jQuery1.9.1源码分析--Ajax模块

//Serialize an array of form elements or a set of
    //key/values into a query string
    // 将数组形式的表单元素或者哈希表序列化成字符串
    jQuery.param = function(a, traditional) {
        var prefix, s = [],
            add = function(key, value) {
                // If value is a function, invoke it and return its value
                // 如果value是函数就执行并返回执行结果
                value = jQuery.isFunction(value) ? value() : (value == null ? '' : value);
                s[s.length] = encodeURIComponent(key) + '=' +
                    encodeURIComponent(value);
            };

        // Set traditional to true for jQuery <= 1.3.2 behavior
        if (traditional === undefined) {
            traditional = jQuery.ajaxSettings && jQuery.ajaxSettings.traditional;
        }

        // If an array was passed in, assume that it is an array of form elements.
        // 如果传进来的是数组，假设是表单元素
        if (jQuery.isArray(a) || (a.jquery && !jQuery.isPlainObject(a))) {
            // 序列化表单元素
            jQuery.each(a, function() {
                add(this.name, this.value);
            });

        } else {
            // If traditional, encode the "old" way (the way 1.3.2 or older
            // did it), otherwise encode params recursively.
            for (prefix in a) {
                buildParams(prefix, a[prefix], traditional, add);
            }
        }

        // Return the resulting serialization
        return s.join('&').replace(r20, '+');
    };

    function buildParams(prefix, obj, traditional, add) {
        var name;

        if (jQuery.isArray(obj)) {
            // Serialize array item.
            jQuery.each(obj, function(i, v) {
                if (traditional || rbracket.test(prefix)) {
                    // Treat each array item as a scalar.
                    add(prefix, v);

                } else {
                    // Item is non-scalar (array or object), encode its numeric index
                    buildParams(prefix + '[' + (typeof v === 'object' ? i : '') + ']', v, traditional, add);
                }
            });

        } else if (!traditional && jQuery.type(obj) === 'object') {
            // Serialize object item
            for (name in obj) {
                buildParams(prefix + '[' + name + ']', obj[name], traditional, add);
            }

        } else {
            // Serialize scalar item
            add(prefix, obj);
        }
    }

    

    var
    // Document location
    ajaxLocParts,
        ajaxLocation,
        ajax_nonce = jQuery.now(),

        // 匹配“？”
        ajax_rquery = /\?/,
        // 匹配hash，“#”开头的字符串
        rhash = /#.*$/,
        // 匹配“_=”开头到“&”结尾的字符串
        rts = /([?&])_=[^&]*/,
        // 匹配头部信息，获取headerName，和headerValue
        rheaders = /^(.*?):[ \t]*([^\r\n]*)\r?$/mg,
        // IE leaves an \r character at EOL
        // #7653, #8125, #8152: local protocol detection
        // 匹配协议， 可以判断是否为本地协议
        rlocalProtocol = /^(?:about|app|app-storage|.+-extension|file|res|widget):$/,
        // 匹配请求是否有内容
        rnoContent = /^(?:GET|HEAD)$/,
        // 匹配“//”开头的字符
        rprotocol = /^\/\//,
        // 匹配url，例如匹配http://www.baidu.com:8080
        // 将会获取"http:", "www.baidu.com", "8080"
        rurl = /^([\w.+-]+:)(?:\/\/([^\/?#:]*)(?::(\d+)|)|)/,

        // Keep a copy of the old load method
        // jQuery.fn.load旧版本的方法的拷贝
        _load = jQuery.fn.load,

        /* Prefilters
         * 1) They are useful to introduce custom dataTypes (see ajax/jsonp.js for an example)
         * 2) These are called:
         *    - BEFORE asking for a transport
         *    - AFTER param serialization (s.data is a string if s.processData is true)
         * 3) key is the dataType
         * 4) the catchall symbol "*" can be used
         * 5) execution will start with transport dataType and THEN continue down to "*" if needed
         */
        // 前置过滤器
        prefilters = {},

        /* Transports bindings
         * 1) key is the dataType
         * 2) the catchall symbol "*" can be used
         * 3) selection will start with transport dataType and THEN go to "*" if needed
         */
        // 请求分发器
        transports = {},

        // Avoid comment-prolog char sequence (#10098); must appease lint and evade compression
        // "*/*"
        allTypes = "*/".concat("*");

    // #8138， IE may throw an exception when accessing
    // a field from window.location if document.domain has been set
    // 在IE中，如果document.domain被设置了，获取window.location会报错
    try {
        ajaxLocation = location.href;
    } catch (e) {
        // Use the href attribute of an A element
        // since IE will modify it given document.location
        // 因为IE会修改A标签元素的href属性，添加window.location字符串
        ajaxLocation = document.createElement('a');
        ajaxLocation.href = '';
        ajaxLocation = ajaxLocation.href;
    }

    // Segment location into parts
    // [URL， http, host, port]
    ajaxLocParts = rurl.exec(ajaxLocation.toLowerCase()) || [];

    // Base "constructor" for jQuery.ajaxPrefilter and jQuery.ajaxTransport
    // 返回一个函数，为prefilters或者transport添加属性,
    // 该属性值是一个数组，里面存放的是函数func，
    // 可以给dataTypeExpression字符串添加标识，表示是添加到数组头部还是尾部

    function addToPrefiltersOrTransports(structure) {

        // dataTypeExpression is optional and defaults to "*"
        return function(dataTypeExpression, func) {
            // 如果dataTypeExpression不是字符串，将它赋值给func
            // 然后把"*"赋值给dataTypeExpression
            if (typeof dataTypeExpression !== 'string') {
                func = dataTypeExpression;
                dataTypeExpression = '*';
            }

            var dataType, i = 0,
                // 返回空格分隔的dataTypeExpression数组
                dataTypes = dataTypeExpression.toLowerCase().match(core_rnotwhite) || [];

            if (jQuery.isFunction(func)) {
                // For each detaType in the dataTypeExpression
                // 遍历dataTypes数组
                while ((dataType = dataTypes[i++])) {
                    // Prepend if requested
                    // 如果第一个字符是“+”，截取“+”后面的字符串或者
                    // 默认为“*”，即匹配所有，
                    // 给structure的属性dataType数组头部添加func
                    if (dataType[0] === '+') {
                        dataType = dataType.slice(1) || '*';
                        (structure[dataType] = structure[dataType] || []).unshift(func);

                        // Otherwise append
                        // 否则第一个字符不是“*”就给structure的属性dataType数组
                        // 尾部添加func
                    } else {
                        (structure[dataType] = structure[dataType] || []).push(func);
                    }
                }
            }
        };
    }

    // Base inspection function for prefilters and transports

    function inspectPrefiltersOrTransports(structure, options, originalOptions, jqXHR) {

        var inspected = {},
            seekingTransport = (structure === transports);

        // 遍历structure[dataType]数组，并执行回调，
        // prefilterOrFactory为函数数组元素，
        // 执行该函数如果返回的结果dataTypeOrTransport是字符串且时prefilters且没有被inspected过，
        // 就给options.dataTypes数组头部添加该字符串，
        // 继续递归dataTypeOrTransport(当我们使用json/jsonp的时候会返回“script”，于是会执行“script”相关的回调)。
        // 如果是transport就返回dataTypeOrTransport的假结果

        function inspect(dataType) {
            var selected;
            inspected[dataType] = true;
            jQuery.each(structure[dataType] || [], function(_, prefilterOrFactory) {
                var dataTypeOrTransport = prefilterOrFactory(options, originalOptions, jqXHR);
                if (typeof dataTypeOrTransport === "string" && !seekingTransport && !inspected[dataTypeOrTransport]) {
                    options.dataTypes.unshift(dataTypeOrTransport);
                    inspect(dataTypeOrTransport);
                    return false;
                } else if (seekingTransport) {
                    // 如果是查找transport，selected是一个对象，
                    // !selected返回的是false，表示退出each循环
                    return !(selected = dataTypeOrTransport);
                }
            });
            return selected;
        }

        return inspect(options.dataTypes[0]) || !inspected["*"] && inspect("*");
    }

    // A special extend for ajax options
    // that takes "flats" options (not to be deep extended)
    // 对ajax配置项进行扩展
    // 如果jQuery.ajaxSettings.flatOptions存在src对应的key值，
    // 就直接给target添加（覆盖）相应key/value，
    // flatOptions对象里的属性是不需要被深度拷贝的
    // 否则创建一个deep对象，将src的key/value添加给deep，
    // 然后深度克隆deep对象到target.
    // 最后都返回target

    function ajaxExtend(target, src) {
        var deep, key, flatOptions = jQuery.ajaxSettings.flatOptions || {};

        for (key in src) {
            if (src[key] !== undefined) {
                // 如果jQuery.ajaxSettings.flatOptions存在src对应的key值，
                // 就直接给target添加（覆盖）相应key/value，
                // 否则创建一个deep对象，将src的key/value添加给deep
                (flatOptions[key] ? target : (deep || (deep = {})))[key] = src[key];
            }
        }
        // 深度克隆deep对象到target
        if (deep) {
            jQuery.extend(true, target, deep);
        }

        return target;
    }

    jQuery.fn.load = function(url, params, callback) {
        // 早期版本的jQuery.fn.load的接口
        if (typeof url !== 'string' && _load) {
            return _load.apply(this, arguments);
        }

        var selector, response, type, self = this,
            off = url.indexOf(' ');

        // 如果url有空格，空格前面是url，后面是选择器selector
        if (off >= 0) {
            selector = url.slice(off, url.length);
            url = url.slice(0, off);
        }

        // If it's a function
        // load(url, function(){})
        if (jQuery.isFunction(params)) {

            // We assume that it's the callback
            callback = params;
            params = undefined;

            // Otherwise, build a param string
            // 否则如果param是对象，则把ajax类型type设置为“POST”，
            // 说明是发送数据到服务器
        } else if (params && typeof params === 'object') {
            type = 'POST';
        }

        // If we have elements to modify, make the request
        // 必须要有元素集
        if (self.length) {
            // 调用底层ajax方法， 其中用了Deferred对象
            jQuery.ajax({
                url: url,

                // If "type" variable is undefined, then "GET" method will be used
                type: type,
                dataType: 'html',
                data: params
            }).done(function(responseText) {
                // 请求成功后
                // Save response for use in complete callback
                response = arguments;

                self.html(selector ?

                    // If a selector was specified, locate the right elements in a dummy div
                    // Exclude scripts to avoid IE 'Permission Denied' errors
                    // 如果有选择器，则创建一个div节点，
                    // 将返回的数据解析成HTML，然后在该HTML下找到选择器匹配的部分，
                    // 填充到div中
                    jQuery('<div>').append(jQuery.parseHTML(responseText)).find(selector) :

                    // Otherwise use the full result
                    // 否则直接填充
                    responseText);

            }).complete(callback && function(jqXHR, status) {
                // 请求完成后
                // 遍历每个DOM元素，执行callback， 第二个参数是callback的参数
                self.each(callback, response || [jqXHR.responseText, status, jqXHR]);
            });
        }

        return this;
    };

    // Attach a bunch of functions for handling common AJAX events
    // 添加一些AJAX事件方法，这里用的是观察者模式，
    // jQuery.fn.on方法订阅事件，
    // jQuery.fn.trigger方法则可以发布事件
    jQuery.each(["ajaxStart", "ajaxStop", "ajaxComplete", "ajaxError", "ajaxSuccess", "ajaxSend"], function(i, type) {
        jQuery.fn[type] = function(fn) {
            return this.on(type, fn);
        };
    });

    // 添加jQuery.get和jQuery.post方法, 区别在于ajax的类型是get还是post
    jQuery.each(['get', 'post'], function(i, method) {
        jQuery[method] = function(url, data, callback, type) {
            // shift arguments if data argument was omitted
            if (jQuery.jsFunction(data)) {
                type = type || callback;
                callback = data;
                data = undefined;
            }

            return jQuery.ajax({
                url: url,
                type: method,
                dataType: type,
                data: data,
                success: callback
            });
        };
    });

    jQuery.extend({
        // Counter for holding the number of active queries
        active: 0,
        // Last-Modified header cache for next request
        // 上一次被修改的头部的缓存信息
        lastModified: {},
        etag: {},
        // ajax配置项
        ajaxSettings: {
            // 个用来包含发送请求的URL字符串。
            url: ajaxLocation,
            /**
             * (默认: "GET") 请求方式 ("POST" 或 "GET")， 默认为 "GET"。注意：其它 HTTP 请求方法，如 PUT 和 DELETE 也可以使用，但仅部分浏览器支持。
             * @type {String}
             */
            type: "GET",
            /**
             * 是否为本地
             * 默认: 取决于当前的位置协议
             * 允许当前环境被认定为“本地”，（如文件系统），即使jQuery默认情况下不会承认它。以下协议目前公认为本地：file, *-extension, and widget。如果isLocal设置需要修改，建议在$.ajaxSetup()方法中这样做一次。
             * @type {Boolean}
             */
            isLocal: rlocalProtocol.test(ajaxLocParts[1]),
            /**
             * (默认: true) 是否触发全局 AJAX 事件。设置为 false 将不会触发全局 AJAX 事件，如 ajaxStart 或 ajaxStop 可用于控制不同的 Ajax 事件。
             * @type {Boolean}
             */
            global: true,
            /**
             * (默认: true) 默认情况下，通过data选项传递进来的数据，如果是一个对象(技术上讲只要不是字符串)，都会处理转化成一个查询字符串，以配合默认内容类型 "application/x-www-form-urlencoded"。如果要发送 DOM 树信息或其它不希望转换的信息，请设置为 false。
             * @type {Boolean}
             */
            processData: true,
            /*
             (默认: true) 默认设置下，所有请求均为异步请求。如果需要发送同步请求，请将此选项设置为 false。注意，同步请求将锁住浏览器，用户其它操作必须等待请求完成才可以执行。
             */
            async: true,
            /**
             * (默认: "application/x-www-form-urlencoded") 发送信息至服务器时内容编码类型。默认值适合大多数情况。如果你明确地传递了一个content-type给 $.ajax() 那么他必定会发送给服务器（即使没有数据要发送）
             * @type {String}
             */
            contentType: "application/x-www-form-urlencoded; charset=UTF-8",

            /*
             // 设置请求超时时间（毫秒）。此设置将覆盖全局设置。
             timeout: 0,
             data: null,
             dataType: null,
             // 用于响应HTTP访问认证请求的用户名
             username: null,
             // 用于响应HTTP访问认证请求的密码
             password: null,
             // (默认: true,dataType为script和jsonp时默认为false)，设置为 false 将不缓存此页面。
             cache: null,
             throws: false,
             // 如果你想要用传统的方式来序列化数据，那么就设置为true
             traditional: false,
             // 个额外的"{键:值}"对映射到请求一起发送。此设置被设置之前beforeSend函数被调用;因此，消息头中的值设置可以在覆盖beforeSend函数范围内的任何设置。
             headers: {},
             */
            /**
             * 接受的数据的type类型
             * 内容类型发送请求头，告诉服务器什么样的响应会接受返回。如果accepts设置需要修改，推荐在$.ajaxSetup()方法中做一次。
             * @type {Object}
             */
            accepts: {
                "*": allTypes,
                text: "text/plain",
                html: "text/html",
                xml: "application/xml, text/xml",
                json: "application/json, text/javascript"
            },
            /**
             * 一个以"{字符串:正则表达式}"配对的对象，用来确定jQuery将如何解析响应，给定其内容类型。
             * @type {Object}
             */
            contents: {
                xml: /xml/,
                html: /html/,
                json: /json/
            },
            responseFields: {
                xml: "responseXML",
                text: "responseText"
            },
            // Data converters
            // Keys separate source (or catchall "*") and destination types with a single space
            /**
             * 数据转换器
             * 一个数据类型对数据类型转换器的对象。每个转换器的值是一个函数，返回响应的转化值
             * @type {Object}
             */
            converters: {
                // Convert anything to text
                "* text": window.String,
                // Text to html (true = no transformation)
                // true为不转换
                "text html": true,
                // Evaluate text as a json expression
                "text json": jQuery.parseJSON,
                // Parse text as xml
                "text xml": jQuery.parseXML
            },
            // For options that shouldn't be deep extended:
            // you can add your own custom options here if
            // and when you create one that shouldn't be
            // deep extended (see ajaxExtend)
            // 不会被深度拷贝的配置项
            flatOptions: {
                url: true,
                context: true
            }
        },
        // Creates a full fledged settings object into target
        // with both ajaxSettings and settings fields.
        // If target is omitted, writes into ajaxSettings.
        // 创建更健壮的配置项到target中，包含了ajaxSettings和settings参数
        // 如果只有一个参数，直接添加到jQuery.ajaxSettings中
        ajaxSetup: function(target, settings) {
            return settings ?
            // Building a settings object
            ajaxExtend(ajaxExtend(target, jQuery.ajaxSettings), settings) :
            // Extending ajaxSettings
            ajaxExtend(jQuery.ajaxSettings, target);
        },

        ajaxPrefilter: addToPrefiltersOrTransports(prefilters),
        ajaxTransport: addToPrefiltersOrTransports(transports),

        // Main method
        ajax: function(url, options) {
            // If url is an object, simulate pre-1.5 signature
            if (typeof url === "object") {
                options = url;
                url = undefined;
            }

            // Force options to be an object
            options = options || {};

            var // Cross-domain detection vars
            // 跨域检测变量
            parts,
                // Loop variable
                i,
                // URL without anti-cache param
                // 没有破坏缓存参数的url，即不会
                cacheURL,
                // Response headers as string
                responseHeadersString,
                // timeout handle
                // 计时器
                timeoutTimer,

                // To know if global events are to be dispatched
                // 全局事件是否该被触发
                fireGlobals,

                transport,
                // Response headers
                responseHeaders,
                // Create the final options object
                // 最终的ajax配置项
                s = jQuery.ajaxSetup({}, options),
                // Callbacks context
                // 回调函数的上下文
                callbackContext = s.context || s,
                // Context for global events is callbackContext if it is a DOM node or jQuery collection
                // 如果配置项有context则用jQuery对象，否则使用jQuery.event对象
                globalEventContext = s.context && (callbackContext.nodeType || callbackContext.jquery) ? jQuery(callbackContext) : jQuery.event,
                // Deferreds
                // 创建一个延迟对象
                deferred = jQuery.Deferred(),
                // 完成延迟的回调列表
                completeDeferred = jQuery.Callbacks("once memory"),
                // Status-dependent callbacks
                // 状态码， 根据status来决定回调
                statusCode = s.statusCode || {},
                // Headers (they are sent all at once)
                requestHeaders = {},
                requestHeadersNames = {},
                // The jqXHR state
                // jqXHR的状态
                state = 0,
                // Default abort message
                // 默认退出信息
                strAbort = "canceled",
                // Fake xhr
                // 伪装的xhr对象
                jqXHR = {
                    readyState: 0,
                    // Builds headers hashtable if needed
                    // 如果需要就创建头部哈希表
                    getResponseHeader: function(key) {
                        var match;
                        // 如果jqXHR的状态为2
                        if (state === 2) {
                            if (!responseHeaders) {
                                responseHeaders = {};
                                // 逐个获取已有的头部信息responseHeadersString的key和value值，
                                // 给responseHeaders对象添加key属性和相应的value
                                while ((match = rheaders.exec(responseHeadersString))) {
                                    responseHeaders[match[1].toLowerCase()] = match[2];
                                }
                            }
                            // 给responseHeaders添加参数key的属性
                            match = responseHeaders[key.toLowerCase()];
                        }
                        // 返回responseHeaders
                        return match == null ? null : match;
                    },

                    // Raw string
                    getAllResponseHeaders: function() {
                        return state === 2 ? responseHeadersString : null;
                    },

                    // Caches the header
                    // 缓存头部
                    setRequestHeader: function(name, value) {
                        var lname = name.toLowerCase();
                        // 如果jqXHR的state小于等于0，
                        // 获取requestHeadersNames的name属性值（没有该属性就添加），
                        // 添加requestHeaders的name属性和对应的value值
                        if (!state) {
                            name = requestHeadersNames[lname] = requestHeadersNames[lname] || name;
                            requestHeaders[name] = value;
                        }
                        return this;
                    },

                    // Overrides response content-type header
                    // 重写响应的mimeType
                    overrideMimeType: function(type) {
                        if (!state) {
                            s.mimeType = type;
                        }
                        return this;
                    },

                    // Status-dependent callbacks
                    statusCode: function(map) {
                        var code;
                        // 如果存在map对象
                        if (map) {
                            // 如果jqXHR的状态小于2，
                            // 说明没有完成
                            if (state < 2) {
                                // 遍历map
                                for (code in map) {
                                    // Lazy-add the new callback in a way that preserves old ones
                                    // 给statusCode对象添加code属性，值为数组，
                                    // 第一个元素存放着旧的值
                                    statusCode[code] = [statusCode[code], map[code]];
                                }

                                // 如果jqXHR大于等于2
                            } else {
                                // Execute the appropriate callbacks
                                // 执行相应的回调
                                jqXHR.always(map[jqXHR.status]);
                            }
                        }
                        return this;
                    },
                    // Cancel the request
                    // 取消请求
                    abort: function(statusText) {
                        var finalText = statusText || strAbort;
                        if (transport) {
                            transport.abort(finalText);
                        }
                        done(0, finalText);
                        return this;
                    }
                };

            // Attach deferreds
            // 给jqXHR添加promise的属性和方法，
            // 然后添加complete方法，这里用的是回调列表的add方法（即添加回调）
            // 订阅完成回调
            deferred.promise(jqXHR).complete = completeDeferred.add;
            // success/error 方法则是使用promise的done/fail方法
            jqXHR.success = jqXHR.done;
            jqXHR.error = jqXHR.fail;

            // Remove hash character (#7531: and string promotion)
            // Add protocol if not provided (#5866: IE7 issue with protocol-less urls)
            // Handle falsy url in the settings object (#10093: consistency with old signature)
            // We also use the url parameter if available
            s.url = ((url || s.url || ajaxLocation) + "").replace(rhash, "").replace(rprotocol, ajaxLocParts[1] + "//");

            // Alias method option to type as per ticket #12004
            s.type = options.method || options.type || s.method || s.type;

            // Extract dataTypes list
            // dataTypes列表
            s.dataTypes = jQuery.trim(s.dataType || "*").toLowerCase().match(core_rnotwhite) || [""];

            // A cross-domain request is in order when we have a protocol:host:port mismatch
            // 检测是否需要跨域请求
            if (s.crossDomain == null) {
                parts = rurl.exec(s.url.toLowerCase());
                s.crossDomain = !! (parts && (parts[1] !== ajaxLocParts[1] || parts[2] !== ajaxLocParts[2] || (parts[3] || (parts[1] === "http:" ? 80 : 443)) != (ajaxLocParts[3] || (ajaxLocParts[1] === "http:" ? 80 : 443))));
            }

            // Convert data if not already a string
            // 如果data不是字符串则转换为字符串形式“a=1&b=2&c=3”
            if (s.data && s.processData && typeof s.data !== "string") {
                s.data = jQuery.param(s.data, s.traditional);
            }

            // Apply prefilters
            // 遍历prefilters[dataType]数组，并执行回调，
            // prefilterOrFactory为函数数组元素，
            // 执行该函数如果返回的结果dataTypeOrTransport是字符串且时prefilters且没有被inspected过，
            // 就给options.dataTypes数组头部添加该字符串，
            // 继续递归dataTypeOrTransport(当我们使用json/jsonp的时候会返回“script”，于是会执行“script”相关的回调)。
            inspectPrefiltersOrTransports(prefilters, s, options, jqXHR);

            // If request was aborted inside a prefilter, stop there
            if (state === 2) {
                return jqXHR;
            }

            // We can fire global events as of now if asked to
            // 是否触发全局事件
            fireGlobals = s.global;

            // Watch for a new set of requests
            if (fireGlobals && jQuery.active++ === 0) {
                jQuery.event.trigger("ajaxStart");
            }

            // Uppercase the type
            // 将type转换为大写
            s.type = s.type.toUpperCase();

            // Determine if request has content
            // 请求是否有内容
            s.hasContent = !rnoContent.test(s.type);

            // Save the URL in case we're toying with the If-Modified-Since
            // and/or If-None-Match header later on
            // 保存url
            cacheURL = s.url;

            // More options handling for requests with no content
            if (!s.hasContent) {

                // If data is available, append data to url
                // 如果有data，将data字符串加入到url
                if (s.data) {
                    cacheURL = (s.url += (ajax_rquery.test(cacheURL) ? "&" : "?") + s.data);
                    // #9682: remove data so that it's not used in an eventual retry
                    // 删除data属性，防止被重用
                    delete s.data;
                }

                // Add anti-cache in url if needed
                // 如果不需要缓存，则添加破坏缓存时间戳
                if (s.cache === false) {
                    s.url = rts.test(cacheURL) ?
                    // If there is already a '_' parameter, set its value
                    // 如果已经存在_字段，则修改它的值
                    cacheURL.replace(rts, "$1_=" + ajax_nonce++) :
                    // Otherwise add one to the end
                    // 否则就在尾部添加
                    cacheURL + (ajax_rquery.test(cacheURL) ? "&" : "?") + "_=" + ajax_nonce++;
                }
            }

            // Set the If-Modified-Since and/or If-None-Match header, if in ifModified mode.
            /*
             (默认: false) 仅在服务器数据改变时获取新数据。使用 HTTP 包 Last-Modified 头信息判断。他也会检查服务器指定的'etag'来确定数据没有被修改过。
             */
            if (s.ifModified) {
                if (jQuery.lastModified[cacheURL]) {
                    jqXHR.setRequestHeader("If-Modified-Since", jQuery.lastModified[cacheURL]);
                }
                if (jQuery.etag[cacheURL]) {
                    jqXHR.setRequestHeader("If-None-Match", jQuery.etag[cacheURL]);
                }
            }

            // Set the correct header, if data is being sent
            // 设置正确的头信息
            if (s.data && s.hasContent && s.contentType !== false || options.contentType) {
                jqXHR.setRequestHeader("Content-Type", s.contentType);
            }

            // Set the Accepts header for the server, depending on the dataType
            // 为服务端添加Accepts头，取决于dataType，
            // 例如dataType为“html”，则value值为"text/html, */*; q=0.01"， s.accepts['html'],
            // 即"text/html"
            // 没有dataType就用“*/*”
            jqXHR.setRequestHeader("Accept", s.dataTypes[0] && s.accepts[s.dataTypes[0]] ? s.accepts[s.dataTypes[0]] + (s.dataTypes[0] !== "*" ? ", " + allTypes + "; q=0.01" : "") : s.accepts["*"]);

            // Check for headers option
            // 添加配置项headers的头部请求
            for (i in s.headers) {
                jqXHR.setRequestHeader(i, s.headers[i]);
            }

            // Allow custom headers/mimetypes and early abort
            // 执行beforeSend方法，如果该函数返回false则推出
            if (s.beforeSend && (s.beforeSend.call(callbackContext, jqXHR, s) === false || state === 2)) {
                // Abort if not done already and return
                // canceled
                return jqXHR.abort();
            }

            // aborting is no longer a cancellation
            strAbort = "abort";

            // Install callbacks on deferreds
            // 给回调列表添加回调
            for (i in {
                success: 1,
                error: 1,
                complete: 1
            }) {
                jqXHR[i](s[i]);
            }

            // Get transport
            transport = inspectPrefiltersOrTransports(transports, s, options, jqXHR);

            // If no transport, we auto-abort
            if (!transport) {
                done(-1, "No Transport");
            } else {
                // 请求开始
                jqXHR.readyState = 1;

                // Send global event
                // 发送全局事件ajaxSend
                if (fireGlobals) {
                    globalEventContext.trigger("ajaxSend", [jqXHR, s]);
                }
                // Timeout
                // 如果是异步且配置项中timeout有值，
                // 则设置定时器， 当定时期结束时就会执行abort方法
                if (s.async && s.timeout > 0) {
                    timeoutTimer = setTimeout(function() {
                        jqXHR.abort("timeout");
                    }, s.timeout);
                }

                try {
                    // 设置jqXHR的状态为1
                    state = 1;
                    // 创建并发送请求，这里才开始调用原生方法
                    transport.send(requestHeaders, done);
                } catch (e) {
                    // Propagate exception as error if not done
                    // 如果未完成则当错误处理
                    if (state < 2) {
                        done(-1, e);
                        // Simply rethrow otherwise
                    } else {
                        throw e;
                    }
                }
            }

            // Callback for when everything is done

            function done(status, nativeStatusText, responses, headers) {
                var isSuccess, success, error, response, modified, statusText = nativeStatusText;

                // Called once
                // 只执行一次
                if (state === 2) {
                    return;
                }

                // State is "done" now
                // jqXHR的状态设置为2
                state = 2;

                // Clear timeout if it exists
                // 清除定时器
                if (timeoutTimer) {
                    clearTimeout(timeoutTimer);
                }

                // Dereference transport for early garbage collection
                // (no matter how long the jqXHR object will be used)
                // 取消引用
                transport = undefined;

                // Cache response headers
                // 缓存响应头
                responseHeadersString = headers || "";

                // Set readyState
                // 设置readyState， 根据state来判断
                jqXHR.readyState = status > 0 ? 4 : 0;

                // Get response data
                // 获取响应的数据， 这里会使用ajaxHandleResponses来处理响应内容
                if (responses) {
                    response = ajaxHandleResponses(s, jqXHR, responses);
                }

                // If successful, handle type chaining
                // 请求成功时
                if (status >= 200 && status < 300 || status === 304) {

                    // Set the If-Modified-Since and/or If-None-Match header, if in ifModified mode.
                    // 如果处于ifModified模式， 设置If-Modified-Since和/或者If-None-Match header
                    if (s.ifModified) {
                        modified = jqXHR.getResponseHeader("Last-Modified");
                        if (modified) {
                            jQuery.lastModified[cacheURL] = modified;
                        }
                        modified = jqXHR.getResponseHeader("etag");
                        if (modified) {
                            jQuery.etag[cacheURL] = modified;
                        }
                    }

                    // if no content
                    // 如果没有内容
                    if (status === 204) {
                        isSuccess = true;
                        statusText = "nocontent";

                        // if not modified
                        // 如果没有被修改
                    } else if (status === 304) {
                        isSuccess = true;
                        statusText = "notmodified";

                        // If we have data, let's convert it
                        // 如果有数据就转换它
                    } else {
                        isSuccess = ajaxConvert(s, response);
                        statusText = isSuccess.state;
                        success = isSuccess.data;
                        error = isSuccess.error;
                        isSuccess = !error;
                    }

                    // 请求失败时
                } else {
                    // We extract error from statusText
                    // then normalize statusText and status for non-aborts
                    error = statusText;
                    if (status || !statusText) {
                        statusText = "error";
                        if (status < 0) {
                            status = 0;
                        }
                    }
                }

                // Set data for the fake xhr object
                // 给伪装的jqXHR附加属性
                jqXHR.status = status;
                jqXHR.statusText = (nativeStatusText || statusText) + "";

                // Success/Error
                // 触发成功/失败回调
                if (isSuccess) {
                    deferred.resolveWith(callbackContext, [success, statusText, jqXHR]);
                } else {
                    deferred.rejectWith(callbackContext, [jqXHR, statusText, error]);
                }

                // Status-dependent callbacks
                // 根据statusCode对应的状态码属性触发相应的回调
                jqXHR.statusCode(statusCode);
                statusCode = undefined;

                if (fireGlobals) {
                    // 触发全局ajaxSuccess方法
                    globalEventContext.trigger(isSuccess ? "ajaxSuccess" : "ajaxError", [jqXHR, s, isSuccess ? success : error]);
                }

                // Complete
                // 触发complete方法
                completeDeferred.fireWith(callbackContext, [jqXHR, statusText]);

                if (fireGlobals) {
                    // 触发全局ajaxComplete方法
                    globalEventContext.trigger("ajaxComplete", [jqXHR, s]);
                    // Handle the global AJAX counter
                    // 如果active<=0，触发ajaxStop方法
                    if (!(--jQuery.active)) {
                        jQuery.event.trigger("ajaxStop");
                    }
                }
            }

            return jqXHR;
        },
        getScript: function(url, callback) {
            return jQuery.get(url, undefined, callback, "script");
        },
        getJSON: function(url, data, callback) {
            return jQuery.get(url, data, callback, "json");
        }
    });

    /* Handles responses to an ajax request:
     * - sets all responseXXX fields accordingly
     * - finds the right dataType (mediates between content-type and expected dataType)
     * - returns the corresponding response
     */
    // 给s.dataTypes头部添加“*”或"text"，给ajaxConvert使用

    function ajaxHandleResponses(s, jqXHR, responses) {
        var firstDataType, ct, finalDataType, type,
            // /xml/ | /html/ | /json/
            contents = s.contents,
            dataTypes = s.dataTypes,
            // responseXML|responseText
            responseFields = s.responseFields;

        // Fill responseXXX fields
        // 给jqXHR填充responseXML/responseText
        for (type in responseFields) {
            if (type in responses) {
                jqXHR[responseFields[type]] = responses[type];
            }
        }

        // Remove auto dataType and get content-type in the process
        // 遍历删除“*”开头的dataTypes数组元素，然后获取content-type
        while (dataTypes[0] === "*") {
            dataTypes.shift();
            if (ct === undefined) {
                ct = s.mimeType || jqXHR.getResponseHeader("Content-Type");
            }
        }

        // Check if we're dealing with a known content-type
        // 检查我们正在处理的是否是一致的content-type
        // 如果是就给dataTypes数组的头部添加type
        /**
         * contents: {
                xml: /xml/,
                html: /html/,
                json: /json/
            }
         */
        if (ct) {
            for (type in contents) {
                if (contents[type] && contents[type].test(ct)) {
                    dataTypes.unshift(type);
                    break;
                }
            }
        }

        // Check to see if we have a response for the expected dataType
        // 检查我们的响应是否是预期的dataType
        if (dataTypes[0] in responses) {
            finalDataType = dataTypes[0];

            // 如果不是， 尝试转换dataTypes
        } else {
            // Try convertible dataTypes
            for (type in responses) {
                // 如果此时dataTypes数组第一个元素没有值或者可以匹配到converters，
                // 取得最终的finalDataType，结束循环
                if (!dataTypes[0] || s.converters[type + " " + dataTypes[0]]) {
                    finalDataType = type;
                    break;
                }
                if (!firstDataType) {
                    firstDataType = type;
                }
            }
            // Or just use first one
            // text | *
            finalDataType = finalDataType || firstDataType;
        }

        // If we found a dataType
        // We add the dataType to the list if needed
        // and return the corresponding response
        // 如果finalDataType不是dataTypes中的第一个元素，
        // 我们将它添加到第一个，
        // 返回responses[finalDataType]
        if (finalDataType) {
            if (finalDataType !== dataTypes[0]) {
                // 给dataTypes数组的第一个元素添加"text"或"*"
                dataTypes.unshift(finalDataType);
            }
            return responses[finalDataType];
        }
    }

    // Chain conversions given the request and the original response
    // 转换响应的数据

    function ajaxConvert(s, response) {
        var conv2, current, conv, tmp, converters = {},
            i = 0,
            // Work with a copy of dataTypes in case we need to modify it for conversion
            dataTypes = s.dataTypes.slice(),
            // "*" |"text"
            prev = dataTypes[0];

        // Apply the dataFilter if provided
        /**
         * 给Ajax返回的原始数据的进行预处理的函数。提供data和type两个参数：data是Ajax返回的原始数据，type是调用jQuery.ajax时提供的dataType参数。函数返回的值将由jQuery进一步处理。
         */
        if (s.dataFilter) {
            response = s.dataFilter(response, s.dataType);
        }

        // Create converters map with lowercased keys
        if (dataTypes[1]) {
            for (conv in s.converters) {
                converters[conv.toLowerCase()] = s.converters[conv];
            }
        }

        // Convert to each sequential dataType, tolerating list modification
        for (;
            (current = dataTypes[++i]);) {

            // There's only work to do if current dataType is non-auto
            if (current !== "*") {

                // Convert response if prev dataType is non-auto and differs from current
                if (prev !== "*" && prev !== current) {

                    // Seek a direct converter
                    conv = converters[prev + " " + current] || converters["* " + current];

                    // If none found, seek a pair
                    if (!conv) {
                        for (conv2 in converters) {

                            // If conv2 outputs current
                            tmp = conv2.split(" ");
                            if (tmp[1] === current) {

                                // If prev can be converted to accepted input
                                conv = converters[prev + " " + tmp[0]] || converters["* " + tmp[0]];
                                if (conv) {
                                    // Condense equivalence converters
                                    if (conv === true) {
                                        conv = converters[conv2];

                                        // Otherwise, insert the intermediate dataType
                                    } else if (converters[conv2] !== true) {
                                        current = tmp[0];
                                        dataTypes.splice(i--, 0, current);
                                    }

                                    break;
                                }
                            }
                        }
                    }

                    // Apply converter (if not an equivalence)
                    if (conv !== true) {

                        // Unless errors are allowed to bubble, catch and return them
                        if (conv && s["throws"]) {
                            response = conv(response);
                        } else {
                            try {
                                response = conv(response);
                            } catch (e) {
                                return {
                                    state: "parsererror",
                                    error: conv ? e : "No conversion from " + prev + " to " + current
                                };
                            }
                        }
                    }
                }

                // Update prev for next iteration
                prev = current;
            }
        }

        return {
            state: "success",
            data: response
        };
    }

    // Install script dataType
    // 给jQuery.ajaxSettings添加相应的数据（深度拷贝）
    jQuery.ajaxSetup({
        accepts: {
            script: "text/javascript, application/javascript, application/ecmascript, application/x-ecmascript"
        },
        contents: {
            script: /(?:java|ecma)script/
        },
        converters: {
            "text script": function(text) {
                jQuery.globalEval(text);
                return text;
            }
        }
    });

    // Handle cache's special case and global
    // 给prefilters['script']数组尾部添加回调,
    jQuery.ajaxPrefilter("script", function(s) {
        if (s.cache === undefined) {
            s.cache = false;
        }
        // 跨域请求限制
        if (s.crossDomain) {
            s.type = "GET";
            s.global = false;
        }
    });

    // Bind script tag hack transport
    // 给transports['script']数组尾部添加回调
    jQuery.ajaxTransport("script", function(s) {

        // This transport only deals with cross domain requests
        // 该分发器只处理跨域请求
        if (s.crossDomain) {

            var script,
                head = document.head || jQuery("head")[0] || document.documentElement;

            return {

                send: function(_, callback) {
                    // 创建一个script标签，添加相应属性
                    script = document.createElement('script');

                    script.async = true;

                    if (s.scriptCharset) {
                        script.charset = s.scriptCharset;
                    }

                    script.src = s.url;

                    // Attach handlers for all browsers
                    // 为所有浏览器添加事件处理器
                    script.onload = script.onreadystatechange = function(_, isAbort) {
                        // 如果script加载完毕或者isAbort为true
                        if (isAbort || !script.readyState || /loaded|complete/.test(script.readyState)) {

                            // Handle memory leak in IE
                            // 处理IE的内存泄露
                            script.onload = script.onreadystatechange = null;

                            // Remove the script
                            // 删除script标签
                            if (script.parentNode) {
                                script.parentNode.removeChild(script);
                            }

                            // Dereference the script
                            script = null;

                            // Callback if not abort
                            // 如果isAbort部位true执行callback
                            if (!isAbort) {
                                callback(200, "success");
                            }
                        }
                    };

                    // Circumvent IE6 bugs with base elements (#2709 and #4378) by prepending
                    // Use native DOM manipulation to avoid our domManip AJAX trickery
                    // 插入到头部避免IE6的bug
                    head.insertBefore(script, head.firstChild);
                },
                // 退出的时候执行的回调
                abort: function() {
                    if (script) {
                        script.onload(undefined, true);
                    }
                }
            };
        }
    });
    var oldCallbacks = [],
        // 匹配=?或者??
        rjsonp = /(=)\?(?=&|$)|\?\?/;

    // Default jsonp settings
    // 为jQuery.ajaxSettings添加jsonp属性和jsonpCallback方法
    jQuery.ajaxSetup({
        jsonp: "callback",
        jsonpCallback: function() {
            // TODO
            var callback = oldCallbacks.pop() || (jQuery.expando + "_" + (ajax_nonce++));
            this[callback] = true;
            return callback;
        }
    });

    // Detect, normalize options and install callbacks for jsonp requests
    // 给prefilters['json']和prefilters['jsonp']数组添加回调
    jQuery.ajaxPrefilter("json jsonp", function(s, originalSettings, jqXHR) {

        var callbackName, overwritten, responseContainer,
            /*
         在一个jsonp请求中重写回调函数的名字。这个值用来替代在"callback=?"这种GET或POST请求中URL参数里的"callback"部分，比如{jsonp:'onJsonPLoad'}会导致将"onJsonPLoad=?"传给服务器。
         */
            // 如果url中包含了=?或者??，用“url”,否则如果data中有就用"data"
            jsonProp = s.jsonp !== false && (rjsonp.test(s.url) ?
                "url" :
                typeof s.data === "string" && !(s.contentType || "").indexOf("application/x-www-form-urlencoded") && rjsonp.test(s.data) && "data"
            );

        // Handle if the expected data type is "jsonp" or we have a parameter to set
        // 如果有jsonProp或者第一个data-type是"jsonp"
        if (jsonProp || s.dataTypes[0] === "jsonp") {

            // Get callback name, remembering preexisting value associated with it
            // 获取回调名称，如果是函数就将函数返回值当做名称
            callbackName = s.jsonpCallback = jQuery.isFunction(s.jsonpCallback) ?
                s.jsonpCallback() :
                s.jsonpCallback;

            // Insert callback into url or form data
            // 如果有jsonProp，将s[jsonProp]的=?替换成=callbackName
            if (jsonProp) {
                s[jsonProp] = s[jsonProp].replace(rjsonp, "$1" + callbackName);
            } else if (s.jsonp !== false) {
                // 否则如果s.jsonp为true,直接在url上面添加
                s.url += (ajax_rquery.test(s.url) ? "&" : "?") + s.jsonp + "=" + callbackName;
            }

            // Use data converter to retrieve json after script execution
            // 当script执行后使用converter取回json对象
            s.converters["script json"] = function() {
                if (!responseContainer) {
                    jQuery.error(callbackName + " was not called");
                }
                return responseContainer[0];
            };

            // force json dataType
            // 强制将dataType数组第一个元素设为“json”
            s.dataTypes[0] = "json";

            // Install callback
            // 安装回调
            // 先用个变量保存以前callback的内容
            overwritten = window[callbackName];
            window[callbackName] = function() {
                // responseContainer为参数，其中第一个参数是服务端返回的json对象
                responseContainer = arguments;
            };

            // Clean-up function (fires after converters)
            jqXHR.always(function() {
                // Restore preexisting value
                window[callbackName] = overwritten;

                // Save back as free
                if (s[callbackName]) {
                    // make sure that re-using the options doesn't screw things around
                    s.jsonpCallback = originalSettings.jsonpCallback;

                    // save the callback name for future use
                    oldCallbacks.push(callbackName);
                }

                // Call if it was a function and we have a response
                if (responseContainer && jQuery.isFunction(overwritten)) {
                    overwritten(responseContainer[0]);
                }

                responseContainer = overwritten = undefined;
            });

            // Delegate to script
            return "script";
        }
    });
    var xhrCallbacks, xhrSupported,
        xhrId = 0,
        // #5280: Internet Explorer will keep connections alive if we don't abort on unload
        xhrOnUnloadAbort = window.ActiveXObject && function() {
            // Abort all pending requests
            var key;
            for (key in xhrCallbacks) {
                xhrCallbacks[key](undefined, true);
            }
        };

    // Functions to create xhrs
    // 创建标准的XHR对象

    function createStandardXHR() {
        try {
            return new window.XMLHttpRequest();
        } catch (e) {}
    }

    // 创建IE的XHR对象

    function createActiveXHR() {
        try {
            return new window.ActiveXObject("Microsoft.XMLHTTP");
        } catch (e) {}
    }

    // Create the request object
    // (This is still attached to ajaxSettings for backward compatibility)
    // 如果支持ActiveXObject对象，先试着创建标准的XHR，不行就使用ActiveXObject，
    // 否则就使用标准的xhr对象
    jQuery.ajaxSettings.xhr = window.ActiveXObject ?
    /* Microsoft failed to properly
     * implement the XMLHttpRequest in IE7 (can't request local files),
     * so we use the ActiveXObject when it is available
     * Additionally XMLHttpRequest can be disabled in IE7/IE8 so
     * we need a fallback.
     */

    function() {
        return !this.isLocal && createStandardXHR() || createActiveXHR();
    } :
    // For all other browsers, use the standard XMLHttpRequest object
    createStandardXHR;

    // Determine support properties
    // 检测浏览器是否支持ajax
    xhrSupported = jQuery.ajaxSettings.xhr();
    jQuery.support.cors = !! xhrSupported && ("withCredentials" in xhrSupported);
    xhrSupported = jQuery.support.ajax = !! xhrSupported;

    // Create transport if the browser can provide an xhr
    if (xhrSupported) {
        // 给 transports['*']数组添加回调
        jQuery.ajaxTransport(function(s) {
            // Cross domain only allowed if supported through XMLHttpRequest
            // 不进行跨域请求或者只有支持XMLHttpRequest跨域请求的才符合条件
            if (!s.crossDomain || jQuery.support.cors) {

                var callback;

                return {
                    send: function(headers, complete) {

                        // Get a new xhr
                        var handle, i,
                            // 创建一个xhr的实例
                            xhr = s.xhr();

                        // Open the socket
                        // Passing null username, generates a login popup on Opera (#2865)
                        // 如果有username就传username和password
                        if (s.username) {
                            xhr.open(s.type, s.url, s.async, s.username, s.password);
                        } else {
                            // 否则直接传
                            xhr.open(s.type, s.url, s.async);
                        }

                        // Apply custom fields if provided
                        /*
                         对“文件名-文件值”在本机设置XHR对象。例如，如果需要的话，你可以用它来设置withCredentials为true的跨域请求。
                         */
                        if (s.xhrFields) {
                            for (i in s.xhrFields) {
                                xhr[i] = s.xhrFields[i];
                            }
                        }

                        // Override mime type if needed
                        // 重写mimeType
                        if (s.mimeType && xhr.overrideMimeType) {
                            xhr.overrideMimeType(s.mimeType);
                        }

                        // X-Requested-With header
                        // For cross-domain requests, seeing as conditions for a preflight are
                        // akin to a jigsaw puzzle, we simply never set it to be sure.
                        // (it can always be set on a per-request basis or even using ajaxSetup)
                        // For same-domain requests, won't change header if already provided.
                        // 如果没有跨域且头没有"X-Requested-With"属性，添加
                        if (!s.crossDomain && !headers["X-Requested-With"]) {
                            headers["X-Requested-With"] = "XMLHttpRequest";
                        }

                        // Need an extra try/catch for cross domain requests in Firefox 3
                        // 需要为FF3捕获错误
                        try {
                            for (i in headers) {
                                xhr.setRequestHeader(i, headers[i]);
                            }
                        } catch (err) {}

                        // Do send the request
                        // This may raise an exception which is actually
                        // handled in jQuery.ajax (so no try/catch here)
                        // 真正的发送请求了
                        xhr.send((s.hasContent && s.data) || null);

                        // Listener
                        callback = function(_, isAbort) {
                            var status, responseHeaders, statusText, responses;

                            // Firefox throws exceptions when accessing properties
                            // of an xhr when a network error occurred
                            // http://helpful.knobs-dials.com/index.php/Component_returned_failure_code:_0x80040111_(NS_ERROR_NOT_AVAILABLE)
                            // 当网络发生错误的时候，FF会报错
                            try {

                                // Was never called and is aborted or complete
                                // callback从未被执行且（需要退出请求或者已经请求完毕）
                                if (callback && (isAbort || xhr.readyState === 4)) {

                                    // Only called once
                                    // 重写自己确保只执行一次
                                    callback = undefined;

                                    // Do not keep as active anymore
                                    // 这里的handle时xhrId，
                                    // 我们需要注销有关信息
                                    if (handle) {
                                        xhr.onreadystatechange = jQuery.noop;
                                        if (xhrOnUnloadAbort) {
                                            delete xhrCallbacks[handle];
                                        }
                                    }

                                    // If it's an abort
                                    // 如果需要退出请求，当请求还没执行完毕时，执行abort方法
                                    if (isAbort) {
                                        // Abort it manually if needed
                                        if (xhr.readyState !== 4) {
                                            xhr.abort();
                                        }
                                    } else {
                                        responses = {};
                                        status = xhr.status;
                                        responseHeaders = xhr.getAllResponseHeaders();

                                        // When requesting binary data, IE6-9 will throw an exception
                                        // on any attempt to access responseText (#11426)
                                        if (typeof xhr.responseText === "string") {
                                            responses.text = xhr.responseText;
                                        }

                                        // Firefox throws an exception when accessing
                                        // statusText for faulty cross-domain requests
                                        try {
                                            statusText = xhr.statusText;
                                        } catch (e) {
                                            // We normalize with Webkit giving an empty statusText
                                            statusText = "";
                                        }

                                        // Filter status for non standard behaviors

                                        // If the request is local and we have data: assume a success
                                        // (success with no data won't get notified, that's the best we
                                        // can do given current implementations)
                                        if (!status && s.isLocal && !s.crossDomain) {
                                            status = responses.text ? 200 : 404;
                                            // IE - #1450: sometimes returns 1223 when it should be 204
                                        } else if (status === 1223) {
                                            status = 204;
                                        }
                                    }
                                }
                            } catch (firefoxAccessException) {
                                if (!isAbort) {
                                    complete(-1, firefoxAccessException);
                                }
                            }

                            // Call complete if needed
                            if (responses) {
                                complete(status, statusText, responses, responseHeaders);
                            }
                        };

                        if (!s.async) {
                            // if we're not in sync mode we fire the callback
                            // 如果是同步请求，我们立刻执行回调
                            callback();
                        } else if (xhr.readyState === 4) {
                            // (IE6 & IE7) if it's in cache and has been
                            // retrieved directly we need to fire the callback
                            // 如果请求成功后直接执行callback
                            setTimeout(callback);
                        } else {
                            handle = ++xhrId;
                            // IE会保持连接，只有在unload事件中退出请求
                            if (xhrOnUnloadAbort) {
                                // Create the active xhrs callbacks list if needed
                                // and attach the unload handler
                                if (!xhrCallbacks) {
                                    xhrCallbacks = {};
                                    jQuery(window).unload(xhrOnUnloadAbort);
                                }
                                // Add to list of active xhrs callbacks
                                // 给激活的xhr列表添加回调
                                xhrCallbacks[handle] = callback;
                            }
                            xhr.onreadystatechange = callback;
                        }
                    },

                    abort: function() {
                        if (callback) {
                            callback(undefined, true);
                        }
                    }
                };
            }
        });
    }