Tornado源码浅析

初识tornado

经典的hello world 案例:

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(8888)
    tornado.ioloop.IOLoop.instance().start()

程序执行流程:

1、创建Application 对象，把正则表达式和类名MainHandler传入构造函数，即:  tornado.web.Application(...)

2、执行Application 对象的 listen(...) 方法，即: application.listen(8888)

3、执行IOLoop类的类的 start() 方法，即：tornado.ioloop.IOLoop.instance().start()

程序实质：创建一个socket 监听8888端口，当请求到来时，根据请求的url和请求方式(get,post,put)来指定相应的类中的方法来处理本次请求。

在浏览器上访问：http://127.0.0.1:8888/index，则服务器给浏览器就会返回 Hello,world ，否则返回 404: Not Found（tornado内部定义的值）， 即完成一次http请求和响应。

由上述分析，我们将整个Web框架分为两大部分：

• 待请求阶段（程序启动阶段），即：创建服务端socket并监听端口

• 处理请求阶段，即：当有客户端连接时，接受请求，并根据请求的不同做出相应的相应

待请求阶段（程序启动阶段）

import tornado.ioloop
import tornado.web

class MainHandler(tornado.web.RequestHandler):
    def get(self):
        self.write("Hello, world")

application = tornado.web.Application([             ##  <=======  1
    (r"/index", MainHandler),
])

if __name__ == "__main__":
    application.listen(8888)                        ##  <======== 2
    tornado.ioloop.IOLoop.instance().start()        ##  <======== 3

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

def __init__(self, handlers=None, default_host="", transforms=None,
             **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("compress_response") or settings.get("gzip"):
            self.transforms.append(GZipContentEncoding)
    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 = {}
    # 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/")
        static_handler_class = settings.get("static_handler_class",
                                            StaticFileHandler)
        static_handler_args = settings.get("static_handler_args", {})
        static_handler_args['path'] = path
        # 在参数中传入的handlers前再添加三个映射：
        # 【/static/.*】            -->  StaticFileHandler
        # 【/(favicon\.ico)】    -->  StaticFileHandler
        # 【/(robots\.txt)】        -->  StaticFileHandler
        for pattern in [re.escape(static_url_prefix) + r"(.*)",
                        r"/(favicon\.ico)", r"/(robots\.txt)"]:
            handlers.insert(0, (pattern, static_handler_class,
                                static_handler_args))
    # 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'):
        self.settings.setdefault('autoreload', True)
        self.settings.setdefault('compiled_template_cache', False)
        self.settings.setdefault('static_hash_cache', False)
        self.settings.setdefault('serve_traceback', True)

    if self.settings.get('autoreload'):
        from tornado import autoreload
        autoreload.start()

Application.__init__

def add_handlers(self, host_pattern, host_handlers):
    """Appends the given handlers to our handler list.

    Host patterns are processed sequentially in the order they were
    added. All matching patterns will be considered.
    """
    # 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.alex.com 和 http://safe.alex.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 == '.*$':
        self.handlers.insert(-1, (re.compile(host_pattern), handlers))
    else:
        self.handlers.append((re.compile(host_pattern), handlers))
        # [(re.compile('.*$'), [])]

    # 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 isinstance(spec, (tuple, list)):
            assert len(spec) in (2, 3, 4)
            spec = URLSpec(*spec)  # <==============
        handlers.append(spec)
        if spec.name:
            if spec.name in self.named_handlers:
                app_log.warning(
                    "Multiple handlers named %s; replacing previous value",
                    spec.name)
            self.named_handlers[spec.name] = spec

Application.add_handlers

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

2. application.listen(...)

def listen(self, port, address="", **kwargs):
    from tornado.httpserver import HTTPServer
    server = HTTPServer(self, **kwargs)
    server.listen(port, address)
    return server

这步执行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 HTTPServer(TCPServer, Configurable,httputil.HTTPServerConnectionDelegate):

HttpServer 类继承了TCPServer类,

TCPServer类listen方法:

def listen(self, port, address=""):
    sockets = bind_sockets(port, address=address)
    self.add_sockets(sockets)

1.def bind_sockets 建立socket连接

bind_sockets

2.add_sockets(sockets) 添加socket,进行监听

def add_sockets(self, sockets):
    if self.io_loop is None:
        self.io_loop = IOLoop.current()         #1 <==============
        # tornado.platform.select.SelectIOLoop object
        # 设置成员变量self.io_loop为IOLoop的实例，注：IOLoop使用单例模式创建

    for sock in sockets:
        self._sockets[sock.fileno()] = sock
        add_accept_handler(sock, self._handle_connection,io_loop=self.io_loop)       #2 <=============
        # 执行IOLoop的add_accept_handler方法，将socket句柄、self._handle_connection方法和io_loop对象当参数传入

def _handle_connection(self, connection, address):
    if self.ssl_options is not None:
        assert ssl, "Python 2.6+ and OpenSSL required for SSL"
        try:
            connection = ssl_wrap_socket(connection,
                                         self.ssl_options,
                                         server_side=True,
                                         do_handshake_on_connect=False)
        except ssl.SSLError as err:
            if err.args[0] == ssl.SSL_ERROR_EOF:
                return connection.close()
            else:
                raise
        except socket.error as err:
            # If the connection is closed immediately after it is created
            # (as in a port scan), we can get one of several errors.
            # wrap_socket makes an internal call to getpeername,
            # which may return either EINVAL (Mac OS X) or ENOTCONN
            # (Linux).  If it returns ENOTCONN, this error is
            # silently swallowed by the ssl module, so we need to
            # catch another error later on (AttributeError in
            # SSLIOStream._do_ssl_handshake).
            # To test this behavior, try nmap with the -sT flag.
            # https://github.com/tornadoweb/tornado/pull/750
            if errno_from_exception(err) in (errno.ECONNABORTED, errno.EINVAL):
                return connection.close()
            else:
                raise
    try:
        if self.ssl_options is not None:
            stream = SSLIOStream(connection, io_loop=self.io_loop,
                                 max_buffer_size=self.max_buffer_size,
                                 read_chunk_size=self.read_chunk_size)
        else:
            stream = IOStream(connection, io_loop=self.io_loop,
                              max_buffer_size=self.max_buffer_size,
                              read_chunk_size=self.read_chunk_size)
        future = self.handle_stream(stream, address)
        if future is not None:
            self.io_loop.add_future(future, lambda f: f.result())
    except Exception:
        app_log.error("Error in connection callback", exc_info=True)

self._handle_connection

#1 创建的IOLoop对象为: SelectIOLoop (windows下)

def configurable_default(cls):
    if hasattr(select, "epoll"):
        from tornado.platform.epoll import EPollIOLoop
        return EPollIOLoop
    if hasattr(select, "kqueue"):
        # Python 2.6+ on BSD or Mac
        from tornado.platform.kqueue import KQueueIOLoop
        return KQueueIOLoop
    from tornado.platform.select import SelectIOLoop
    return SelectIOLoop          # <============

#2 然后执行add_accept_handler()

def add_accept_handler(sock, callback, io_loop=None):
    """Adds an `.IOLoop` event handler to accept new connections on ``sock``.

    When a connection is accepted, ``callback(connection, address)`` will
    be run (``connection`` is a socket object, and ``address`` is the
    address of the other end of the connection).  Note that this signature
    is different from the ``callback(fd, events)`` signature used for
    `.IOLoop` handlers.

    .. versionchanged:: 4.1
       The ``io_loop`` argument is deprecated.
    """
    if io_loop is None:
        io_loop = IOLoop.current()

    def accept_handler(fd, events):
        # More connections may come in while we're handling callbacks;
        # to prevent starvation of other tasks we must limit the number
        # of connections we accept at a time.  Ideally we would accept
        # up to the number of connections that were waiting when we
        # entered this method, but this information is not available
        # (and rearranging this method to call accept() as many times
        # as possible before running any callbacks would have adverse
        # effects on load balancing in multiprocess configurations).
        # Instead, we use the (default) listen backlog as a rough
        # heuristic for the number of connections we can reasonably
        # accept at once.
        for i in xrange(_DEFAULT_BACKLOG):
            try:
                connection, address = sock.accept()
            except socket.error as e:
                # _ERRNO_WOULDBLOCK indicate we have accepted every
                # connection that is available.
                if errno_from_exception(e) in _ERRNO_WOULDBLOCK:
                    return
                # ECONNABORTED indicates that there was a connection
                # but it was closed while still in the accept queue.
                # (observed on FreeBSD).
                if errno_from_exception(e) == errno.ECONNABORTED:
                    continue
                raise
            callback(connection, address)
    io_loop.add_handler(sock, accept_handler, IOLoop.READ)

add_accept_handler

执行其中的 io_loop.add_handler  io_loop对象为: SelectIOLoop 所以执行它的add_handler()

class SelectIOLoop(PollIOLoop):      # <============
    def initialize(self, **kwargs):
        super(SelectIOLoop, self).initialize(impl=_Select(), **kwargs)

执行PollIOLoop 的add_handler()

def add_handler(self, fd, handler, events):
    fd, obj = self.split_fd(fd)
    self._handlers[fd] = (obj, stack_context.wrap(handler))     #1 <==========
    self._impl.register(fd, events | self.ERROR)

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。

def wrap(fn):
    """Returns a callable object that will restore 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).
    """
    # Check if function is already wrapped
    if fn is None or hasattr(fn, '_wrapped'):
        return fn

    # Capture current stack head
    # TODO: Any other better way to store contexts and update them in wrapped function?
    cap_contexts = [_state.contexts]

    if not cap_contexts[0][0] and not cap_contexts[0][1]:
        # Fast path when there are no active contexts.
        def null_wrapper(*args, **kwargs):
            try:
                current_state = _state.contexts
                _state.contexts = cap_contexts[0]
                return fn(*args, **kwargs)
            finally:
                _state.contexts = current_state
        null_wrapper._wrapped = True
        return null_wrapper

    def wrapped(*args, **kwargs):
        ret = None
        try:
            # Capture old state
            current_state = _state.contexts

            # Remove deactivated items
            cap_contexts[0] = contexts = _remove_deactivated(cap_contexts[0])

            # Force new state
            _state.contexts = contexts

            # Current exception
            exc = (None, None, None)
            top = None

            # Apply stack contexts
            last_ctx = 0
            stack = contexts[0]

            # Apply state
            for n in stack:
                try:
                    n.enter()
                    last_ctx += 1
                except:
                    # Exception happened. Record exception info and store top-most handler
                    exc = sys.exc_info()
                    top = n.old_contexts[1]

            # Execute callback if no exception happened while restoring state
            if top is None:
                try:
                    ret = fn(*args, **kwargs)
                except:
                    exc = sys.exc_info()
                    top = contexts[1]

            # If there was exception, try to handle it by going through the exception chain
            if top is not None:
                exc = _handle_exception(top, exc)
            else:
                # Otherwise take shorter path and run stack contexts in reverse order
                while last_ctx > 0:
                    last_ctx -= 1
                    c = stack[last_ctx]

                    try:
                        c.exit(*exc)
                    except:
                        exc = sys.exc_info()
                        top = c.old_contexts[1]
                        break
                else:
                    top = None

                # If if exception happened while unrolling, take longer exception handler path
                if top is not None:
                    exc = _handle_exception(top, exc)

            # If exception was not handled, raise it
            if exc != (None, None, None):
                raise_exc_info(exc)
        finally:
            _state.contexts = current_state
        return ret

    wrapped._wrapped = True
    return wrapped

stack_context.wrap

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_connection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

3. tornado.ioloop.IOLoop.instance().start()

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，则触发操作

def start(self):
    if self._running:
        raise RuntimeError("IOLoop is already running")
    self._setup_logging()
    if self._stopped:
        self._stopped = False
        return
    old_current = getattr(IOLoop._current, "instance", None)
    IOLoop._current.instance = self
    self._thread_ident = thread.get_ident()
    self._running = True

    # signal.set_wakeup_fd closes a race condition in event loops:
    # a signal may arrive at the beginning of select/poll/etc
    # before it goes into its interruptible sleep, so the signal
    # will be consumed without waking the select.  The solution is
    # for the (C, synchronous) signal handler to write to a pipe,
    # which will then be seen by select.
    #
    # In python's signal handling semantics, this only matters on the
    # main thread (fortunately, set_wakeup_fd only works on the main
    # thread and will raise a ValueError otherwise).
    #
    # If someone has already set a wakeup fd, we don't want to
    # disturb it.  This is an issue for twisted, which does its
    # SIGCHLD processing in response to its own wakeup fd being
    # written to.  As long as the wakeup fd is registered on the IOLoop,
    # the loop will still wake up and everything should work.
    old_wakeup_fd = None
    if hasattr(signal, 'set_wakeup_fd') and os.name == 'posix':
        # requires python 2.6+, unix.  set_wakeup_fd exists but crashes
        # the python process on windows.
        try:
            old_wakeup_fd = signal.set_wakeup_fd(self._waker.write_fileno())
            if old_wakeup_fd != -1:
                # Already set, restore previous value.  This is a little racy,
                # but there's no clean get_wakeup_fd and in real use the
                # IOLoop is just started once at the beginning.
                signal.set_wakeup_fd(old_wakeup_fd)
                old_wakeup_fd = None
        except ValueError:
            # Non-main thread, or the previous value of wakeup_fd
            # is no longer valid.
            old_wakeup_fd = None

    try:
        while True:

            # Prevent IO event starvation by delaying new callbacks
            # to the next iteration of the event loop.
            with self._callback_lock:
                callbacks = self._callbacks
                self._callbacks = []

            # Add any timeouts that have come due to the callback list.
            # Do not run anything until we have determined which ones
            # are ready, so timeouts that call add_timeout cannot
            # schedule anything in this iteration.
            due_timeouts = []
            if self._timeouts:
                now = self.time()
                while self._timeouts:
                    if self._timeouts[0].callback is None:
                        # The timeout was cancelled.  Note that the
                        # cancellation check is repeated below for timeouts
                        # that are cancelled by another timeout or callback.
                        heapq.heappop(self._timeouts)
                        self._cancellations -= 1
                    elif self._timeouts[0].deadline <= now:
                        due_timeouts.append(heapq.heappop(self._timeouts))
                    else:
                        break
                if (self._cancellations > 512 and
                        self._cancellations > (len(self._timeouts) >> 1)):
                    # Clean up the timeout queue when it gets large and it's
                    # more than half cancellations.
                    self._cancellations = 0
                    self._timeouts = [x for x in self._timeouts
                                      if x.callback is not None]
                    heapq.heapify(self._timeouts)

            for callback in callbacks:
                self._run_callback(callback)
            for timeout in due_timeouts:
                if timeout.callback is not None:
                    self._run_callback(timeout.callback)
            # Closures may be holding on to a lot of memory, so allow
            # them to be freed before we go into our poll wait.
            callbacks = callback = due_timeouts = timeout = None

            if self._callbacks:
                # If any callbacks or timeouts called add_callback,
                # we don't want to wait in poll() before we run them.
                poll_timeout = 0.0
            elif self._timeouts:
                # If there are any timeouts, schedule the first one.
                # Use self.time() instead of 'now' to account for time
                # spent running callbacks.
                poll_timeout = self._timeouts[0].deadline - self.time()
                poll_timeout = max(0, min(poll_timeout, _POLL_TIMEOUT))
            else:
                # No timeouts and no callbacks, so use the default.
                poll_timeout = _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 as 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 errno_from_exception(e) == 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 modify self._events
            self._events.update(event_pairs)
            while self._events:
                fd, events = self._events.popitem()
                try:
                    fd_obj, handler_func = self._handlers[fd]
                    handler_func(fd_obj, events)
                except (OSError, IOError) as e:
                    if errno_from_exception(e) == errno.EPIPE:
                        # Happens when the client closes the connection
                        pass
                    else:
                        self.handle_callback_exception(self._handlers.get(fd))
                except Exception:
                    self.handle_callback_exception(self._handlers.get(fd))
            fd_obj = handler_func = None

    finally:
        # 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)
        IOLoop._current.instance = old_current
        if old_wakeup_fd is not None:
            signal.set_wakeup_fd(old_wakeup_fd)

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

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_connection方法和相关上下文的stack_context.wrap(handler)

请求来了

轮询过程中，当有请求到达时，先执行, accept_handler 接收请求地址，调用callback方法,即: _handle_connection()

def add_accept_handler(sock, callback, io_loop=None):
    if io_loop is None:
        io_loop = IOLoop.current()

    def accept_handler(fd, events):

        for i in xrange(_DEFAULT_BACKLOG):
            try:
                connection, address = sock.accept()
            except socket.error as e:
                # _ERRNO_WOULDBLOCK indicate we have accepted every
                # connection that is available.
                if errno_from_exception(e) in _ERRNO_WOULDBLOCK:
                    return
                # ECONNABORTED indicates that there was a connection
                # but it was closed while still in the accept queue.
                # (observed on FreeBSD).
                if errno_from_exception(e) == errno.ECONNABORTED:
                    continue
                raise
            callback(connection, address)      # <=============
    io_loop.add_handler(sock, accept_handler, IOLoop.READ)

创建封装了客户端socket对象和IOLoop对象的IOStream实例（用于之后获取或输出数据）。

def _handle_connection(self, connection, address):
    # ... 省略
    try:
        if self.ssl_options is not None:
            # ...省略
        else:
            stream = IOStream(connection, io_loop=self.io_loop,
                              max_buffer_size=self.max_buffer_size,
                              read_chunk_size=self.read_chunk_size)
        future = self.handle_stream(stream, address)
        if future is not None:
            self.io_loop.add_future(future, lambda f: f.result())
    except Exception:
        app_log.error("Error in connection callback", exc_info=True)

class IOStream(BaseIOStream):
    def __init__(self, socket, *args, **kwargs):
        self.socket = socket
        self.socket.setblocking(False)
        super(IOStream, self).__init__(*args, **kwargs)

IOStream

class BaseIOStream(object):
    """A utility class to write to and read from a non-blocking file or socket.

    We support a non-blocking ``write()`` and a family of ``read_*()`` methods.
    All of the methods take an optional ``callback`` argument and return a
    `.Future` only if no callback is given.  When the operation completes,
    the callback will be run or the `.Future` will resolve with the data
    read (or ``None`` for ``write()``).  All outstanding ``Futures`` will
    resolve with a `StreamClosedError` when the stream is closed; users
    of the callback interface will be notified via
    `.BaseIOStream.set_close_callback` instead.

    When a stream is closed due to an error, the IOStream's ``error``
    attribute contains the exception object.

    Subclasses must implement `fileno`, `close_fd`, `write_to_fd`,
    `read_from_fd`, and optionally `get_fd_error`.
    """
    def __init__(self, io_loop=None, max_buffer_size=None,
                 read_chunk_size=None, max_write_buffer_size=None):
        """`BaseIOStream` constructor.

        :arg io_loop: The `.IOLoop` to use; defaults to `.IOLoop.current`.
                      Deprecated since Tornado 4.1.
        :arg max_buffer_size: Maximum amount of incoming data to buffer;
            defaults to 100MB.
        :arg read_chunk_size: Amount of data to read at one time from the
            underlying transport; defaults to 64KB.
        :arg max_write_buffer_size: Amount of outgoing data to buffer;
            defaults to unlimited.

        .. versionchanged:: 4.0
           Add the ``max_write_buffer_size`` parameter.  Changed default
           ``read_chunk_size`` to 64KB.
        """
        self.io_loop = io_loop or ioloop.IOLoop.current()
        self.max_buffer_size = max_buffer_size or 104857600
        # A chunk size that is too close to max_buffer_size can cause
        # spurious failures.
        self.read_chunk_size = min(read_chunk_size or 65536,
                                   self.max_buffer_size // 2)
        self.max_write_buffer_size = max_write_buffer_size
        self.error = None
        self._read_buffer = collections.deque()
        self._write_buffer = collections.deque()
        self._read_buffer_size = 0
        self._write_buffer_size = 0
        self._write_buffer_frozen = False
        self._read_delimiter = None
        self._read_regex = None
        self._read_max_bytes = None
        self._read_bytes = None
        self._read_partial = False
        self._read_until_close = False
        self._read_callback = None
        self._read_future = None
        self._streaming_callback = None
        self._write_callback = None
        self._write_future = None
        self._close_callback = None
        self._connect_callback = None
        self._connect_future = None
        # _ssl_connect_future should be defined in SSLIOStream
        # but it's here so we can clean it up in maybe_run_close_callback.
        # TODO: refactor that so subclasses can add additional futures
        # to be cancelled.
        self._ssl_connect_future = None
        self._connecting = False
        self._state = None
        self._pending_callbacks = 0
        self._closed = False

BaseIOStream

1.tornado.web.RequestHandler

这是所有业务处理handler需要继承的父类，接下来，介绍一些RequestHandler类中常用的一些方法：

#1 initialize

def __init__(self, application, request, **kwargs):
    super(RequestHandler, self).__init__()

    self.application = application
    self.request = request
    self._headers_written = False
    self._finished = False
    self._auto_finish = True
    self._transforms = None  # will be set in _execute
    self._prepared_future = None
    self._headers = None  # type: httputil.HTTPHeaders
    self.path_args = None
    self.path_kwargs = None
    self.ui = ObjectDict((n, self._ui_method(m)) for n, m in
                         application.ui_methods.items())
    # UIModules are available as both `modules` and `_tt_modules` in the
    # template namespace.  Historically only `modules` was available
    # but could be clobbered by user additions to the namespace.
    # The template {% module %} directive looks in `_tt_modules` to avoid
    # possible conflicts.
    self.ui["_tt_modules"] = _UIModuleNamespace(self,
                                                application.ui_modules)
    self.ui["modules"] = self.ui["_tt_modules"]
    self.clear()
    self.request.connection.set_close_callback(self.on_connection_close)
    self.initialize(**kwargs)

def initialize(self):
    pass

从源码中可以看出initialize函数会在RequestHandler类初始化的时候执行，但是源码中initialize函数并没有做任何事情，

这其实是tornado为我们预留的修改源码的地方，这就允许程序在执行所有的handler前首先执行我们在initialize中定义的方法。

#2 write

def write(self, chunk):
    if self._finished:
        raise RuntimeError("Cannot write() after finish()")
    if not isinstance(chunk, (bytes, unicode_type, dict)):
        message = "write() only accepts bytes, unicode, and dict objects"
        if isinstance(chunk, list):
            message += ". Lists not accepted for security reasons; see http://www.tornadoweb.org/en/stable/web.html#tornado.web.RequestHandler.write"
        raise TypeError(message)
    if isinstance(chunk, dict):
        chunk = escape.json_encode(chunk)
        self.set_header("Content-Type", "application/json; charset=UTF-8")

    chunk = utf8(chunk)
    self._write_buffer.append(chunk)

write方法接收字典和字符串类型的参数，如果用户传来的数据是字典类型，源码中会自动用json对字典进行序列化，最终序列化成字符串。

self._write_buffer是源码中定义的一个临时存放需要输出的字符串的地方，是列表形式。

#3 flush

def flush(self, include_footers=False, callback=None):
    chunk = b"".join(self._write_buffer)
    self._write_buffer = []
    if not self._headers_written:
        self._headers_written = True
        for transform in self._transforms:
            self._status_code, self._headers, chunk = \
                transform.transform_first_chunk(
                    self._status_code, self._headers,
                    chunk, include_footers)

        if self.request.method == "HEAD":
            chunk = None
        if hasattr(self, "_new_cookie"):
            for cookie in self._new_cookie.values():
                self.add_header("Set-Cookie", cookie.OutputString(None))

        start_line = httputil.ResponseStartLine('',
                                                self._status_code,
                                                self._reason)
        return self.request.connection.write_headers(
            start_line, self._headers, chunk, callback=callback)
    else:
        for transform in self._transforms:
            chunk = transform.transform_chunk(chunk, include_footers)

        if self.request.method != "HEAD":
            return self.request.connection.write(chunk, callback=callback)
        else:
            future = Future()
            future.set_result(None)
            return future

flush方法会self._write_buffer列表中的所有元素拼接成字符串，并赋值给chunk，然后清空self._write_buffer列表，然后设置请求头，最终调用request.write方法在前端页面显示。

#4 render

def render(self, template_name, **kwargs):
    """Renders the template with the given arguments as the response."""
    if self._finished:
        raise RuntimeError("Cannot render() after finish()")
    html = self.render_string(template_name, **kwargs)

    # Insert the additional JS and CSS added by the modules on the page
    js_embed = []
    js_files = []
    css_embed = []
    css_files = []
    html_heads = []
    html_bodies = []

由上述源码可看出render方法是根据参数渲染模板：

js和css部分的源码：

for module in getattr(self, "_active_modules", {}).values():
    embed_part = module.embedded_javascript()
    if embed_part:
        js_embed.append(utf8(embed_part))
    file_part = module.javascript_files()
    if file_part:
        if isinstance(file_part, (unicode_type, bytes)):
            js_files.append(file_part)
        else:
            js_files.extend(file_part)
    embed_part = module.embedded_css()
    if embed_part:
        css_embed.append(utf8(embed_part))
    file_part = module.css_files()
    if file_part:
        if isinstance(file_part, (unicode_type, bytes)):
            css_files.append(file_part)
        else:
            css_files.extend(file_part)
    head_part = module.html_head()
    if head_part:
        html_heads.append(utf8(head_part))
    body_part = module.html_body()
    if body_part:
        html_bodies.append(utf8(body_part))

由上述源码可看出，静态文件（以JavaScript为例，css是类似的）的渲染流程是：

首先通过module.embedded_javascript() 获取需要插入JavaScript字符串，添加到js_embed 列表中；

进而通过module.javascript_files()获取已有的列表格式的JavaScript files，最终将它加入js_files.

下面对js_embed和js_files做进一步介绍：

js_embed源码：

if js_embed:
    js = b'<script type="text/javascript">\n//<![CDATA[\n' + \
        b'\n'.join(js_embed) + b'\n//]]>\n</script>'
    sloc = html.rindex(b'</body>')
    html = html[:sloc] + js + b'\n' + html[sloc:]

上图源码即生成script标签，这是一些我们自己定义的一些JavaScript代码；最终是通过字符串拼接方式插入到整个html中。

js_files源码：

if js_files:
    # Maintain order of JavaScript files given by modules
    paths = []
    unique_paths = set()
    for path in js_files:
        if not is_absolute(path):
            path = self.static_url(path)
        if path not in unique_paths:
            paths.append(path)
            unique_paths.add(path)
    js = ''.join('<script src="' + escape.xhtml_escape(p) +
                 '" type="text/javascript"></script>'
                 for p in paths)
    sloc = html.rindex(b'</body>')
    html = html[:sloc] + utf8(js) + b'\n' + html[sloc:]

上述源码即生成script标签，这是一些需要引入的JavaScript代码块；最终是通过字符串拼接方式插入到整个html中。

需要注意的是:其中静态路径是调用self.static_url(path)实现的。

def static_url(self, path, include_host=None, **kwargs):
    self.require_setting("static_path", "static_url")
    get_url = self.settings.get("static_handler_class",
                                StaticFileHandler).make_static_url

    if include_host is None:
        include_host = getattr(self, "include_host", False)

    if include_host:
        base = self.request.protocol + "://" + self.request.host
    else:
        base = ""

    return base + get_url(self.settings, path, **kwargs)

由上述代码可看出：源码首先会判断用户有没有设置静态路径的前缀，然后将静态路径与相对路径进行拼接成绝对路径，

接下来按照绝对路径打开文件，并对文件内容（f.read（））做md5加密，最终将根目录+静态路径前缀+相对路径拼接在前端html中展示。

#5 render_string

1. 创建Loader对象，并执行load方法

　　-- 通过open函数打开html文件并读取内容，并将内容作为参数又创建一个 Template 对象

　　-- 当执行Template的 __init__ 方法时，根据模板语言的标签 {{}}、{%%}等分割并html文件，最后生成一个字符串表示的函数

2. 获取所有要嵌入到html模板中的变量，包括：用户返回和框架默认

3. 执行Template对象的generate方法

　　-- 编译字符串表示的函数，并将用户定义的值和框架默认的值作为全局变量

　　-- 执行被编译的函数获取被嵌套了数据的内容，然后将内容返回（用于响应给请求客户端）

def render_string(self, template_name, **kwargs):
    template_path = self.get_template_path()
    if not template_path:
        frame = sys._getframe(0)
        web_file = frame.f_code.co_filename
        while frame.f_code.co_filename == web_file:
            frame = frame.f_back
        template_path = os.path.dirname(frame.f_code.co_filename)
    with RequestHandler._template_loader_lock:
        if template_path not in RequestHandler._template_loaders:
            loader = self.create_template_loader(template_path)
            RequestHandler._template_loaders[template_path] = loader
        else:
            loader = RequestHandler._template_loaders[template_path]
    t = loader.load(template_name)
    namespace = self.get_template_namespace()
    namespace.update(kwargs)
    return t.generate(**namespace)

示例html：

源码模板语言处理部分的截图：