__

class Application(object):
    def add_handlers(self, host_pattern, host_handlers):
        #如果主机模型最后没有结尾符，那么就为他添加一个结尾符。
        if not host_pattern.endswith("$"):
            host_pattern += "$"
        handlers = []
        #对主机名先做一层路由映射，例如：http://www.wupeiqi.com 和 http://safe.wupeiqi.com
        #即：safe对应一组url映射，www对应一组url映射，那么当请求到来时，先根据它做第一层匹配，之后再继续进入内部匹配。
 
        #对于第一层url映射来说，由于.*会匹配所有的url，所将 .* 的永远放在handlers列表的最后，不然 .* 就会截和了...
        #re.complie是编译正则表达式，以后请求来的时候只需要执行编译结果的match方法就可以去匹配了
        if self.handlers and self.handlers[-1][0].pattern == '.*$':
            self.handlers.insert(-1, (re.compile(host_pattern), handlers))
        else:
            self.handlers.append((re.compile(host_pattern), handlers))
 
        #遍历我们设置的和构造函数中添加的【url->Handler】映射，将url和对应的Handler封装到URLSpec类中(构造函数中会对url进行编译)
        #并将所有的URLSpec对象添加到handlers列表中，而handlers列表和主机名模型组成一个元祖，添加到self.Handlers列表中。
        for spec in host_handlers:
            if type(spec) is type(()):
                assert len(spec) in (2, 3)
                pattern = spec[0]
                handler = spec[1]
                if len(spec) == 3:
                    kwargs = spec[2]
                else:
                    kwargs = {}
                spec = URLSpec(pattern, handler, kwargs)
            handlers.append(spec)
 
            if spec.name:
                #未使用该功能，默认spec.name = None
                if spec.name in self.named_handlers:
                    logging.warning("Multiple handlers named %s; replacing previous value",spec.name)
                self.named_handlers[spec.name] = spec

Application.add_handlers

class URLSpec(object):
    def __init__(self, pattern, handler_class, kwargs={}, name=None):
        if not pattern.endswith('$'):
            pattern += '$'
        self.regex = re.compile(pattern)
        self.handler_class = handler_class
        self.kwargs = kwargs
        self.name = name
        self._path, self._group_count = self._find_groups()

URLSpec

上述代码主要完成了以下功能：加载配置信息和生成url映射，并且把所有的信息封装在一个application对象中。

加载的配置信息包括：

• 编码和返回方式信息
• 静态文件路径
• ui_modules（模版语言中使用，暂时忽略）
• ui_methods（模版语言中使用，暂时忽略）
• 是否debug模式运行

　　以上的所有配置信息，都可以在settings中配置，然后在创建Application对象时候，传入参数即可。如：application = tornado.web.Application([(r"/index", MainHandler),],**settings)

生成url映射：

• 将url和对应的Handler添加到对应的主机前缀中，如：safe.index.com、www.auto.com

 封装数据：

　　将配置信息和url映射关系封装到Application对象中，信息分别保存在Application对象的以下字段中：

• self.transforms，保存着编码和返回方式信息
• self.settings，保存着配置信息
• self.ui_modules，保存着ui_modules信息
• self.ui_methods，保存这ui_methods信息
• self.handlers，保存着所有的主机名对应的Handlers，每个handlers则是url正则对应的Handler

二、application.listen(xxx)

　　第一步操作将配置和url映射等信息封装到了application对象中，而这第二步执行application对象的listen方法，该方法内部又把之前包含各种信息的application对象封装到了一个HttpServer对象中，然后继续调用HttpServer对象的liseten方法。

class Application(object):
    #创建服务端socket，并绑定IP和端口并添加相应设置，注：未开始通过while监听accept，等待客户端连接
    def listen(self, port, address="", **kwargs):
        from tornado.httpserver import HTTPServer
        server = HTTPServer(self, **kwargs)
        server.listen(port, address) 

详细代码：

class HTTPServer(object):
    def __init__(self, request_callback, no_keep_alive=False, io_loop=None,xheaders=False, ssl_options=None):
        #Application对象
        self.request_callback = request_callback
        #是否长连接
        self.no_keep_alive = no_keep_alive
        #IO循环
        self.io_loop = io_loop
        self.xheaders = xheaders
        #Http和Http
        self.ssl_options = ssl_options
        self._socket = None
        self._started = False
 
    def listen(self, port, address=""):
        self.bind(port, address)
        self.start(1)
 
    def bind(self, port, address=None, family=socket.AF_UNSPEC):
        assert not self._socket
        #创建服务端socket对象，IPV4和TCP连接
        self._socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM, 0)
        flags = fcntl.fcntl(self._socket.fileno(), fcntl.F_GETFD)
        flags |= fcntl.FD_CLOEXEC
        fcntl.fcntl(self._socket.fileno(), fcntl.F_SETFD, flags)
        #配置socket对象
        self._socket.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
        self._socket.setblocking(0)
        #绑定IP和端口
        self._socket.bind((address, port))
        #最大阻塞数量
        self._socket.listen(128)
 
    def start(self, num_processes=1):
        assert not self._started
        self._started = True
        if num_processes is None or num_processes <= 0:
            num_processes = _cpu_count()
        if num_processes > 1 and ioloop.IOLoop.initialized():
            logging.error("Cannot run in multiple processes: IOLoop instance "
                          "has already been initialized. You cannot call "
                          "IOLoop.instance() before calling start()")
            num_processes = 1
        #如果进程数大于1
        if num_processes > 1:
            logging.info("Pre-forking %d server processes", num_processes)
            for i in range(num_processes):
                if os.fork() == 0:
                    import random
                    from binascii import hexlify
                    try:
                        # If available, use the same method as
                        # random.py
                        seed = long(hexlify(os.urandom(16)), 16)
                    except NotImplementedError:
                        # Include the pid to avoid initializing two
                        # processes to the same value
                        seed(int(time.time() * 1000) ^ os.getpid())
                    random.seed(seed)
                    self.io_loop = ioloop.IOLoop.instance()
                    self.io_loop.add_handler(
                        self._socket.fileno(), self._handle_events,
                        ioloop.IOLoop.READ)
                    return
            os.waitpid(-1, 0)
        #进程数等于1，默认
        else:
            if not self.io_loop:
                #设置成员变量self.io_loop为IOLoop的实例，注：IOLoop使用methodclass完成了一个单例模式
                self.io_loop = ioloop.IOLoop.instance()
            #执行IOLoop的add_handler方法，将socket句柄、self._handle_events方法和IOLoop.READ当参数传入
            self.io_loop.add_handler(self._socket.fileno(),
                                     self._handle_events,
                                     ioloop.IOLoop.READ)
    def _handle_events(self, fd, events):
        while True:
            try:
                #====important=====#
                connection, address = self._socket.accept()
            except socket.error, e:
                if e.args[0] in (errno.EWOULDBLOCK, errno.EAGAIN):
                    return
                raise
            if self.ssl_options is not None:
                assert ssl, "Python 2.6+ and OpenSSL required for SSL"
                try:
                    #====important=====#
                    connection = ssl.wrap_socket(connection,server_side=True,do_handshake_on_connect=False,**self.ssl_options)
                except ssl.SSLError, err:
                    if err.args[0] == ssl.SSL_ERROR_EOF:
                        return connection.close()
                    else:
                        raise
                except socket.error, err:
                    if err.args[0] == errno.ECONNABORTED:
                        return connection.close()
                    else:
                        raise
            try:
                if self.ssl_options is not None:
                    stream = iostream.SSLIOStream(connection, io_loop=self.io_loop)
                else:
                    stream = iostream.IOStream(connection, io_loop=self.io_loop)
                #====important=====#
                HTTPConnection(stream, address, self.request_callback,self.no_keep_alive, self.xheaders)
            except:
                logging.error("Error in connection callback", exc_info=True)

HTTPServer

class IOLoop(object):
    # Constants from the epoll module
    _EPOLLIN = 0x001
    _EPOLLPRI = 0x002
    _EPOLLOUT = 0x004
    _EPOLLERR = 0x008
    _EPOLLHUP = 0x010
    _EPOLLRDHUP = 0x2000
    _EPOLLONESHOT = (1 << 30)
    _EPOLLET = (1 << 31)
 
    # Our events map exactly to the epoll events
    NONE = 0
    READ = _EPOLLIN
    WRITE = _EPOLLOUT
    ERROR = _EPOLLERR | _EPOLLHUP | _EPOLLRDHUP
 
    def __init__(self, impl=None):
        self._impl = impl or _poll()
        if hasattr(self._impl, 'fileno'):
            self._set_close_exec(self._impl.fileno())
        self._handlers = {}
        self._events = {}
        self._callbacks = []
        self._timeouts = []
        self._running = False
        self._stopped = False
        self._blocking_signal_threshold = None
 
        # Create a pipe that we send bogus data to when we want to wake
        # the I/O loop when it is idle
        if os.name != 'nt':
            r, w = os.pipe()
            self._set_nonblocking(r)
            self._set_nonblocking(w)
            self._set_close_exec(r)
            self._set_close_exec(w)
            self._waker_reader = os.fdopen(r, "rb", 0)
            self._waker_writer = os.fdopen(w, "wb", 0)
        else:
            self._waker_reader = self._waker_writer = win32_support.Pipe()
            r = self._waker_writer.reader_fd
        self.add_handler(r, self._read_waker, self.READ)
 
    @classmethod
    def instance(cls):
        if not hasattr(cls, "_instance"):
            cls._instance = cls()
        return cls._instance
 
    def add_handler(self, fd, handler, events):
        """Registers the given handler to receive the given events for fd."""
        self._handlers[fd] = stack_context.wrap(handler)
        self._impl.register(fd, events | self.ERROR)

IOLoop

def wrap(fn):
    '''Returns a callable object that will resore the current StackContext
    when executed.
 
    Use this whenever saving a callback to be executed later in a
    different execution context (either in a different thread or
    asynchronously in the same thread).
    '''
    if fn is None:
      return None
    # functools.wraps doesn't appear to work on functools.partial objects
    #@functools.wraps(fn)
    def wrapped(callback, contexts, *args, **kwargs):
        # If we're moving down the stack, _state.contexts is a prefix
        # of contexts.  For each element of contexts not in that prefix,
        # create a new StackContext object.
        # If we're moving up the stack (or to an entirely different stack),
        # _state.contexts will have elements not in contexts.  Use
        # NullContext to clear the state and then recreate from contexts.
        if (len(_state.contexts) > len(contexts) or
            any(a[1] is not b[1]
                for a, b in itertools.izip(_state.contexts, contexts))):
            # contexts have been removed or changed, so start over
            new_contexts = ([NullContext()] +
                            [cls(arg) for (cls,arg) in contexts])
        else:
            new_contexts = [cls(arg)
                            for (cls, arg) in contexts[len(_state.contexts):]]
        if len(new_contexts) > 1:
            with contextlib.nested(*new_contexts):
                callback(*args, **kwargs)
        elif new_contexts:
            with new_contexts[0]:
                callback(*args, **kwargs)
        else:
            callback(*args, **kwargs)
    if getattr(fn, 'stack_context_wrapped', False):
        return fn
    contexts = _state.contexts
    result = functools.partial(wrapped, fn, contexts)
    result.stack_context_wrapped = True
    return result

stack_context.wrap

备注：stack_context.wrap其实就是对函数进行一下封装，即：函数在不同情况下上下文信息可能不同。

上述代码本质上就干了以下这么四件事：

1. 把包含了各种配置信息的application对象封装到了HttpServer对象的request_callback字段中
2. 创建了服务端socket对象
3. 单例模式创建IOLoop对象，然后将socket对象句柄作为key，被封装了的函数_handle_events作为value，添加到IOLoop对象的_handlers字段中
4. 向epoll中注册监听服务端socket对象的读可用事件

目前，我们只是看到上述代码大致干了这四件事，而其目的有什么？他们之间的联系又是什么呢？

答：现在不妨先来做一个猜想，待之后再在源码中确认验证是否正确！猜想：通过epoll监听服务端socket事件，一旦请求到达时，则执行3中被封装了的_handle_events函数，该函数又利用application中封装了的各种配置信息对客户端url来指定判定，然后指定对应的Handler处理该请求。

注意：使用epoll创建服务端socket

import socket, select 
 
EOL1 = b'/n/n'
EOL2 = b'/n/r/n'
response  = b'HTTP/1.0 200 OK/r/nDate: Mon, 1 Jan 1996 01:01:01 GMT/r/n'
response += b'Content-Type: text/plain/r/nContent-Length: 13/r/n/r/n'
response += b'Hello, world!' 
 
serversocket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
serversocket.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
serversocket.bind(('0.0.0.0', 8080))
serversocket.listen(1)
serversocket.setblocking(0) 
 
epoll = select.epoll()
epoll.register(serversocket.fileno(), select.EPOLLIN) 
 
try:
   connections = {}; requests = {}; responses = {}
   while True:
      events = epoll.poll(1)
      for fileno, event in events:
         if fileno == serversocket.fileno():
            connection, address = serversocket.accept()
            connection.setblocking(0)
            epoll.register(connection.fileno(), select.EPOLLIN)
            connections[connection.fileno()] = connection
            requests[connection.fileno()] = b''
            responses[connection.fileno()] = response
         elif event & select.EPOLLIN:
            requests[fileno] += connections[fileno].recv(1024)
            if EOL1 in requests[fileno] or EOL2 in requests[fileno]:
               epoll.modify(fileno, select.EPOLLOUT)
               print('-'*40 + '/n' + requests[fileno].decode()[:-2])
         elif event & select.EPOLLOUT:
            byteswritten = connections[fileno].send(responses[fileno])
            responses[fileno] = responses[fileno][byteswritten:]
            if len(responses[fileno]) == 0:
               epoll.modify(fileno, 0)
               connections[fileno].shutdown(socket.SHUT_RDWR)
         elif event & select.EPOLLHUP:
            epoll.unregister(fileno)
            connections[fileno].close()
            del connections[fileno]
finally:
   epoll.unregister(serversocket.fileno())
   epoll.close()
   serversocket.close() 

Code

上述，其实就是利用epoll对象的poll(timeout)方法去轮询已经注册在epoll中的socket句柄，当有读可用的信息时候，则返回包含当前句柄和Event Code的序列，然后在通过句柄对客户端的请求进行处理

三、tornado.ioloop.IOLoop.instance().start()

上一步中创建了socket对象并使得socket对象和epoll建立了关系，该步骤则就来执行epoll的epoll方法去轮询已经注册在epoll对象中的socket句柄，当有读可用信息时，则触发一些操作什么的....

class IOLoop(object):
	def add_handler(self, fd, handler, events):
		#HttpServer的Start方法中会调用该方法
        self._handlers[fd] = stack_context.wrap(handler)
        self._impl.register(fd, events | self.ERROR)
 
    def start(self):
        while True:
            poll_timeout = 0.2
            try:
				#epoll中轮询
                event_pairs = self._impl.poll(poll_timeout)
            except Exception, e:
				#省略其他
			#如果有读可用信息，则把该socket对象句柄和Event Code序列添加到self._events中
            self._events.update(event_pairs)
			#遍历self._events，处理每个请求
            while self._events:
                fd, events = self._events.popitem()
                try:
					#以socket为句柄为key，取出self._handlers中的stack_context.wrap(handler)，并执行
					#stack_context.wrap(handler)包装了HTTPServer类的_handle_events函数的一个函数
					#是在上一步中执行add_handler方法时候，添加到self._handlers中的数据。
                    self._handlers[fd](fd, events)
                except:
					#省略其他

class IOLoop(object):
    def start(self):
        """Starts the I/O loop.
 
        The loop will run until one of the I/O handlers calls stop(), which
        will make the loop stop after the current event iteration completes.
        """
        if self._stopped:
            self._stopped = False
            return
        self._running = True
        while True:
            # Never use an infinite timeout here - it can stall epoll
            poll_timeout = 0.2
 
            # Prevent IO event starvation by delaying new callbacks
            # to the next iteration of the event loop.
            callbacks = self._callbacks
            self._callbacks = []
            for callback in callbacks:
                self._run_callback(callback)
 
            if self._callbacks:
                poll_timeout = 0.0
 
            if self._timeouts:
                now = time.time()
                while self._timeouts and self._timeouts[0].deadline <= now:
                    timeout = self._timeouts.pop(0)
                    self._run_callback(timeout.callback)
                if self._timeouts:
                    milliseconds = self._timeouts[0].deadline - now
                    poll_timeout = min(milliseconds, poll_timeout)
 
            if not self._running:
                break
 
            if self._blocking_signal_threshold is not None:
                # clear alarm so it doesn't fire while poll is waiting for
                # events.
                signal.setitimer(signal.ITIMER_REAL, 0, 0)
 
            try:
                event_pairs = self._impl.poll(poll_timeout)
            except Exception, e:
                # Depending on python version and IOLoop implementation,
                # different exception types may be thrown and there are
                # two ways EINTR might be signaled:
                # * e.errno == errno.EINTR
                # * e.args is like (errno.EINTR, 'Interrupted system call')
                if (getattr(e, 'errno', None) == errno.EINTR or
                    (isinstance(getattr(e, 'args', None), tuple) and
                     len(e.args) == 2 and e.args[0] == errno.EINTR)):
                    continue
                else:
                    raise
 
            if self._blocking_signal_threshold is not None:
                signal.setitimer(signal.ITIMER_REAL,
                                 self._blocking_signal_threshold, 0)
 
            # Pop one fd at a time from the set of pending fds and run
            # its handler. Since that handler may perform actions on
            # other file descriptors, there may be reentrant calls to
            # this IOLoop that update self._events
            self._events.update(event_pairs)
            while self._events:
                fd, events = self._events.popitem()
                try:
                    self._handlers[fd](fd, events)
                except (KeyboardInterrupt, SystemExit):
                    raise
                except (OSError, IOError), e:
                    if e.args[0] == errno.EPIPE:
                        # Happens when the client closes the connection
                        pass
                    else:
                        logging.error("Exception in I/O handler for fd %d",
                                      fd, exc_info=True)
                except:
                    logging.error("Exception in I/O handler for fd %d",
                                  fd, exc_info=True)
        # reset the stopped flag so another start/stop pair can be issued
        self._stopped = False
        if self._blocking_signal_threshold is not None:
            signal.setitimer(signal.ITIMER_REAL, 0, 0)

对于上述代码，执行start方法后，程序就进入“死循环”，也就是会一直不停的轮询的去检查是否有请求到来，如果有请求到达，则执行封装了HttpServer类的_handle_events方法和相关上下文的stack_context.wrap(handler)（其实就是执行HttpServer类的_handle_events方法），详细见下篇博文，简要代码如下：

class HTTPServer(object):
    def _handle_events(self, fd, events):
        while True:
            try:
                connection, address = self._socket.accept()
            except socket.error, e:
                if e.args[0] in (errno.EWOULDBLOCK, errno.EAGAIN):
                    return
                raise
            if self.ssl_options is not None:
                assert ssl, "Python 2.6+ and OpenSSL required for SSL"
                try:
                    connection = ssl.wrap_socket(connection,
                                                 server_side=True,
                                                 do_handshake_on_connect=False,
                                                 **self.ssl_options)
                except ssl.SSLError, err:
                    if err.args[0] == ssl.SSL_ERROR_EOF:
                        return connection.close()
                    else:
                        raise
                except socket.error, err:
                    if err.args[0] == errno.ECONNABORTED:
                        return connection.close()
                    else:
                        raise
            try:
                if self.ssl_options is not None:
                    stream = iostream.SSLIOStream(connection, io_loop=self.io_loop)
                else:
                    stream = iostream.IOStream(connection, io_loop=self.io_loop)
                HTTPConnection(stream, address, self.request_callback,
                               self.no_keep_alive, self.xheaders)
            except:
                logging.error("Error in connection callback", exc_info=True) 

结束

本篇博文介绍了“待请求阶段”的所作所为，简要来说其实就是三件事：其一、把setting中的各种配置以及url和Handler之间的映射关系封装到来application对象中（application对象又被封装到了HttpServer对象的request_callback字段中）；其二、结合epoll创建服务端socket；其三、当请求到达时交由HttpServer类的_handle_events方法处理请求，即：处理请求的入口。对于处理请求的详细，请参见下篇博客（客官莫急，加班编写中...）