使Web API支持namespace

淡淡的烟草味﹌ 2021-12-19 16:19 90阅读 0赞

[使Web API支持namespace][Web API_namespace]

# 问题描述 #

假设我有一个应用场景：Core Framework可以用于任何区域的站点，其中的CustomersController有个取customer的fullname的方法GetFullName()，可想而知，这个api在中国和美国的站点上，应该得到不同的返回值。如下图所示：

[![web api namespace][]][web api namespace 1]

这样的设计可以带来两个好处：

1、利用了OO的思想，可以封装各个区域customer service相关的一些公共逻辑

2、使得client端可以一致的接口访问服务，如：[http://hostname/api/customers][http_hostname_api_customers]

这看上去不错，但是为了达到我的目的，就必须让web api支持在不同的namespace（或者area）中，存在相同名称的controller。但是web api默认情况下是不支持的，那么是否可以通过某种方法，使web api支持这种效果呢？答案是肯定的。

# 查找原因 #

为了让web api支持namespace（或者area），就必须找到为什么默认情况下web api不支持，在这个过程中，也许能找到切入点。为了能找到原因，我做了如下工作：

### 1、Server-Side Handlers ###

从mvc4官网，找到了server端的request处理过程，如下图所示：

[![web api handlers][]][web api handlers 1]

从图中我们可以看到，controller是通过HttpControllerDispatcher调度器，来处理的。

### 2、HttpControllerDispatcher ###

HttpControllerDispatcher 位于System.Web.Http.Dispatcher命名空间中，其源代码中有一个私有属性：

private IHttpControllerSelector ControllerSelector
    {
        get
        {
            if (this._controllerSelector == null)
            {
                this._controllerSelector = this._configuration.Services.GetHttpControllerSelector();
            }
            return this._controllerSelector;
        }
    }

从代码中可以看出，该属性，只是简单的从services容器中，得到IHttpControllerSelector类型的一个对象，所以问题现在转移到在这个controllerselector对象上。

### 3、DefaultHttpControllerSelector ###

在this.\_configuration.Services.GetHttpControllerSelector();这条语句中，\_configuration其实就是System.Web.Http.HttpConfiguration，在其构造函数中，可以看到：

this.Services = new DefaultServices(this);

DefaultServices为ServicesContainer的一个子类，所以可以称之为服务容器，定义在System.Web.Http.Services命名空间下，在其构造函数中，有如下代码：

......
    this.SetSingle<IHttpControllerActivator>(new DefaultHttpControllerActivator());
    
    this.SetSingle<IHttpControllerTypeResolver>(new DefaultHttpControllerTypeResolver());
                ......

从红色部分这正是我所需要的找的controllerselector。

### 4、问题所在 ###

从第2步中，如果通过源代码，可以发现：HttpControllerDispatcher 在处理request时，需要通过HttpControllerDescriptor对象的CreateController方法，才能最终实例化一个ApiController。而HttpControllerDescriptor是通过IHttpControllerSelector（默认就是DefaultHttpControllerSelector）的SelectController方法构造的。我进一步在DefaultHttpControllerSelector源码中，发现如下代码：

private ConcurrentDictionary<string, HttpControllerDescriptor> InitializeControllerInfoCache()
    {
        ConcurrentDictionary<string, HttpControllerDescriptor> dictionary = new ConcurrentDictionary<string, HttpControllerDescriptor>(StringComparer.OrdinalIgnoreCase);
        HashSet<string> set = new HashSet<string>();
        foreach (KeyValuePair<string, ILookup<string, Type>> pair in this._controllerTypeCache.Cache)
        {
            string key = pair.Key;
            foreach (IGrouping<string, Type> grouping in pair.Value)
            {
                foreach (Type type in grouping)
                {
                    if (dictionary.Keys.Contains(key))
                    {
                        set.Add(key);
                        break;
                    }
                    dictionary.TryAdd(key, new HttpControllerDescriptor(this._configuration, key, type));
                }
            }
        }
        foreach (string str2 in set)
        {
            HttpControllerDescriptor descriptor;
            dictionary.TryRemove(str2, out descriptor);
        }
        return dictionary;
    }

和HttpControllerTypeCache这样一个cache辅助类里的：

private Dictionary<string, ILookup<string, Type>> InitializeCache()
    {
        IAssembliesResolver assembliesResolver = this._configuration.Services.GetAssembliesResolver();
        return this._configuration.Services.GetHttpControllerTypeResolver().GetControllerTypes(assembliesResolver).
            GroupBy<Type, string>(t => t.Name.Substring(0, t.Name.Length - DefaultHttpControllerSelector.ControllerSuffix.Length), StringComparer.OrdinalIgnoreCase).
            ToDictionary<IGrouping<string, Type>, string, ILookup<string, Type>>(g => g.Key, 
            g => g.ToLookup<Type, string>(t => (t.Namespace ?? string.Empty), StringComparer.OrdinalIgnoreCase), StringComparer.OrdinalIgnoreCase);
    }

这就是问题所在，导致在同一个assembly中，不能有两个相同名字的api controller。否则就会执行：

ICollection<Type> controllerTypes = this._controllerTypeCache.GetControllerTypes(controllerName);
    if (controllerTypes.Count == 0)
    {
        throw new HttpResponseException(request.CreateErrorResponse(HttpStatusCode.NotFound, System.Web.Http.Error.Format(SRResources.ResourceNotFound, new object[] { request.RequestUri }), System.Web.Http.Error.Format(SRResources.DefaultControllerFactory_ControllerNameNotFound, new object[] { controllerName })));
    }
    throw CreateAmbiguousControllerException(request.GetRouteData().Route, controllerName, controllerTypes);

controllerTypes.Count大于0，导致抛出“Multiple types were found that match the controller named…”异常。

# 解决问题 #

到现在为止，其实解决办法已经出来了：就是不用上面的两个方法，来构造HttpControllerDispatcher。那么我们就只能自定义IHttpControllerSelector了。所以我自定义了一个NamespaceHttpControllerSelector用于支持namespace，源代码如下：

public class NamespaceHttpControllerSelector : DefaultHttpControllerSelector
    {
        private const string NamespaceRouteVariableName = "Namespace";
        private readonly HttpConfiguration _configuration;
        private readonly Lazy<ConcurrentDictionary<string, Type>> _apiControllerCache;
    
        public NamespaceHttpControllerSelector(HttpConfiguration configuration)
            : base(configuration)
        {
            _configuration = configuration;
            _apiControllerCache = new Lazy<ConcurrentDictionary<string, Type>>(new Func<ConcurrentDictionary<string,Type>>(InitializeApiControllerCache));
        }
    
        private ConcurrentDictionary<string, Type> InitializeApiControllerCache()
        {
            IAssembliesResolver assembliesResolver = this._configuration.Services.GetAssembliesResolver();
            var types = this._configuration.Services.GetHttpControllerTypeResolver().GetControllerTypes(assembliesResolver).ToDictionary(t => t.FullName, t => t);
    
            return new ConcurrentDictionary<string, Type>(types);
        }
    
        public IEnumerable<string> GetControllerFullName(HttpRequestMessage request, string controllerName)
        {
            object namespaceName;
            var data = request.GetRouteData();
            IEnumerable<string> keys = _apiControllerCache.Value.ToDictionary<KeyValuePair<string, Type>, string, Type>(t => t.Key, 
                    t => t.Value, StringComparer.CurrentCultureIgnoreCase).Keys.ToList();
            if (data.Route.DataTokens == null ||
                !data.Route.DataTokens.TryGetValue(NamespaceRouteVariableName, out namespaceName))
            {
                return from k in keys
                        where k.EndsWith(string.Format(".{0}{1}", controllerName, DefaultHttpControllerSelector.ControllerSuffix), StringComparison.CurrentCultureIgnoreCase)
                        select k;
            }
    
            //get the defined namespace
            string[] namespaces = (string[])namespaceName;
            return from n in namespaces
                    join k in keys on string.Format("{0}.{1}{2}", n, controllerName, DefaultHttpControllerSelector.ControllerSuffix).ToLower() equals k.ToLower()
                    select k;
        }
    
        public override HttpControllerDescriptor SelectController(HttpRequestMessage request)
        {
            Type type;
            if (request == null)
            {
                throw new ArgumentNullException("request");
            }
            string controllerName = this.GetControllerName(request);
            if (string.IsNullOrEmpty(controllerName))
            {
                throw new HttpResponseException(request.CreateErrorResponse(HttpStatusCode.NotFound,
                    string.Format("No route providing a controller name was found to match request URI '{0}'", new object[] { request.RequestUri })));
            }
            IEnumerable<string> fullNames = GetControllerFullName(request, controllerName);
            if (fullNames.Count() == 0)
            {
                throw new HttpResponseException(request.CreateErrorResponse(HttpStatusCode.NotFound,
                        string.Format("No route providing a controller name was found to match request URI '{0}'", new object[] { request.RequestUri })));
            }
    
            if (this._apiControllerCache.Value.TryGetValue(fullNames.First(), out type))
            {
                return new HttpControllerDescriptor(_configuration, controllerName, type);
            }
            throw new HttpResponseException(request.CreateErrorResponse(HttpStatusCode.NotFound,
                string.Format("No route providing a controller name was found to match request URI '{0}'", new object[] { request.RequestUri })));
        }
    }

其实代码并不难懂，核心部分就是：

1、InitializeApiControllerCache方法，用于通过fullname为key，构造一个controller type的一个集合

2、GetControllerFullName方法，从namespace数组和\_apiControllerCache集合中，取到符合条件的controller的fullname

到目前为止，我们的工作还剩下最后一步，就是用NamespaceHttpControllerSelector替换DefaultHttpControllerSelector，使其生效。通过以上的分析，其实也很明显了，只需要在Application\_Start方法中，用DefaultServices继承下来的Replace即可：

GlobalConfiguration.Configuration.Services.Replace(typeof(IHttpControllerSelector), 
        new NamespaceHttpControllerSelector(GlobalConfiguration.Configuration));

至此，才算大功告成！

posted on 2013-01-03 10:15 [Xavier Zhang][] 阅读( ...) 评论( ...) [编辑][Link 1] 收藏

转载于:https://www.cnblogs.com/champoin/archive/2013/01/03/2842714.html

[Web API_namespace]: https://www.cnblogs.com/champoin/archive/2013/01/03/2842714.html
[web api namespace]: /images/20211219/3eb8e3208fef4bfca6e29b2122cf5e07.png
[web api namespace 1]: https://images0.cnblogs.com/blog/38021/201301/03101506-3222714cdd1c4413b3727b2c8f9624c6.jpg
[http_hostname_api_customers]: http://hostname/api/customers
[web api handlers]: /images/20211219/48501a116476410993eaac2e8d6d893b.png
[web api handlers 1]: https://images0.cnblogs.com/blog/38021/201301/03101508-de4b3a710e2f48858db6ee6a83bb482c.png
[Xavier Zhang]: https://www.cnblogs.com/champoin/
[Link 1]: https://i.cnblogs.com/EditPosts.aspx?postid=2842714