在上一章中，详细介绍了 ASP.NET Core 中的授权策略，在需要授权时，只需要在对应的Controler或者Action上面打上[Authorize]特性，并指定要执行的策略名称即可，但是，授权策略是怎么执行的呢？怀着一颗好奇的心，忍不住来探索一下它的执行流程。
目录
- MVC中的授权
- AuthorizationApplicationModelProvider
- AuthorizeFilter
- IPolicyEvaluator
- AuthenticateAsync(AuthenticationSchemes)
- AuthorizeAsync(Requirements)
- IAuthorizationService
- IAuthorizationPolicyProvider
- IAuthorizationHandlerContextFactory
- IAuthorizationHandlerProvider
- IAuthorizationEvaluator
在《(上一章》中提到，AuthorizeAttribute只是一个简单的实现了IAuthorizeData接口的特性，并且在 ASP.NET Core 授权系统中并没有使用到它。我们知道在认证中，还有一个UseAuthentication扩展方法来激活认证系统，但是在授权中并没有类似的机制。
这是因为当我们使用[Authorize]通常是在MVC中，由MVC来负责激活授权系统。本来在这个系列的文章中，我并不想涉及到MVC的知识，但是为了能更好的理解授权系统的执行，就来简单介绍一下MVC中与授权相关的知识。
MVC中的授权
当我们使用MVC时，首先会调用MVC的AddMvc扩展方法，用来注册一些MVC相关的服务：
public static IMvcBuilder AddMvc(this IServiceCollection services)
{var builder = services.AddMvcCore();builder.AddAuthorization();...
}public static IMvcCoreBuilder AddAuthorization(this IMvcCoreBuilder builder)
{AddAuthorizationServices(builder.Services);return builder;
}internal static void AddAuthorizationServices(IServiceCollection services)
{services.AddAuthenticationCore();services.AddAuthorization();services.AddAuthorizationPolicyEvaluator();services.TryAddEnumerable(ServiceDescriptor.Transient<IApplicationModelProvider, AuthorizationApplicationModelProvider>());
}在上面AddAuthorizationServices中的前三个方法都属于 ASP.NET Core 《Security》项目中提供的扩展方法，其中前两个在前面几章已经介绍过了，对于AddAuthorizationPolicyEvaluator放到后面再来介绍，我们先来看一下MVC中的AuthorizationApplicationModelProvider。
AuthorizationApplicationModelProvider
在MVC中有一个ApplicationModel的概念，它用来封装Controller, Filter, ApiExplorer等。对应的，在MVC中还提供了一系列的ApplicationModelProvider来初始化ApplicationModel的各个部分，而AuthorizationApplicationModelProvider就是用来初始化与授权相关的部分。
public class AuthorizationApplicationModelProvider : IApplicationModelProvider
{public void OnProvidersExecuting(ApplicationModelProviderContext context){foreach (var controllerModel in context.Result.Controllers){var controllerModelAuthData = controllerModel.Attributes.OfType<IAuthorizeData>().ToArray();if (controllerModelAuthData.Length > 0){controllerModel.Filters.Add(GetFilter(_policyProvider, controllerModelAuthData));}foreach (var attribute in controllerModel.Attributes.OfType<IAllowAnonymous>()){controllerModel.Filters.Add(new AllowAnonymousFilter());}foreach (var actionModel in controllerModel.Actions){var actionModelAuthData = actionModel.Attributes.OfType<IAuthorizeData>().ToArray();if (actionModelAuthData.Length > 0){actionModel.Filters.Add(GetFilter(_policyProvider, actionModelAuthData));}foreach (var attribute in actionModel.Attributes.OfType<IAllowAnonymous>()){actionModel.Filters.Add(new AllowAnonymousFilter());}}}}
}如上，首先查找每个Controller中实现了IAuthorizeData接口的特性，然后将其转化为AuthorizeFilter并添加到Controller的Filter集合中，紧接着再查找实现了IAllowAnonymous接口的特性，将其转化为AllowAnonymousFilter过滤器也添加到Filter集合中，然后以同样的逻辑查找Action上的特性并添加到Action的Filter集合中。
其中的关键点就是将IAuthorizeData（也就是通过我们熟悉的[Authorize]特性）转化为MVC中的AuthorizeFilter过滤器：
public static AuthorizeFilter GetFilter(IAuthorizationPolicyProvider policyProvider, IEnumerable<IAuthorizeData> authData)
{if (policyProvider.GetType() == typeof(DefaultAuthorizationPolicyProvider)){var policy = AuthorizationPolicy.CombineAsync(policyProvider, authData).GetAwaiter().GetResult();return new AuthorizeFilter(policy);}else{return new AuthorizeFilter(policyProvider, authData);}
}CombineAsync在上一章的《AuthorizationPolicy》中已经介绍过了，我们往下看看AuthorizeFilter的实现。
AuthorizeFilter
在MVC中有一个AuthorizeFilter过滤器，类似我们在ASP.NET 4.x中所熟悉的[Authorize]，它实现了IAsyncAuthorizationFilter接口，定义如下：
public class AuthorizeFilter : IAsyncAuthorizationFilter, IFilterFactory
{public AuthorizeFilter(AuthorizationPolicy policy) {}public AuthorizeFilter(IAuthorizationPolicyProvider policyProvider, IEnumerable<IAuthorizeData> authorizeData) : this(authorizeData) {}public AuthorizeFilter(IEnumerable<IAuthorizeData> authorizeData) {}public IEnumerable<IAuthorizeData> AuthorizeData { get; }public AuthorizationPolicy Policy { get; }public virtual async Task OnAuthorizationAsync(AuthorizationFilterContext context){var effectivePolicy = Policy;if (effectivePolicy == null){effectivePolicy = await AuthorizationPolicy.CombineAsync(PolicyProvider, AuthorizeData);}var policyEvaluator = context.HttpContext.RequestServices.GetRequiredService<IPolicyEvaluator>();var authenticateResult = await policyEvaluator.AuthenticateAsync(effectivePolicy, context.HttpContext);if (context.Filters.Any(item => item is IAllowAnonymousFilter)){return;}var authorizeResult = await policyEvaluator.AuthorizeAsync(effectivePolicy, authenticateResult, context.HttpContext, context);... // 如果授权失败，返回ChallengeResult或ForbidResult}
}AuthorizeFilter的OnAuthorizationAsync方法会在Action执行之前触发，其调用IPolicyEvaluator来完成授权，将执行流程切回到 ASP.NET Core 授权系统中。关于MVC中IApplicationModelProvider以及Filter的概念，在以后MVC系列的文章中再来详细介绍，下面就继续介绍 ASP.NET Core 的授权系统，也就是《Security》项目。
IPolicyEvaluator
IPolicyEvaluator是MVC调用授权系统的入口点，其定义如下：
public interface IPolicyEvaluator
{Task<AuthenticateResult> AuthenticateAsync(AuthorizationPolicy policy, HttpContext context);Task<PolicyAuthorizationResult> AuthorizeAsync(AuthorizationPolicy policy, AuthenticateResult authenticationResult, HttpContext context, object resource);
}在上面介绍的AddMVC中，调用了AddAuthorizationPolicyEvaluator扩展方法，它有如下定义：
public static class PolicyServiceCollectionExtensions
{public static IServiceCollection AddAuthorizationPolicyEvaluator(this IServiceCollection services){services.TryAdd(ServiceDescriptor.Transient<IPolicyEvaluator, PolicyEvaluator>());return services;}
}由此可知IPolicyEvaluator的默认实现为PolicyEvaluator，我们就从它入手，来一步一步解剖 ASP.NET Core 授权系统的执行步骤。
在AuthorizeFilter中，依次调到了AuthenticateAsync和AuthorizeAsync方法，我们就一一来看。
AuthenticateAsync(AuthenticationSchemes)
为什么还有一个AuthenticateAsync方法呢，这不是在认证阶段执行的吗？我们看下它的实现：
public class PolicyEvaluator : IPolicyEvaluator
{public virtual async Task<AuthenticateResult> AuthenticateAsync(AuthorizationPolicy policy, HttpContext context){if (policy.AuthenticationSchemes != null && policy.AuthenticationSchemes.Count > 0){ClaimsPrincipal newPrincipal = null;foreach (var scheme in policy.AuthenticationSchemes){var result = await context.AuthenticateAsync(scheme);if (result != null && result.Succeeded){newPrincipal = SecurityHelper.MergeUserPrincipal(newPrincipal, result.Principal);}}if (newPrincipal != null){context.User = newPrincipal;return AuthenticateResult.Success(new AuthenticationTicket(newPrincipal, string.Join(";", policy.AuthenticationSchemes)));}else{context.User = new ClaimsPrincipal(new ClaimsIdentity());return AuthenticateResult.NoResult();}}return (context.User?.Identity?.IsAuthenticated ?? false) ? AuthenticateResult.Success(new AuthenticationTicket(context.User, "context.User")): AuthenticateResult.NoResult();}
}在《上一章》中，我们知道在AuthorizationPolicy中有AuthenticationSchemes和IAuthorizationRequirement两个属性，并详细介绍介绍了Requirement，但是没有提到AuthenticationSchemes的调用。
那么，看到这里，也就大概明白了，它与Requirements的执行是完全独立的，并在它之前执行，用于重置Claims，那么为什么要重置呢？
在认证的章节介绍过，在认证阶段，只会执行默认的认证Scheme，context.User就是使用context.AuthenticateAsync(DefaultAuthenticateScheme)来赋值的，当我们希望使用非默认的Scheme，或者是想合并多个认证Scheme的Claims时，就需要使用基于Scheme的授权来重置Claims了。
它的实现也很简单，直接使用我们在授权策略中指定的Schemes来依次调用认证服务的AuthenticateAsync方法，并将生成的Claims合并，最后返回我们熟悉的AuthenticateResult认证结果。
AuthorizeAsync(Requirements)
接下来再看一下PolicyEvaluator的AuthorizeAsync方法：
public class PolicyEvaluator : IPolicyEvaluator
{private readonly IAuthorizationService _authorization;public PolicyEvaluator(IAuthorizationService authorization){_authorization = authorization;}public virtual async Task<PolicyAuthorizationResult> AuthorizeAsync(AuthorizationPolicy policy, AuthenticateResult authenticationResult, HttpContext context, object resource){var result = await _authorization.AuthorizeAsync(context.User, resource, policy);if (result.Succeeded) return PolicyAuthorizationResult.Success();return (authenticationResult.Succeeded) ? PolicyAuthorizationResult.Forbid() : PolicyAuthorizationResult.Challenge();}
}该方法会根据Requirements来完成授权，具体的实现是通过调用IAuthorizationService来实现的。
最终返回的是一个PolicyAuthorizationResult对象，并在授权失败时，根据认证结果来返回Forbid(未授权)或Challenge(未登录)。
public class PolicyAuthorizationResult
{private PolicyAuthorizationResult() { }public bool Challenged { get; private set; }public bool Forbidden { get; private set; }public bool Succeeded { get; private set; }
}IAuthorizationService
然后就到了授权的核心对象AuthorizationService，也可以称为授权的外交官，我们也可以直接在应用代码中调用该对象来实现授权，它有如下定义：
public interface IAuthorizationService
{ Task<AuthorizationResult> AuthorizeAsync(ClaimsPrincipal user, object resource, string policyName);Task<AuthorizationResult> AuthorizeAsync(ClaimsPrincipal user, object resource, IEnumerable<IAuthorizationRequirement> requirements);
}在
AuthorizeAsync中还涉及到一个resource对象，用来实现面向资源的授权，放在《下一章》中再来介绍，而在本章与《前一章》的示例中，该值均为null。
ASP.NET Core 中还为IAuthorizationService提供了几个扩展方法：
public static class AuthorizationServiceExtensions
{public static Task<AuthorizationResult> AuthorizeAsync(this IAuthorizationService service, ClaimsPrincipal user, string policyName) {}public static Task<AuthorizationResult> AuthorizeAsync(this IAuthorizationService service, ClaimsPrincipal user, AuthorizationPolicy policy) {}public static Task<AuthorizationResult> AuthorizeAsync(this IAuthorizationService service, ClaimsPrincipal user, object resource, IAuthorizationRequirement requirement) {}public static Task<AuthorizationResult> AuthorizeAsync(this IAuthorizationService service, ClaimsPrincipal user, object resource, AuthorizationPolicy policy) {}
}其默认实现为DefaultAuthorizationService:
public class DefaultAuthorizationService : IAuthorizationService
{private readonly AuthorizationOptions _options;private readonly IAuthorizationHandlerContextFactory _contextFactory;private readonly IAuthorizationHandlerProvider _handlers;private readonly IAuthorizationEvaluator _evaluator;private readonly IAuthorizationPolicyProvider _policyProvider;public async Task<AuthorizationResult> AuthorizeAsync(ClaimsPrincipal user, object resource, string policyName){ var policy = await _policyProvider.GetPolicyAsync(policyName);return await this.AuthorizeAsync(user, resource, policy);}public async Task<AuthorizationResult> AuthorizeAsync(ClaimsPrincipal user, object resource, IEnumerable<IAuthorizationRequirement> requirements){var authContext = _contextFactory.CreateContext(requirements, user, resource);var handlers = await _handlers.GetHandlersAsync(authContext);foreach (var handler in handlers){await handler.HandleAsync(authContext);if (!_options.InvokeHandlersAfterFailure && authContext.HasFailed){break;}}return _evaluator.Evaluate(authContext);}
}通过上面代码可以看出，在《上一章》中介绍的授权策略，在这里获取到它的Requirements，后续便不再需要了。而在AuthorizationService中是通过调用四大核心对象来完成授权，我们一一来看。
IAuthorizationPolicyProvider
由于在[Authorize]中，我们指定的是策略的名称，因此需要使用IAuthorizationPolicyProvider来根据名称获取到策略对象，默认实现为DefaultAuthorizationPolicyProvider：
public class DefaultAuthorizationPolicyProvider : IAuthorizationPolicyProvider
{private readonly AuthorizationOptions _options;public Task<AuthorizationPolicy> GetDefaultPolicyAsync(){return Task.FromResult(_options.DefaultPolicy);}public virtual Task<AuthorizationPolicy> GetPolicyAsync(string policyName){return Task.FromResult(_options.GetPolicy(policyName));}
}在上一章中介绍过，我们定义的策略都保存在《AuthorizationOptions》的字典中，因此在这里只是简单的将AuthorizationOptions中的同名方法异步化。
IAuthorizationHandlerContextFactory
授权上下文是我们接触较多的对象，当我们自定义授权Handler时就会用到它，它是使用简单工厂模式来创建的：
public class DefaultAuthorizationHandlerContextFactory : IAuthorizationHandlerContextFactory
{public virtual AuthorizationHandlerContext CreateContext(IEnumerable<IAuthorizationRequirement> requirements, ClaimsPrincipal user, object resource){return new AuthorizationHandlerContext(requirements, user, resource);}
}授权上下文中主要包含用户的Claims和授权策略的Requirements：
public class AuthorizationHandlerContext
{private HashSet<IAuthorizationRequirement> _pendingRequirements;private bool _failCalled;private bool _succeedCalled;public AuthorizationHandlerContext(IEnumerable<IAuthorizationRequirement> requirements, ClaimsPrincipal user, object resource){Requirements = requirements; User = user; Resource = resource;_pendingRequirements = new HashSet<IAuthorizationRequirement>(requirements);}public virtual bool HasFailed { get { return _failCalled; } }public virtual bool HasSucceeded => !_failCalled && _succeedCalled && !_pendingRequirements.Any();public virtual void Fail(){_failCalled = true;}public virtual void Succeed(IAuthorizationRequirement requirement){_succeedCalled = true;_pendingRequirements.Remove(requirement);}
}如上，_pendingRequirements中保存着所有待验证的Requirements，验证成功的Requirement则从中移除。
IAuthorizationHandlerProvider
兜兜转转，终于进入到了授权的最终验证逻辑中了，首先，使用IAuthorizationHandlerProvider来获取到所有的授权Handler。
IAuthorizationHandlerProvider的默认实现为DefaultAuthorizationHandlerProvider:
public class DefaultAuthorizationHandlerProvider : IAuthorizationHandlerProvider
{private readonly IEnumerable<IAuthorizationHandler> _handlers;public DefaultAuthorizationHandlerProvider(IEnumerable<IAuthorizationHandler> handlers){_handlers = handlers;}public Task<IEnumerable<IAuthorizationHandler>> GetHandlersAsync(AuthorizationHandlerContext context)=> Task.FromResult(_handlers);
}在《上一章》中，我们还介绍到，我们定义的Requirement，可以直接实现IAuthorizationHandler接口，也可以单独定义Handler，但是需要注册到DI系统中去。
在默认的AuthorizationHandlerProvider中，会从DI系统中获取到我们注册的所有Handler，最终调用其HandleAsync方法。
我们在实现IAuthorizationHandler接口时，通常是继承自AuthorizationHandler<TRequirement>来实现，它有如下定义：
public abstract class AuthorizationHandler<TRequirement> : IAuthorizationHandler where TRequirement : IAuthorizationRequirement
{public virtual async Task HandleAsync(AuthorizationHandlerContext context){foreach (var req in context.Requirements.OfType<TRequirement>()){await HandleRequirementAsync(context, req);}}protected abstract Task HandleRequirementAsync(AuthorizationHandlerContext context, TRequirement requirement);
}如上，首先会在HandleAsync过滤出与Requirement对匹配的Handler，然后再调用其HandleRequirementAsync方法。
那我们定义的直接实现IAuthorizationHandler了接口的Requirement又是如何执行的呢？
在AddAuthorization扩展方法中可以看到，默认还为IAuthorizationHandler注册了一个PassThroughAuthorizationHandler，定义如下：
public class PassThroughAuthorizationHandler : IAuthorizationHandler
{public async Task HandleAsync(AuthorizationHandlerContext context){foreach (var handler in context.Requirements.OfType<IAuthorizationHandler>()){await handler.HandleAsync(context);}}
}它负责调用该策略中所有实现了IAuthorizationHandler接口的Requirement。
IAuthorizationEvaluator
最后，通过调用IAuthorizationEvaluator接口，来完成最终的授权结果，默认实现为DefaultAuthorizationEvaluator:
public class DefaultAuthorizationEvaluator : IAuthorizationEvaluator
{public AuthorizationResult Evaluate(AuthorizationHandlerContext context)=> context.HasSucceeded? AuthorizationResult.Success(): AuthorizationResult.Failed(context.HasFailed? AuthorizationFailure.ExplicitFail(): AuthorizationFailure.Failed(context.PendingRequirements));
}当我们在一个策略中指定多个Requirement时，只有全部验证通过时，授权上下文中的HasSucceeded才会为True，而HasFailed代表授权结果的显式失败。
这里根据授权上下文的验证结果来生成授权结果：
public class AuthorizationResult
{public bool Succeeded { get; private set; }public AuthorizationFailure Failure { get; private set; }public static AuthorizationResult Success() => new AuthorizationResult { Succeeded = true };public static AuthorizationResult Failed(AuthorizationFailure failure) => new AuthorizationResult { Failure = failure };public static AuthorizationResult Failed() => new AuthorizationResult { Failure = AuthorizationFailure.ExplicitFail() };
}public class AuthorizationFailure
{private AuthorizationFailure() { }public bool FailCalled { get; private set; }public IEnumerable<IAuthorizationRequirement> FailedRequirements { get; private set; }public static AuthorizationFailure ExplicitFail(){return new AuthorizationFailure { FailCalled = true, FailedRequirements = new IAuthorizationRequirement[0] };}public static AuthorizationFailure Failed(IEnumerable<IAuthorizationRequirement> failed)=> new AuthorizationFailure { FailedRequirements = failed };}整个授权流程的结构大致如下：
总结
通过对 ASP.NET Core 授权系统执行流程的探索，可以看出授权是主要是通过调用IAuthorizationService来完成的，而授权策略的本质是提供 Requirement ，我们完全可以使用它们两个来完成各种灵活的授权方式，而不用局限于策略。在 ASP.NET Core 中，还提供了基于资源的授权，放在《下一章》中来介绍，并会简单演示一下在一个通用权限管理系统中如何来授权。