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ASP.NET Web API 如果采用Web Host方式来寄宿，在请求进入Web API 消息处理管道之前，就会用ASP.NET 自身的路由系统根据注册的路由表，解析出当前请求的HttpController和Action的名称，以及与目标Action方法某个参数进行绑定的路由变量。
ASP.NET路由系统包括两方面应用：
- 注册路由映射，即注册路由模板和物理文件的匹配关系，实现请求URL地址和处理请求的物理地址的分离，可以提高请求URL可读性，SEO优化，灵活性，即请求URL和处理请求的物理文件的变化互不影响
- URL生成，根据注册的路由规则生成相应URL，使用这种URL，刚好利用了路由注册的灵活性，可以使原来生成的URL不中断，只需要修改路由配置的处理文件。
一、涉及的类及源码分析
涉及的主要类型都在System.Web.Routing中，类及主要成员和相互关系如下图：
1、RouteBase
public abstract class RouteBase
{
private bool _routeExistingFiles = true;
//根据路由模板与请求的URL进行匹配，如果成功返回RouteData，否则返回NULL
public abstract RouteData GetRouteData(HttpContextBase httpContext);
//采用指定的路由变量列表（包含名称和值）与URL路由模板进行匹配，若匹配成功，返回完整URL，否则返回NULL
public abstract VirtualPathData GetVirtualPath(RequestContext requestContext, RouteValueDictionary values);
//表示是否对现有的物理文件实施路由，默认值为true,即通过地址“/employees/hr/index.aspx” 是访问不到 Index.aspx页面
//但是有时候我们希望以物理文件路径方式来访问对应的物理文件，可以设置该值为false,就可以访问到Index.aspx页面
public bool RouteExistingFiles
{
get
{
return this._routeExistingFiles;
}
set
{
this._routeExistingFiles = value;
}
}
}
2、Route
RouteBase唯一子类，基于路由模板模式的路由匹配规则就定义在其中，向全局路由表中添加的就是一个Route对象。
public class Route : RouteBase
{
private const string HttpMethodParameterName = "httpMethod";
private string _url;
private ParsedRoute _parsedRoute;
//构造函数，前边省略N个重载
public Route(string url, RouteValueDictionary defaults, RouteValueDictionary constraints, RouteValueDictionary dataTokens, IRouteHandler routeHandler)
{
this.Url = url;
this.Defaults = defaults;
this.Constraints = constraints;
this.DataTokens = dataTokens;
this.RouteHandler = routeHandler;
}
//为路由模板中的变量以正则表达式的形武设定一些约束条件，Key为变量名，Value为正则表达式
//如果有定义，匹配也要满足该约束
public RouteValueDictionary Constraints { get; set; }
//存储额外的变量值
public RouteValueDictionary DataTokens { get; set; }
//路由变量默认值，也不一定要定义在路由模板中
public RouteValueDictionary Defaults { get; set; }
public IRouteHandler RouteHandler { get; set; }
//路由模板，如/weather/{areacode}/{days}，请求的URL就是跟该模板进行匹配，/分割成多个段，每个段又分变量（areacode，days）和字面量（weather）
//匹配的两个条件，段数和路由模板相同，以及对应文本段内容也要相同，注，URL大小写不敏感
public string Url
{
get
{
return this._url ?? string.Empty;
}
set
{
this._parsedRoute = RouteParser.Parse(value);
this._url = value;
}
}
//根据路由模板与请求的URL进行匹配，如果成功返回RouteData，否则返回NULL
public override RouteData GetRouteData(HttpContextBase httpContext)
{
RouteValueDictionary values = this._parsedRoute.Match(httpContext.Request.AppRelativeCurrentExecutionFilePath.Substring(2) + httpContext.Request.PathInfo, this.Defaults);
if (values == null)
return (RouteData)null;
RouteData routeData = new RouteData((RouteBase)this, this.RouteHandler);
if (!this.ProcessConstraints(httpContext, values, RouteDirection.IncomingRequest))
return (RouteData)null;
foreach (KeyValuePair<string, object> keyValuePair in values)
routeData.Values.Add(keyValuePair.Key, keyValuePair.Value);
if (this.DataTokens != null)
{
foreach (KeyValuePair<string, object> dataToken in this.DataTokens)
routeData.DataTokens[dataToken.Key] = dataToken.Value;
}
return routeData;
}
//采用指定的路由变量列表（包含名称和值）与URL路由模板进行匹配，若匹配成功，返回完整URL，否则返回NULL
public override VirtualPathData GetVirtualPath(RequestContext requestContext, RouteValueDictionary values)
{
BoundUrl boundUrl = this._parsedRoute.Bind(requestContext.RouteData.Values, values, this.Defaults, this.Constraints);
if (boundUrl == null)
return (VirtualPathData)null;
if (!this.ProcessConstraints(requestContext.HttpContext, boundUrl.Values, RouteDirection.UrlGeneration))
return (VirtualPathData)null;
VirtualPathData virtualPathData = new VirtualPathData((RouteBase)this, boundUrl.Url);
if (this.DataTokens != null)
{
foreach (KeyValuePair<string, object> dataToken in this.DataTokens)
virtualPathData.DataTokens[dataToken.Key] = dataToken.Value;
}
return virtualPathData;
}
//处理约束
protected virtual bool ProcessConstraint(HttpContextBase httpContext, object constraint, string parameterName, RouteValueDictionary values, RouteDirection routeDirection)
{
IRouteConstraint routeConstraint = constraint as IRouteConstraint;
if (routeConstraint != null)
return routeConstraint.Match(httpContext, this, parameterName, values, routeDirection);
string str = constraint as string;
if (str == null)
throw new InvalidOperationException(string.Format((IFormatProvider)CultureInfo.CurrentUICulture, System.Web.SR.GetString("Route_ValidationMustBeStringOrCustomConstraint"), new object[2]
{
(object) parameterName,
(object) this.Url
}));
object obj;
values.TryGetValue(parameterName, out obj);
return Regex.IsMatch(Convert.ToString(obj, (IFormatProvider)CultureInfo.InvariantCulture), "^(" + str + ")$", RegexOptions.IgnoreCase | RegexOptions.CultureInvariant);
}
private bool ProcessConstraints(HttpContextBase httpContext, RouteValueDictionary values, RouteDirection routeDirection)
{
if (this.Constraints != null)
{
foreach (KeyValuePair<string, object> constraint in this.Constraints)
{
if (!this.ProcessConstraint(httpContext, constraint.Value, constraint.Key, values, routeDirection))
return false;
}
}
return true;
}
}
另外，RequestContext是对Http请求上下文和路由数据的封装
public class RequestContext
{
public RequestContext()
{
}
public RequestContext(HttpContextBase httpContext, RouteData routeData)
{
if (httpContext == null)
throw new ArgumentNullException(nameof(httpContext));
if (routeData == null)
throw new ArgumentNullException(nameof(routeData));
this.HttpContext = httpContext;
this.RouteData = routeData;
}
//请求上下文
public virtual HttpContextBase HttpContext { get; set; }
//路由数据
public virtual RouteData RouteData { get; set; }
}
3、RouteData
RouteBase的GetRouteData方法的返回类型，用于封装解析后路由数据，其用RouteValueDictionary保存路由变量数据，RouteValueDictionary是—个字典,其 Key和 Value分别表示路由变量的名称和值，定义如下：
public class RouteValueDictionary : IDictionary<string, object>
{
}
public class RouteData
{
private RouteValueDictionary _values = new RouteValueDictionary();
private RouteValueDictionary _dataTokens = new RouteValueDictionary();
private IRouteHandler _routeHandler;
public RouteData()
{
}
public RouteData(RouteBase route, IRouteHandler routeHandler)
{
this.Route = route;
this.RouteHandler = routeHandler;
}
//是直接附加到Route上的自定义变量。
public RouteValueDictionary DataTokens
{
get
{
return this._dataTokens;
}
}
//解析它的 Route对象
public RouteBase Route { get; set; }
//提供最 终用 于处理请求的HttpHandIer对象(通过调用其GetHttpHandler方法获取)
//可以在构造函数中传入，也可以属性赋值
public IRouteHandler RouteHandler
{
get
{
return this._routeHandler;
}
set
{
this._routeHandler = value;
}
}
//其中的路由变量是Route通过对请求URL的解析得到的
public RouteValueDictionary Values
{
get
{
return this._values;
}
}
//获取包含某些固定名称的变量值（如controller和action）对应的值
public string GetRequiredString(string valueName)
{
object obj;
if (this.Values.TryGetValue(valueName, out obj))
{
string str = obj as string;
if (!string.IsNullOrEmpty(str))
return str;
}
//不存在直接抛出异常
throw new InvalidOperationException(string.Format((IFormatProvider)CultureInfo.CurrentUICulture, System.Web.SR.GetString("RouteData_RequiredValue"), new object[1]
{
(object) valueName
}));
}
}
public interface IRouteHandler
{
IHttpHandler GetHttpHandler(RequestContext requestContext);
}
4、VirtualPathData
RouteBase的GeVirtualPathData方法的返回类型
public class VirtualPathData
{
private RouteValueDictionary _dataTokens = new RouteValueDictionary();
private string _virtualPath;
public VirtualPathData(RouteBase route, string virtualPath)
{
this.Route = route;
this.VirtualPath = virtualPath;
}
//来源于附加到 Route的 自定义变量集合
public RouteValueDictionary DataTokens
{
get
{
return this._dataTokens;
}
}
//当时解析的路由对象
public RouteBase Route { get; set; }
//完整虚拟路径
public string VirtualPath
{
get
{
return this._virtualPath ?? string.Empty;
}
set
{
this._virtualPath = value;
}
}
}
5、RouteTable Routes RouteCollection
全局路由表，即RouteTable类的静态属性Routes 类型为RouteCollection，通过其中的MapPageRoute方法进行路由映射
public class RouteTable
{
private static RouteCollection _instance = new RouteCollection();
public static RouteCollection Routes
{
get
{
return RouteTable._instance;
}
}
}
RouteCollection 是Route的集合
主要逻辑：
RouteCollection的GetRouteData和GetVirtalPath方法会遍历集合中的每个Route对象，并传入给定的参数调用同名方法直到找到一个匹配的Route(返回值不为Null),并返回相应的RouteData和VirtaulPathData对象，如果集合中任何一个Route都不匹配，最终返回NULL
读写锁的应用：
RouteCollection这个集合对象不是线程安全的，使用ReaderWriterLockSlim进行线程读写同步，多个线程可以同时读，其他情况都不允许，一个线程写时，其他的线程不能读或写，一个线程在读时候，其他线程只能读，不能写，即多个线程只能读读，不能读写、写写；
在对集合进行读取或更新时候，会调用GetReadLock和GetWriteLock方法获取读锁或写锁，返回的是内嵌私有类型:ReadLockDispsoabled和WriteLockDispsoabled,他们实现了接口IDispsoabled,是对ReaderWriterLockSlim的读写锁功能的封装。
public class RouteCollection : Collection<RouteBase>
{
private Dictionary<string, RouteBase> _namedMap = new Dictionary<string, RouteBase>((IEqualityComparer<string>)StringComparer.OrdinalIgnoreCase);
private ReaderWriterLockSlim _rwLock = new ReaderWriterLockSlim();
public RouteCollection()
{
}
public bool RouteExistingFiles { get; set; }
public void Add(string name, RouteBase item)
{
//...
this.Add(item);
if (!string.IsNullOrEmpty(name))
this._namedMap[name] = item;
Route route = item as Route;
if (route == null || route.RouteHandler == null)
return;
}
//省略N个重载方法
public Route MapPageRoute(string routeName, string routeUrl, string physicalFile, bool checkPhysicalUrlAccess, RouteValueDictionary defaults, RouteValueDictionary constraints, RouteValueDictionary dataTokens)
{
if (routeUrl == null)
throw new ArgumentNullException(nameof(routeUrl));
//新建路由对象,IRouteHandler直接new PageRouteHandler
Route route = new Route(routeUrl, defaults, constraints, dataTokens, (IRouteHandler)new PageRouteHandler(physicalFile, checkPhysicalUrlAccess));
//添加进去
this.Add(routeName, (RouteBase)route);
return route;
}
protected override void ClearItems()
{
this._namedMap.Clear();
base.ClearItems();
}
public IDisposable GetReadLock()
{
this._rwLock.EnterReadLock();
return (IDisposable)new System.Web.Routing.RouteCollection.ReadLockDisposable(this._rwLock);
}
private RequestContext GetRequestContext(RequestContext requestContext)
{
if (requestContext != null)
return requestContext;
HttpContext current = HttpContext.Current;
if (current == null)
throw new InvalidOperationException(System.Web.SR.GetString("RouteCollection_RequiresContext"));
return new RequestContext((HttpContextBase)new HttpContextWrapper(current), new RouteData());
}
private bool IsRouteToExistingFile(HttpContextBase httpContext)
{
string executionFilePath = httpContext.Request.AppRelativeCurrentExecutionFilePath;
if (!(executionFilePath != "~/") || this.VPP == null)
return false;
if (!this.VPP.FileExists(executionFilePath))
return this.VPP.DirectoryExists(executionFilePath);
return true;
}
public RouteData GetRouteData(HttpContextBase httpContext)
{
...
using (this.GetReadLock())
{
//遍历集合中所有RouteBase，并调用其GetRouteData方法，找到了就马上返回
foreach (RouteBase routeBase in (Collection<RouteBase>)this)
{
RouteData routeData = routeBase.GetRouteData(httpContext);
if (routeData != null)
{
if (!routeBase.RouteExistingFiles)
{
if (!flag2)
flag1 = this.IsRouteToExistingFile(httpContext);
if (flag1)
return (RouteData)null;
}
return routeData;
}
}
}
return (RouteData)null;
}
public VirtualPathData GetVirtualPath(RequestContext requestContext, RouteValueDictionary values)
{
requestContext = this.GetRequestContext(requestContext);
using (this.GetReadLock())
{
//遍历集合中所有RouteBase，并调用其GetVirtualPath方法，找到了就马上返回
foreach (RouteBase routeBase in (Collection<RouteBase>)this)
{
VirtualPathData virtualPath = routeBase.GetVirtualPath(requestContext, values);
if (virtualPath != null)
{
virtualPath.VirtualPath = this.NormalizeVirtualPath(requestContext, virtualPath.VirtualPath);
return virtualPath;
}
}
}
return (VirtualPathData)null;
}
public VirtualPathData GetVirtualPath(RequestContext requestContext, string name, RouteValueDictionary values)
{
requestContext = this.GetRequestContext(requestContext);
if (string.IsNullOrEmpty(name))
return this.GetVirtualPath(requestContext, values);
RouteBase routeBase;
bool flag;
using (this.GetReadLock())
flag = this._namedMap.TryGetValue(name, out routeBase);
if (flag)
{
VirtualPathData virtualPath = routeBase.GetVirtualPath(requestContext, values);
if (virtualPath == null)
return (VirtualPathData)null;
virtualPath.VirtualPath = this.NormalizeVirtualPath(requestContext, virtualPath.VirtualPath);
return virtualPath;
}
throw new ArgumentException(string.Format((IFormatProvider)CultureInfo.CurrentUICulture, System.Web.SR.GetString("RouteCollection_NameNotFound"), new object[1]
{
(object) name
}), nameof(name));
}
public IDisposable GetWriteLock()
{
this._rwLock.EnterWriteLock();
return (IDisposable)new System.Web.Routing.RouteCollection.WriteLockDisposable(this._rwLock);
}
//忽略路由
public void Ignore(string url)
{
this.Ignore(url, (object)null);
}
public void Ignore(string url, object constraints)
{
if (url == null)
throw new ArgumentNullException(nameof(url));
System.Web.Routing.RouteCollection.IgnoreRouteInternal ignoreRouteInternal = new System.Web.Routing.RouteCollection.IgnoreRouteInternal(url);
RouteValueDictionary routeValueDictionary = new RouteValueDictionary(constraints);
ignoreRouteInternal.Constraints = routeValueDictionary;
this.Add((RouteBase)ignoreRouteInternal);
}
protected override void InsertItem(int index, RouteBase item)
{
if (item == null)
throw new ArgumentNullException(nameof(item));
if (this.Contains(item))
throw new ArgumentException(string.Format((IFormatProvider)CultureInfo.CurrentCulture, System.Web.SR.GetString("RouteCollection_DuplicateEntry"), new object[0]), nameof(item));
base.InsertItem(index, item);
}
protected override void RemoveItem(int index)
{
this.RemoveRouteName(index);
base.RemoveItem(index);
}
private void RemoveRouteName(int index)
{
RouteBase routeBase = this[index];
foreach (KeyValuePair<string, RouteBase> named in this._namedMap)
{
if (named.Value == routeBase)
{
this._namedMap.Remove(named.Key);
break;
}
}
}
protected override void SetItem(int index, RouteBase item)
{
if (item == null)
throw new ArgumentNullException(nameof(item));
if (this.Contains(item))
throw new ArgumentException(string.Format((IFormatProvider)CultureInfo.CurrentCulture, System.Web.SR.GetString("RouteCollection_DuplicateEntry"), new object[0]), nameof(item));
this.RemoveRouteName(index);
base.SetItem(index, item);
}
private class ReadLockDisposable : IDisposable
{
private ReaderWriterLockSlim _rwLock;
public ReadLockDisposable(ReaderWriterLockSlim rwLock)
{
this._rwLock = rwLock;
}
void IDisposable.Dispose()
{
this._rwLock.ExitReadLock();
}
}
private class WriteLockDisposable : IDisposable
{
private ReaderWriterLockSlim _rwLock;
public WriteLockDisposable(ReaderWriterLockSlim rwLock)
{
this._rwLock = rwLock;
}
void IDisposable.Dispose()
{
this._rwLock.ExitWriteLock();
}
}
private sealed class IgnoreRouteInternal : Route
{
public IgnoreRouteInternal(string url)
: base(url, (IRouteHandler)new StopRoutingHandler())
{
}
public override VirtualPathData GetVirtualPath(RequestContext requestContext, RouteValueDictionary routeValues)
{
return (VirtualPathData)null;
}
}
}
6、IRouteConstraint
除了用正则表达式对某个变量进行约束外，还可以用一个实现了IRouteConstraint接口的对象对整个请求进行约束
public interface IRouteConstraint
{
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