在这里可以看到，如果要使用Aspnet Core自身的认证体系，需先注册Scheme，并且该Scheme必须指定一个类型为IAuthenticationHandler的Handler，否则会抛出异常。（这个其实在AddxxxScheme的时候已经指定了AuthenticationHandler）
我们再看一下IAuthenticationSchemeProvider的GetRequestHandlerSchemesAsync方法做了什么
public virtual Task<IEnumerable<AuthenticationScheme>> GetRequestHandlerSchemesAsync() { return Task.FromResult<IEnumerable<AuthenticationScheme>>((IEnumerable<AuthenticationScheme>) this._requestHandlers); }
这东西返回了_requestHandlers，这是什么？看代码
public class AuthenticationSchemeProvider : IAuthenticationSchemeProvider { private readonly object _lock = new object(); private readonly AuthenticationOptions _options; private readonly IDictionary<string, AuthenticationScheme> _schemes; private readonly List<AuthenticationScheme> _requestHandlers; /// <summary> /// Creates an instance of <see cref="T:Microsoft.AspNetCore.Authentication.AuthenticationSchemeProvider" /> /// using the specified <paramref />, /// </summary> public AuthenticationSchemeProvider(IOptions<AuthenticationOptions> options) : this(options, (IDictionary<string, AuthenticationScheme>) new Dictionary<string, AuthenticationScheme>((IEqualityComparer<string>) StringComparer.Ordinal)) { } /// <summary> /// Creates an instance of <see cref="T:Microsoft.AspNetCore.Authentication.AuthenticationSchemeProvider" /> /// using the specified <paramref /> and <paramref />. /// </summary> protected AuthenticationSchemeProvider( IOptions<AuthenticationOptions> options, IDictionary<string, AuthenticationScheme> schemes) { this._options = options.Value; IDictionary<string, AuthenticationScheme> dictionary = schemes; if (dictionary == null) throw new ArgumentNullException(nameof (schemes)); this._schemes = dictionary; this._requestHandlers = new List<AuthenticationScheme>(); foreach (AuthenticationSchemeBuilder scheme in this._options.Schemes) this.AddScheme(scheme.Build()); } public virtual void AddScheme(AuthenticationScheme scheme) { if (this._schemes.ContainsKey(scheme.Name)) throw new InvalidOperationException("Scheme already exists: " + scheme.Name); lock (this._lock) { if (this._schemes.ContainsKey(scheme.Name)) throw new InvalidOperationException("Scheme already exists: " + scheme.Name); if (typeof (IAuthenticationRequestHandler).IsAssignableFrom(scheme.HandlerType)) this._requestHandlers.Add(scheme); this._schemes[scheme.Name] = scheme; } } ..... }
这东西就是把我们在认证注册服务中指定的scheme，通过解析出的AuthenticationSchemeProvider 的构造函数加载来的，进而返回一系列的List<AuthenticationScheme>，OK拿到这些scheme之后有什么用呢？这里引出了我们的第二个对象AuthenticationHandlerProvider，下面我们来了解一下。
IAuthenticationHandlerProvider
我们看到，AuthenticationMiddleware中用到了IAuthenticationHandlerProvider的GetHandlerAsync方法，那我们先看一下这个方法的作用
public class AuthenticationHandlerProvider : IAuthenticationHandlerProvider { private Dictionary<string, IAuthenticationHandler> _handlerMap = new Dictionary<string, IAuthenticationHandler>((IEqualityComparer<string>) StringComparer.Ordinal); /// <summary>Constructor.</summary> public AuthenticationHandlerProvider(IAuthenticationSchemeProvider schemes) { this.Schemes = schemes; } /// <summary> /// The <see cref="T:Microsoft.AspNetCore.Authentication.IAuthenticationHandlerProvider" />. /// </summary> public IAuthenticationSchemeProvider Schemes { get; } /// <summary>Returns the handler instance that will be used.</summary> public async Task<IAuthenticationHandler> GetHandlerAsync( HttpContext context, string authenticationScheme) { if (this._handlerMap.ContainsKey(authenticationScheme)) return this._handlerMap[authenticationScheme]; AuthenticationScheme schemeAsync = await this.Schemes.GetSchemeAsync(authenticationScheme); if (schemeAsync == null) return (IAuthenticationHandler) null; IAuthenticationHandler handler = (context.RequestServices.GetService(schemeAsync.HandlerType) ?? ActivatorUtilities.CreateInstance(context.RequestServices, schemeAsync.HandlerType)) as IAuthenticationHandler; if (handler != null) { await handler.InitializeAsync(schemeAsync, context); this._handlerMap[authenticationScheme] = handler; } return handler; } }