var xmlhttp = new XMLHttpRequest();
xmlhttp.onreadystatechange = function() {
  if(xmlhttp.readystate == 4 && xmlhttp.status == 200) {
    var response = xmlhttp.responseText;
  }else if(xmlhttp.status == 400) { //或者可以是任何以4开头的状态码
    //优雅地处理错误
  }
};
//建立连接
xmlhttp.open("GET", "http://myserver/api", true);
//发起请求
xmlhttp.send();

$http.get('api/user', {params: {id:'5'}
}).success(function(data, status, headers, config) {
  //加载成功之后做一些事
}).error(function(data, status, headers, config) {
  //处理错误
});

var postData = {text:'long blob of text'};
//下面这一行会被当成参数附加到URL后面，所以post请求最终会变成/api/user?id=5
var config = {params: {id: '5'}};
$http.post('api/user', postData, config
).success(function(data, status, headers, config) {
  //成功之后做一些事情
}).error(function(data, status, headers, config) {
  //处理错误
});

$http(config)

$http({
  method: string,
  url: string,
  params: object,
  data: string or object,
  headers: object,
  transformRequest: function transform(data, headersGetter) or an array of functions,
  transformResponse: function transform(data, headersGetter) or an array of functions,
  cache: boolean or Cache object,
  timeout: number,
  withCredentials: boolean
});

[{key1: 'value1', key2: 'value2'}]

?key1=value&key2=value2

angular.module('MyApp', []).
  config(function($httpProvider) {
    //删除AngularJS默认的X-Request-With头
    delete $httpProvider.default.headers.common['X-Requested-With'];
    //为所有GET请求设置DO NOT TRACK
    $httpProvider.default.headers.get['DNT'] = '1';
});

$http.get('api/user', {
   //设置Authorization(授权)头。在真实的应用中，你需要到一个服务里面去获取auth令牌
   headers: {'Authorization': 'Basic Qzsda231231'},
   params: {id:5}
}).success(function() {//处理成功的情况 });

$http.get('http://server/myapi', {
  cache: true
}).success(function() {//处理成功的情况});

var module = angular.module('myApp');
module.config(function($httpProvider) {
  $httpProvider.defaults.transformRequest = function(data) {
     //使用jQuery的param方法把JSON数据转换成字符串形式
     return $.param(data);
   };
});

angular.module('app', [
  'ngAnimate',
  'ngCookies',
  'ngResource',
  'ngRoute',
  'ngSanitize',
  'ngTouch'
],function ($httpProvider) {
  // 头部配置
  $httpProvider.defaults.headers.post['Content-Type'] = 'application/x-www-form-urlencoded;charset=utf-8';
  $httpProvider.defaults.headers.post['Accept'] = 'application/json, text/javascript, */*; q=0.01';
  $httpProvider.defaults.headers.post['X-Requested-With'] = 'XMLHttpRequest';
  /**
   * 重写angular的param方法，使angular使用jquery一样的数据序列化方式 The workhorse; converts an object to x-www-form-urlencoded serialization.
   * @param {Object} obj
   * @return {String}
   */
  var param = function (obj) {
    var query = '', name, value, fullSubName, subName, subValue, innerObj, i;
    for (name in obj) {
      value = obj[name];
      if (value instanceof Array) {
        for (i = 0; i < value.length; ++i) {
          subValue = value[i];
          fullSubName = name + '[' + i + ']';
          innerObj = {};
          innerObj[fullSubName] = subValue;
          query += param(innerObj) + '&';
        }
      }
      else if (value instanceof Object) {
        for (subName in value) {
          subValue = value[subName];
          fullSubName = name + '[' + subName + ']';
          innerObj = {};
          innerObj[fullSubName] = subValue;
          query += param(innerObj) + '&';
        }
      }
      else if (value !== undefined && value !== null)
        query += encodeURIComponent(name) + '=' + encodeURIComponent(value) + '&';
    }
    return query.length ? query.substr(0, query.length - 1) : query;
  };
  // Override $http service's default transformRequest
  $httpProvider.defaults.transformRequest = [function (data) {
    return angular.isObject(data) && String(data) !== '[object File]' ? param(data) : data;
  }];
}).config(function ($routeProvider) {
    $routeProvider
      .when('/', {
        templateUrl: 'views/main.html',
        controller: 'MainCtrl'
      })
      .when('/about', {
        templateUrl: 'views/about.html',
        controller: 'AboutCtrl'
      })
      .otherwise({
        redirectTo: '/'
      });
  });