Django中模块的加载原理

Django中的module的加载是通过反射来完成的，借助importlib中的import_module函数来实现的动态加载。import_module的内部通过使用了递归和线程锁，字符串的切割，实现模块的加载。

主要有以下几个步骤：

• 根据字符串的形式导入模块
• 去模块中找到相应的类
• 根据相应的类实例化
• 执行对象.方法

根据字符串的形式导入模块

假设在有以下的类跟方法：

path = "handler.basic.BaseHandler"

func_name = "process_request"

导入目标函数：

import importlib
 
# 通过切割拿到模块和类名
module_path,cls_name = path.rsplit('.', 1)
# 导入模块
module = importlib.import_module(module_path)
print(module)

去模块中找到相应的类

通过反射拿到类。

cls =  getattr(module, cls)

根据相应的类实例化

实例化对象：

obj = cls()

执行对象方法

通过反射拿到方法，

func = getattr(obj, func_name)
fuc()

Django中间件源码

    def load_middleware(self):
        """
        Populate middleware lists from settings.MIDDLEWARE (or the deprecated
        MIDDLEWARE_CLASSES).
 
        Must be called after the environment is fixed (see __call__ in subclasses).
        """
        self._request_middleware = []
        self._view_middleware = []
        self._template_response_middleware = []
        self._response_middleware = []
        self._exception_middleware = []
 
        if settings.MIDDLEWARE is None:
            warnings.warn(
                "Old-style middleware using settings.MIDDLEWARE_CLASSES is "
                "deprecated. Update your middleware and use settings.MIDDLEWARE "
                "instead.", RemovedInDjango20Warning
            )
            handler = convert_exception_to_response(self._legacy_get_response)
            for middleware_path in settings.MIDDLEWARE_CLASSES:
                mw_class = import_string(middleware_path)
                try:
                    mw_instance = mw_class()
                except MiddlewareNotUsed as exc:
                    if settings.DEBUG:
                        if six.text_type(exc):
                            logger.debug('MiddlewareNotUsed(%r): %s', middleware_path, exc)
                        else:
                            logger.debug('MiddlewareNotUsed: %r', middleware_path)
                    continue
 
                if hasattr(mw_instance, 'process_request'):
                    self._request_middleware.append(mw_instance.process_request)
                if hasattr(mw_instance, 'process_view'):
                    self._view_middleware.append(mw_instance.process_view)
                if hasattr(mw_instance, 'process_template_response'):
                    self._template_response_middleware.insert(0, mw_instance.process_template_response)
                if hasattr(mw_instance, 'process_response'):
                    self._response_middleware.insert(0, mw_instance.process_response)
                if hasattr(mw_instance, 'process_exception'):
                    self._exception_middleware.insert(0, mw_instance.process_exception)
        else:
            handler = convert_exception_to_response(self._get_response)
            for middleware_path in reversed(settings.MIDDLEWARE):
                # 从setting中的中间件的配置，切割拿到导入的模块和类名，通过反射拿到对应的类
                middleware = import_string(middleware_path)
                try:
                    # 中间件实例化对象，
                    mw_instance = middleware(handler)
                except MiddlewareNotUsed as exc:
                    if settings.DEBUG:
                        if six.text_type(exc):
                            logger.debug('MiddlewareNotUsed(%r): %s', middleware_path, exc)
                        else:
                            logger.debug('MiddlewareNotUsed: %r', middleware_path)
                    continue
 
                if mw_instance is None:
                    raise ImproperlyConfigured(
                        'Middleware factory %s returned None.' % middleware_path
                    )
 
                if hasattr(mw_instance, 'process_view'):
                    self._view_middleware.insert(0, mw_instance.process_view)
                if hasattr(mw_instance, 'process_template_response'):
                    self._template_response_middleware.append(mw_instance.process_template_response)
                if hasattr(mw_instance, 'process_exception'):
                    self._exception_middleware.append(mw_instance.process_exception)
 
                handler = convert_exception_to_response(mw_instance)
 
        # We only assign to this when initialization is complete as it is used
        # as a flag for initialization being complete.
        self._middleware_chain = handler
 
##############################################################################
 
def import_string(dotted_path):
    """
    Import a dotted module path and return the attribute/class designated by the
    last name in the path. Raise ImportError if the import failed.
    """
    try:
        # module_path: django.middleware.csrf
        # class_name: CsrfViewMiddleware
        module_path, class_name = dotted_path.rsplit('.', 1)
    except ValueError:
        msg = "%s doesn't look like a module path" % dotted_path
        six.reraise(ImportError, ImportError(msg), sys.exc_info()[2])
 
    # module_path: django.middleware.csrf
    # 通过module_path导入模块
    module = import_module(module_path)
 
    try:
        # 拿到当前类的类名
        return getattr(module, class_name)
    except AttributeError:
        msg = 'Module "%s" does not define a "%s" attribute/class' % (
            module_path, class_name)
        six.reraise(ImportError, ImportError(msg), sys.exc_info()[2])
 
#####################################################################
 
def import_module(name, package=None):
    """Import a module.
 
    The 'package' argument is required when performing a relative import. It
    specifies the package to use as the anchor point from which to resolve the
    relative import to an absolute import.
 
    """
    level = 0
    if name.startswith('.'):
        if not package:
            msg = ("the 'package' argument is required to perform a relative "
                   "import for {!r}")
            raise TypeError(msg.format(name))
        for character in name:
            if character != '.':
                break
            level += 1
    # _gcd_import中使用了递归和线程锁，字符串的切割，实现模块的加载。
    return _bootstrap._gcd_import(name[level:], package, level)