源码解析.Net中DependencyInjection的实现
前言
笔者的这篇文章和上篇文章思路一样，不注重依赖注入的使用方法，更加注重源码的实现，我尽量的表达清楚内容，让读者能够真正的学到东西。如果有不太清楚依赖注入是什么或怎么在.Net项目中使用的话，请点击这里，这是微软的官方文档，把用法介绍的很清晰了，相信你会有很大收获。那么废话不多说，咱们进入正题（可能篇幅有点长，耐心读完你会有收获的????）。
DependencyInjection类之间的关系
下图中只列举重要的类和接口(实际的类和接口有很多)，里面的方法和属性也只列出重要的，这里只是让你有个大概的印象，看不懂没关系，继续往下读。
源码解析
首先我们要知道，.Net程序在启动的时候会构建一个Host(Host.Build().Run()),在其Build方法中来构建容器(只构建根容器，下面会解释)，具体在哪构建容器，源码如下图：
public class HostBuilder : IHostBuilder{ private List<Action<IConfigurationBuilder>> _configureHostConfigActions = new List<Action<IConfigurationBuilder>>(); public IHostBuilder ConfigureAppConfiguration(Action<HostBuilderContext, IConfigurationBuilder> configureDelegate)
{
_configureAppConfigActions.Add(configureDelegate ?? throw new ArgumentNullException(nameof(configureDelegate))); return this;
} //即我们在main函数中看到的Build方法
public IHost Build()
{ //只能执行一次这个方法
if (_hostBuilt)
{ throw new InvalidOperationException(SR.BuildCalled);
}
_hostBuilt = true; using var diagnosticListener = new DiagnosticListener("Microsoft.Extensions.Hosting"); const string hostBuildingEventName = "HostBuilding"; const string hostBuiltEventName = "HostBuilt"; if (diagnosticListener.IsEnabled() && diagnosticListener.IsEnabled(hostBuildingEventName))
{
Write(diagnosticListener, hostBuildingEventName, this);
} //执行Host配置(应用程序执行路径，增加_dotnet环境变量，获取命令行参数,加载预配置)
BuildHostConfiguration(); //设置主机环境变量
CreateHostingEnvironment(); //设置上下文
CreateHostBuilderContext(); //构建程序配置(加载appsetting.json)
BuildAppConfiguration(); //构造容器，加载服务
CreateServiceProvider(); var host = _appServices.GetRequiredService<IHost>(); if (diagnosticListener.IsEnabled() && diagnosticListener.IsEnabled(hostBuiltEventName))
{
Write(diagnosticListener, hostBuiltEventName, host);
} return host;
}
private void CreateServiceProvider()
{ //构建服务集合
var services = new ServiceCollection();
services.AddSingleton<IHostingEnvironment>(_hostingEnvironment);
services.AddSingleton<IHostEnvironment>(_hostingEnvironment);
services.AddSingleton(_hostBuilderContext);
services.AddSingleton(_ => _appConfiguration);
services.AddSingleton<IApplicationLifetime>(s => (IApplicationLifetime)s.GetService<IHostApplicationLifetime>());
services.AddSingleton<IHostApplicationLifetime, ApplicationLifetime>();
AddLifetime(services);
services.AddSingleton<IHost>(_ =>
{ return new Internal.Host(_appServices,
_hostingEnvironment,
_defaultProvider,
_appServices.GetRequiredService<IHostApplicationLifetime>(),
_appServices.GetRequiredService<ILogger<Internal.Host>>(),
_appServices.GetRequiredService<IHostLifetime>(),
_appServices.GetRequiredService<IOptions<HostOptions>>());
});
services.AddOptions().Configure<HostOptions>(options => { options.Initialize(_hostConfiguration); });
services.AddLogging(); //加载StartUp里面的ConfigService方法
foreach (Action<HostBuilderContext, IServiceCollection> configureServicesAction in _configureServicesActions)
{
configureServicesAction(_hostBuilderContext, services);
} //构建容器
object containerBuilder = _serviceProviderFactory.CreateBuilder(services); //配置容器适配器，可以构建自己的容器
foreach (IConfigureContainerAdapter containerAction in _configureContainerActions)
{
containerAction.ConfigureContainer(_hostBuilderContext, containerBuilder);
} //生成容器服务
_appServices = _serviceProviderFactory.CreateServiceProvider(containerBuilder); if (_appServices == null)
{ throw new InvalidOperationException(SR.NullIServiceProvider);
} //在根容器中先生成IConfiguration
_ = _appServices.GetService<IConfiguration>();
}
}从上面可以看出，_serviceProviderFactory实际上就是DefaultServiceProviderFactory对象实例，它通过调用自己的CreateServiceProvider方法，来构建默认容器，而默认容器是作为根容器存在，全局只有一个。
_serviceProviderFactory.CreateBuilder(services)方法只是简单的转换(把IServiceCollection转化成Object)，为什么他要返回object呢？因为在.net中默认实现是个IServiceCollection，那么如果要是需要自定义容器的话，则需要通过自己的ProviderFactory返回自己的Builder，来生成容器，在这里就不细讲了，以后其他文章会详细说下怎么自定义一个IOC，这里只讲默认实现，接下来我们具体讲它是怎么生成容器服务的，请看CreateServiceProvider方法实现：
//.net提供的默认的ProviderFactorypublic class DefaultServiceProviderFactory : IServiceProviderFactory<IServiceCollection>
{ private readonly ServiceProviderOptions _options; public DefaultServiceProviderFactory() : this(ServiceProviderOptions.Default)
{
} public DefaultServiceProviderFactory(ServiceProviderOptions options)
{ if (options == null)
{ throw new ArgumentNullException(nameof(options));
}
_options = options;
} public IServiceCollection CreateBuilder(IServiceCollection services)
{ return services;
} //通过调用扩展方法,来实现ServiceProvider
public IServiceProvider CreateServiceProvider(IServiceCollection containerBuilder)
{ return containerBuilder.BuildServiceProvider(_options);
}
}public static class ServiceCollectionContainerBuilderExtensions{ public static ServiceProvider BuildServiceProvider(this IServiceCollection services, ServiceProviderOptions options)
{ if (services == null)
{ throw new ArgumentNullException(nameof(services));
} if (options == null)
{ throw new ArgumentNullException(nameof(options));
}
return new ServiceProvider(services, options);
}
}public sealed class ServiceProvider : IServiceProvider, IDisposable, IAsyncDisposable{ //验证Scoped服务是否有在Singleton服务中存在，默认是关闭的，因为可能会把Scoped服务提升为Singleton服务，那么这个本身就是不正常的调用，建议修改调用方法。
private readonly CallSiteValidator _callSiteValidator; //用来获取服务
private readonly Func<Type, Func<ServiceProviderEngineScope, object>> _createServiceAccessor; //服务提供的引擎
internal ServiceProviderEngine _engine; //是否已被释放
private bool _disposed; //对每种type做了一个缓存，来提高效率
private ConcurrentDictionary<Type, Func<ServiceProviderEngineScope, object>> _realizedServices; //服务描述工厂，保存着服务描述的集合，描述服务的类型
internal CallSiteFactory CallSiteFactory { get; } //根容器
internal ServiceProviderEngineScope Root { get; } internal static bool VerifyOpenGenericServiceTrimmability { get; } =
AppContext.TryGetSwitch("Microsoft.Extensions.DependencyInjection.VerifyOpenGenericServiceTrimmability", out bool verifyOpenGenerics) ? verifyOpenGenerics : false; internal ServiceProvider(ICollection<ServiceDescriptor> serviceDescriptors, ServiceProviderOptions options)
{ //根容器就是本身，并且IsRootScope是true标识
Root = new ServiceProviderEngineScope(this, isRootScope: true);
//获取用那种引擎来获取服务
_engine = GetEngine();
//获取服务时调用的函数
_createServiceAccessor = CreateServiceAccessor;
_realizedServices = new ConcurrentDictionary<Type, Func<ServiceProviderEngineScope, object>>(); //主要是生成服务的ServiceCallSite这个很重要，用来描述服务类型
CallSiteFactory = new CallSiteFactory(serviceDescriptors); //额外的一些内部服务描述
CallSiteFactory.Add(typeof(IServiceProvider), new ServiceProviderCallSite());
CallSiteFactory.Add(typeof(IServiceScopeFactory), new ConstantCallSite(typeof(IServiceScopeFactory), Root));
CallSiteFactory.Add(typeof(IServiceProviderIsService), new ConstantCallSite(typeof(IServiceProviderIsService), CallSiteFactory)); if (options.ValidateScopes)
{
_callSiteValidator = new CallSiteValidator();
} //在build阶段就验证服务注入是否正确
if (options.ValidateOnBuild)
{
List<Exception> exceptions = null; foreach (ServiceDescriptor serviceDescriptor in serviceDescriptors)
{ try
{
ValidateService(serviceDescriptor);
}
catch (Exception e)
{
exceptions = exceptions ?? new List<Exception>();
exceptions.Add(e);
}
} if (exceptions != null)
{ throw new AggregateException("Some services are not able to be constructed", exceptions.ToArray());
}
}
} public object GetService(Type serviceType) => GetService(serviceType, Root); private void OnCreate(ServiceCallSite callSite)
{
_callSiteValidator?.ValidateCallSite(callSite);
} private void OnResolve(Type serviceType, IServiceScope scope)
{
_callSiteValidator?.ValidateResolution(serviceType, scope, Root);
} internal object GetService(Type serviceType, ServiceProviderEngineScope serviceProviderEngineScope)
{ if (_disposed)
{
ThrowHelper.ThrowObjectDisposedException();
}
Func<ServiceProviderEngineScope, object> realizedService = _realizedServices.GetOrAdd(serviceType, _createServiceAccessor);
OnResolve(serviceType, serviceProviderEngineScope);
DependencyInjectionEventSource.Log.ServiceResolved(this, serviceType); var result = realizedService.Invoke(serviceProviderEngineScope);
System.Diagnostics.Debug.Assert(result is null || CallSiteFactory.IsService(serviceType)); return result;
} private Func<ServiceProviderEngineScope, object> CreateServiceAccessor(Type serviceType)
{
ServiceCallSite callSite = CallSiteFactory.GetCallSite(serviceType, new CallSiteChain()); if (callSite != null)
{
DependencyInjectionEventSource.Log.CallSiteBuilt(this, serviceType, callSite);
OnCreate(callSite); // Optimize singleton case
if (callSite.Cache.Location == CallSiteResultCacheLocation.Root)
{ object value = CallSiteRuntimeResolver.Instance.Resolve(callSite, Root); return scope => value;
} return _engine.RealizeService(callSite);
} return _ => null;
} internal IServiceScope CreateScope()
{ if (_disposed)
{
ThrowHelper.ThrowObjectDisposedException();
} return new ServiceProviderEngineScope(this, isRootScope: false);
} private ServiceProviderEngine GetEngine()
{
ServiceProviderEngine engine;#if NETFRAMEWORK || NETSTANDARD2_0
engine = new DynamicServiceProviderEngine(this);#else
if (RuntimeFeature.IsDynamicCodeCompiled)
{
engine = new DynamicServiceProviderEngine(this);
} else
{
engine = RuntimeServiceProviderEngine.Instance;
}#endif
return engine;
}
}从上面的代码中可以看出，在构建ServiceProvider时，主要是构造默认的根容器和采用哪种引擎来获取服务，并且把服务的类型描述给构建好并缓存起来。我们再来看看GetService方法，里面的调用服务分为Root和非Root，也就是说，需要区分哪些是属于根容器的，哪些不是属于根容器的。在.Net中，默认情况下添加的添加的Singleton类型属于Root,Scoped类型属于Scope，Transient类型属于Dispose，具体请看下面代码：
internal struct ResultCache
{ public ResultCache(ServiceLifetime lifetime, Type type, int slot)
{ switch (lifetime)
{ case ServiceLifetime.Singleton:
Location = CallSiteResultCacheLocation.Root; break; case ServiceLifetime.Scoped:
Location = CallSiteResultCacheLocation.Scope; break; case ServiceLifetime.Transient:
Location = CallSiteResultCacheLocation.Dispose; break; default:
Location = CallSiteResultCacheLocation.None; break;
}
Key = new ServiceCacheKey(type, slot);
} public CallSiteResultCacheLocation Location { get; set; } public ServiceCacheKey Key { get; set; }
}有了基本的了解之后，我们再来看通过GetService方法获取对象实例，当类型是位于根容器时，会将根容器的实例做在缓存里面，而类型如果是Transient，那么则每次获取时，都重新创建，不做缓存处理，至于Scoped方式的类型，稍后再说，我们先看下面代码
internal sealed class CallSiteRuntimeResolver : CallSiteVisitor<RuntimeResolverContext, object>
{ //对于Transient的类型实例
protected override object VisitDisposeCache(ServiceCallSite transientCallSite, RuntimeResolverContext context)
{ //直接构造类型实例，并记录在dispose的集合中，等待容器被Dispose时，同时dispose掉
return context.Scope.CaptureDisposable(VisitCallSiteMain(transientCallSite, context));
} //获取在根容器的对象实例
protected override object VisitRootCache(ServiceCallSite callSite, RuntimeResolverContext context)
{ //对于已经做过缓存的服务，直接返回
if (callSite.Value is object value)
{ return value;
} var lockType = RuntimeResolverLock.Root;
ServiceProviderEngineScope serviceProviderEngine = context.Scope.RootProvider.Root; //对当前服务类型上锁
lock (callSite)
{ //相当于一个双检锁，再查一遍
if (callSite.Value is object resolved)
{ return resolved;
}
resolved = VisitCallSiteMain(callSite, new RuntimeResolverContext
{
Scope = serviceProviderEngine,
AcquiredLocks = context.AcquiredLocks | lockType
});
serviceProviderEngine.CaptureDisposable(resolved); //将获取到的实例做缓存，返回拿到的实例
callSite.Value = resolved; return resolved;
}
}
}而对于注入的Scope类型，我们知道针对每次请求下，针对每个类型的实例是一个，而在ServiceProviderEngineScope类中有一个CreateScope方法，而每次接收到请求时都会构建HttpContext实例，在构建的同时调用CreateScope方法，来构建新的容器作为本次请求的Scope容器，期间获取的Scope类型的实例，都是在里面获取，先做缓存再返回，保证这个作用域内的实例是一个，请看下面代码：
internal sealed class CallSiteRuntimeResolver : CallSiteVisitor<RuntimeResolverContext, object>
{ protected override object VisitScopeCache(ServiceCallSite callSite, RuntimeResolverContext context)
{ //如果是根容器作用域，就在根容器中找，否则就在当前作用域容器中找
return context.Scope.IsRootScope ?
VisitRootCache(callSite, context) :
VisitCache(callSite, context, context.Scope, RuntimeResolverLock.Scope);
} private object VisitCache(ServiceCallSite callSite, RuntimeResolverContext context, ServiceProviderEngineScope serviceProviderEngine, RuntimeResolverLock lockType)
{ bool lockTaken = false; object sync = serviceProviderEngine.Sync;
Dictionary<ServiceCacheKey, object> resolvedServices = serviceProviderEngine.ResolvedServices; if ((context.AcquiredLocks & lockType) == 0)
{
Monitor.Enter(sync, ref lockTaken);
} try
{ //每次查找前，先判断这个实例是否已经创建过，如果在本容器的缓存集合中存在就直接返回
if (resolvedServices.TryGetValue(callSite.Cache.Key, out object resolved))
{ return resolved;
}
resolved = VisitCallSiteMain(callSite, new RuntimeResolverContext
{
Scope = serviceProviderEngine,
AcquiredLocks = context.AcquiredLocks | lockType
});
serviceProviderEngine.CaptureDisposable(resolved); //将类型实例添加到缓存中
resolvedServices.Add(callSite.Cache.Key, resolved); return resolved;
} finally
{ if (lockTaken)
{
Monitor.Exit(sync);
}
}
}
}总结
通过源码可以看出默认的依赖注入有以下特点：
在一个Host中只能存在一个根容器，而其他容器(每次请求创建)都是从根容器中衍生出来的。
Scope的类型可能被提升到Singleton。
对于Singleton的注入类型，都是存放在根容器中，并作缓存。
对于Scoped的注入类型，大部分是存放在每次请求构建的容器中，并作缓存。
对于Transient的注入类型，则不做缓存，每次访问都构建出一个新的对象实例。
关于.Net中默认的依赖注入，上面的代码也只是挑出重点的部分分享给大家，具体想看更多细节，读者可以根据本篇博客直接看源码，因为篇幅问题，实在不能贴太多的代码，主要是把思路给大家说一下。